Question 1

Can a 3-core cable be used for a single-phase supply?

Accepted Answer

Yes. A 3-core cable can supply a single-phase load. The three cores are then used as phase, neutral and earth. For an armoured cable, the armour is usually also bonded to earth as a separate protective path.
Using a 3-core cable this way is common for single-phase motors, pumps, lighting feeders and equipment fed from a distribution board. All three cores are the same size in the normal construction, so the phase and neutral conductors carry the load current while the earth core carries current only during a fault.
The cable must be sized for the load current and the length of the run. A cable sized only for the load can still be too small if voltage drop is high on a long run. A 3-core cable should not be confused with a 3.5-core cable, which has a reduced neutral and is intended for three-phase systems.

Question 2

Can copper-clad aluminium (CCA) LAN cable be used for PoE cameras?

Accepted Answer

Copper-clad aluminium (CCA) cable has an aluminium core with a thin copper coating. It is lighter and cheaper than solid copper, but aluminium has higher electrical resistance than copper, so a CCA conductor of the same size carries power less efficiently.
That matters for Power over Ethernet (PoE) cameras, because the cable carries power as well as data. The higher resistance means more voltage drop and more heat in the cable, and the problem grows on long runs or in bundles. Structured cabling standards such as ANSI/TIA-568 and ISO/IEC 11801 are written around solid copper conductors, and CCA cable does not meet them.
APAR recommends solid copper cable for any PoE installation. APAR Tarang Shakti LAN cable uses 24 AWG solid bare copper conductors, not copper-clad.

Question 3

What is the maximum length of an Ethernet (LAN) cable run?

Accepted Answer

The maximum length of a twisted-pair Ethernet (LAN) channel is 100 metres. Structured cabling standards divide this as a permanent link of up to 90 metres between the wall outlet and the patch panel, plus up to 10 metres in total for the patch cords at both ends.
The same 100 metre figure applies to Cat5e and Cat6 for Gigabit Ethernet. Faster speeds can reduce the reach on lower categories. For example, 10 Gigabit Ethernet over Cat6 is limited to a shorter distance than over Cat6A.
Going past 100 metres causes signal loss and errors. For longer runs, use a network switch or media converter partway, or run fibre optic cable instead. APAR Tarang Shakti LAN cable is a Cat 6 UTP cable with 24 AWG solid copper conductors, designed for a 90 metre permanent link within the 100 metre channel.

Question 4

What is the minimum bend radius for fibre optic cable?

Accepted Answer

The minimum bend radius of a fibre optic cable is set by the manufacturer and printed on the datasheet, so that figure always governs. A widely used rule of thumb is about ten times the cable outer diameter for a cable that is installed and at rest, and about twenty times the outer diameter while the cable is under pulling tension.
Bending a fibre tighter than its limit raises signal loss, because light escapes at the bend. A severe bend can also crack the glass, which may not show up until some time after installation. Bend-insensitive fibres, such as the G.657 type used in many drop and indoor cables, tolerate tighter turns, but the cable and its protective sheath still have their own limit.
Keep routes free of kinks, avoid pulling cable round sharp corners, and use proper bend-radius guides at trays and cabinet entries.

Question 5

What do OM1, OM2, OM3, OM4 and OS2 mean on fibre optic cable?

Accepted Answer

The prefix OM stands for optical multimode and OS stands for optical single-mode. OM1 and OM2 are older multimode grades. OM1 has a 62.5 micron core, and OM2 has a 50 micron core. OM3 and OM4 also have a 50 micron core but are laser-optimised, which lets them carry higher-speed signals over longer distances than OM1 and OM2. OM4 supports a longer reach than OM3 at the same speed.
OS2 is the standard grade of single-mode fibre for long-distance and campus backbone links. Single-mode fibre has a core of only 8 to 10 microns, so light travels a single path with low loss.
APAR offers multimode cables using OM1, OM2, OM3 and OM4 fibres, and single-mode cables using G.652D, G.655 and G.657 fibres. The right grade depends on the link length, the speed and the equipment already installed.

Question 6

How big should the earth conductor be compared with the phase conductor?

Accepted Answer

Under common wiring rules such as IEC 60364-5-54, where the earth (protective) conductor is made of the same material as the phase conductors, the usual minimum sizes are as follows. For phase conductors up to 16 sq mm, the earth conductor is the same size as the phase conductor. For phase conductors above 16 and up to 35 sq mm, the earth conductor is 16 sq mm. For phase conductors above 35 sq mm, the earth conductor is half the phase size.
These are minimum values. The earth conductor must also withstand the prospective earth-fault current for the time the protective device takes to clear it, so the figures should be checked against the fault level of the installation and against the local wiring code. In an armoured cable, the armour can serve as the protective conductor only if its cross-section and resistance have been verified for the purpose.

Question 7

Why are the cores of some control cables numbered instead of colour coded?

Accepted Answer

Many control cables identify each core by a printed number on uniformly coloured insulation, often black, rather than by a different colour for each core. The reason is practical. A cable with 12, 24 or 61 cores cannot be separated by colour alone, because only a limited number of colours can be told apart reliably on site, and a number printed on every core allows each one to be matched to a terminal in the panel schedule.
The exact scheme depends on the standard and the manufacturer, so the core identification method should be confirmed on the datasheet before ordering. APAR&#8217;s LV XLPE/PVC control cables are made from 2 up to 61 cores. How many spare cores to include is covered in the control cable core-count and spare-core planning guide.

Question 8

What is the bedding layer in an armoured cable?

Accepted Answer

The bedding is the layer applied over the assembled cores and beneath the armour of an armoured cable. It is also called the inner sheath. Its job is to give the armour a smooth, even surface to sit on, so that the armour wires or strips do not press into the core insulation, and to fill the spaces between the cores so that the cable keeps a round, stable shape.
Without a bedding layer, the armour could bite into the insulation under bending, pulling or clamping loads. The inner and outer sheaths can be chosen to suit the installation, and APAR&#8217;s LV XLPE cables offer PVC, PE, FR, FRLS or LSOH sheath options. The choice of armour itself is covered in the cable armouring guide.

Question 9

What is partial discharge in a medium-voltage cable?

Accepted Answer

Partial discharge is a small electrical discharge that occurs inside a void, impurity or defect in cable insulation without completely bridging the space between conductor and screen. Each discharge is tiny, but repeated discharges slowly erode the insulation around the defect and can eventually lead to breakdown. The effect is of most concern in medium and high voltage cables, where the electrical stress in the insulation is high.
Partial discharge is measured in picocoulombs (pC), and a lower figure means a cleaner insulation system. Voids are reduced by extruding the conductor screen, XLPE insulation and insulation screen together in one pass, which also keeps the interfaces clean. APAR states that its MV/HV XLPE cables are made by simultaneous triple extrusion and are suited to critical applications requiring partial discharge values below 2 pC.

Question 10

What is a 3.5 core cable?

Accepted Answer

A 3.5 core cable has three full-size phase cores and a fourth core, the neutral, of reduced cross-section. The half in the name refers to the smaller neutral, which is commonly about half the phase size. For example, a cable with three 70 sq mm phase cores and a 35 sq mm neutral is described as 3.5 core.
The reduced neutral works because, in a three-phase system with reasonably balanced loads, the neutral carries little current. Where the loads are heavily unbalanced, or where non-linear loads such as drives and switched-mode supplies add harmonic current to the neutral, a full-size four core cable is the safer choice. APAR&#8217;s LV XLPE power cables are available in 2, 3 and 4 core forms with a reduced neutral conductor.

Question 11

What is a breakout in a wiring harness?

Accepted Answer

A breakout is the point in a wiring harness where one or more wires leave the main bundle, called the trunk, and run off as a separate branch to a connector or component. It is also called a branch point.
On the harness drawing each breakout has a defined position, measured along the trunk, and each branch has a defined length. The position matters because the branch is tied down or sleeved at that point, and the wires there carry the bending load when the harness is fitted or moves in service.
A breakout is normally secured with tape, a sleeve, heat-shrink or a moulded boot so that the wires cannot slide or rub at the exit. Placing a breakout too close to a connector leaves no room for the wires to bend without stressing the terminals. APAR&#8217;s harness protection materials chart compares the sleeves, tapes and conduits used at these points.

Question 12

What is connector keying on a wiring harness, and why does it matter?

Accepted Answer

Connector keying is a physical feature of a connector housing, such as a rib, slot or offset shape, that allows it to mate in only one orientation and only with its intended counterpart. It is also called polarisation.
A harness often carries several connectors of the same size close together, for example on a sensor bank or a control module. Without keying, two of them can be plugged into the wrong sockets, or a connector can be forced in rotated. The result is a swapped signal, a blown fuse or a damaged terminal, and the fault may not appear until the vehicle or coach is in service.
Keying is usually backed by colour coding and printed circuit labels, but colour alone is not a reliable control. The harness drawing should name the exact connector part number and keying code for every position. APAR&#8217;s guide to harness labelling and identification covers how branches and ends are marked.

Question 13

What is IPC/WHMA-A-620 and why does it matter for wiring harnesses?

Accepted Answer

IPC/WHMA-A-620 is an industry standard titled Requirements and Acceptance for Cable and Wire Harness Assemblies. It was developed jointly by IPC and the Wire Harness Manufacturers Association, and it describes how finished cable and harness assemblies should look and perform, covering wire preparation, crimping, soldering, connector assembly, routing and marking.
The standard defines acceptance criteria in three classes: Class 1 for general products, Class 2 for dedicated service products and Class 3 for high-performance products. A buyer who wants it applied should state the class on the drawing, because the inspection criteria differ between classes.
APAR&#8217;s harness pages do not name this standard. A buyer who requires it should state the standard and class at enquiry and confirm the requirement with APAR before the quotation is accepted.

Question 14

What span length can covered conductor be used for?

Accepted Answer

APAR&#8217;s covered conductor page cites self-supporting pole spans of 60 to 70 metres without a messenger conductor. This is a typical figure for the product, not a design rule.
The span a particular line can use depends on the conductor type and size, the installed tension, the permitted sag, ground clearance and the wind and ice loading assumed for the route. A line with an ACSR or AL59 ACS core, which carries more of the mechanical load in the steel or high-strength strands, can be designed for a different span from one with an all-aluminium alloy core. The covering adds weight and wind area, and the line designer must include both.
The conductor options and their trade-offs are compared in APAR&#8217;s guide to covered conductor core options. The final span, sag and tension should be confirmed by the utility&#8217;s line designer against the tender specification.

Question 15

Is covered conductor suitable for polluted and salt-laden areas?

Accepted Answer

Yes, a covered conductor line can be used in polluted and salt-laden areas. APAR&#8217;s covered conductor has an anti-tracking, UV-protected outer layer, and the product page lists resistance to pollution and salt among the conditions the outer jacket is designed to withstand. Polluted areas are named on the page as an application.
A bare conductor in the same location relies on insulator surfaces staying clean enough to hold off leakage, while the covering on a covered conductor reduces direct exposure of the live metal to deposits and moisture. Tracking resistance of the outer layer matters here, because surface leakage currents can otherwise cut a conductive path into the jacket over time.
Site conditions vary, so a coastal or industrial route should still be reviewed with the line designer. The site survey should record the pollution source, the distance from the sea and the existing insulator type.

Question 16

What is the difference between a wiring diagram and a harness drawing?

Accepted Answer

A wiring diagram shows how circuits connect electrically. It uses symbols, shows which pin goes to which pin, and ignores physical size, length and position. A harness drawing shows the physical object: the layout of branches, their lengths and tolerances, connector views, wire sizes, protective sleeving, clip positions and labels.
The wiring diagram tells the designer what must be connected. The harness drawing tells the factory what to build and the installer where it must go. A manufacturer cannot build from a wiring diagram alone, because lengths, branch positions and materials are missing. A complete enquiry therefore includes the harness drawing, the from-to wire list and the bill of materials. The data a quote needs is listed in the harness RFQ checklist, and APAR builds to customer drawings on the automotive and EV harness page.

Question 17

What are overmoulding and strain relief on a wiring harness connector?

Accepted Answer

Strain relief is any feature that stops bending, pulling or vibration from reaching the crimp inside a connector. Without it, the wire flexes at the point where it enters the terminal, strands fatigue and the circuit fails, often with the insulation looking intact. Common forms are a rear shroud, a tie-off point, and a moulded boot.
Overmoulding is one way of making that boot. Plastic or rubber is moulded directly around the connector rear and the cable behind it, which bonds the two, seals the entry against moisture and gives a controlled bend radius. APAR&#8217;s rail harness pages list overmoulds and strain relief among the features of complex harnesses. For routing and fixing practice, read the rolling stock harness routing guide, and see the locomotive harness page.

Question 18

What is a splice in a wiring harness, and when is it acceptable?

Accepted Answer

A splice is a permanent joint where one wire is connected to two or more others inside the harness, usually by a crimped or ultrasonically welded joint covered with sealed insulation. It is used to feed several circuits from one supply or ground point without extra connector pins.
Splices are acceptable when the drawing specifies them, the joint is made and tested as part of the process, and the splice is insulated and, where it may see moisture, sealed. They are a weak point if left unprotected, if wire sizes are mixed beyond what the joint is designed for, or if the splice sits at a bend. Repairs on site should use the method the harness drawing allows. See the automotive and EV harness page and the harness testing guide.

Question 19

What is the difference between covered conductor and spacer cable?

Accepted Answer

Both are overhead systems that use covered conductors, but they are built differently. A spacer cable system holds three covered phase conductors close together in a compact arrangement using spacers, and carries them on a separate steel messenger wire that takes the mechanical load. A covered conductor line, by contrast, uses a single-core covered conductor on poles and cross-arms in the same way as a bare line, with each phase self-supporting between poles.
APAR covered conductors are self-supporting and need no messenger wire. They can be installed on an existing bare-conductor network without changing the poles, cross-arms or insulators in the same span. A spacer cable suits very compact corridors, while covered conductor suits reconductoring existing lines. See the covered conductor page and the overhead conductor comparison.

Question 20

What is the maximum operating temperature of a covered conductor?

Accepted Answer

APAR lists an operating temperature of up to 95°C for its covered conductors, and up to 105°C for the AL59 ACS version. The limit applies to the finished conductor, because the XLPE covering restricts the temperature as well as the metal underneath. A bare conductor of the same type may be allowed to run much hotter, so the ampacity from a bare-line table should not be carried over to a covered line without checking.
To test a feeder, compare the expected conductor temperature at peak load and hot ambient conditions with these figures. The conductor types offered are AAAC, ACSR, ACSS and AL59 ACS, in sizes from 100 to 400 mm² and voltage classes from 11 kV to 132 kV. See the APAR covered conductor page for the range.

Question 21

Why are CAN bus wires twisted together in a vehicle harness?

Accepted Answer

CAN bus carries data as a small voltage difference between two wires, called CAN high and CAN low. Twisting the pair keeps the two conductors close together along their whole length, so any external electrical noise couples into both wires almost equally. The receiver looks only at the difference between them, so most of that noise cancels out.
This is why vehicle harnesses use a twisted pair for CAN rather than two loose wires. Pair length, lay and routing away from ignition and high-current cables all affect reliability. The APAR range includes CAN bus cable of the UTP-A type to SAE J1939/15, listed on the automotive and EV harness page.

Question 22

What is the difference between a wiring harness and a cable assembly?

Accepted Answer

A cable assembly is usually a single cable, or a small group of cables, cut to length and fitted with connectors or terminals at the ends. A wiring harness is a larger, organised set of wires, terminals, connectors and protective sleeves that is laid out to a drawing and routes power and signals to many points in a vehicle or machine.
A harness often branches, with several breakouts feeding different components, and may include conduit, grommets and strain relief. APAR builds harnesses to customer drawings for automotive, EV and rail use, as described on the automotive and EV harness page and the railway coach harness page. Simple earthing leads and crimped socket assemblies sit at the cable assembly end of that range.

Question 23

Can birds and animals cause faults on a covered conductor line?

Accepted Answer

They can, but the likelihood is lower than on bare conductor. On a bare line, a bird or monkey bridging two phases, or a phase and an earthed part, can start a fault immediately. A covered conductor places an insulating layer between the animal and the metal, so brief contact is less likely to cause a trip.
The covering is not a guarantee. Gaps at poles, bare fittings and damaged sections can still be points of contact. The APAR covered conductor product page lists forests, dense vegetation and wildlife sanctuaries among its intended applications. Fit matched accessories and insulate any exposed hardware to get the full benefit.

Question 24

Is a covered conductor safe to touch?

Accepted Answer

No. A covered conductor must always be treated as a live bare conductor. The covering is there to cope with brief, accidental contact from branches, animals and neighbouring phases. It is not rated or tested as protection against deliberate human contact, and its surface can be damaged, wet or contaminated.
Line crews should use the same isolation, proving dead and earthing steps they would apply to bare overhead lines. Members of the public should keep clear of any overhead line, covered or not. Details of the APAR covered conductor range are on the product page.

Question 25

What does SS-EN 50397-1 cover for covered conductors?

Accepted Answer

SS-EN 50397-1 is the standard that sets out requirements and tests for covered conductors used in overhead lines. It deals with the conductor and its covering, including how the covering must perform under electrical, mechanical and environmental conditions.
APAR designs its medium voltage covered conductors to this standard, along with BS EN 50182, IS 398 Part 2 and Part 4, and IEC 61089. Accessories for covered conductor lines, such as clamps and piercing connectors, are covered by the companion standard EN 50397-2. When writing a tender, name the standard and the voltage class together, and ask for type test reports against it.

Question 26

What is a formboard in wiring harness manufacturing?

Accepted Answer

A formboard is a full-size layout of the harness, drawn on a board with fixtures, pegs and connector holders placed where each branch, breakout and connector must sit. Wires cut to length are laid along the board, so every branch comes out at the correct position and length before the harness is taped, sleeved or tied.
The board turns a two-dimensional drawing into a repeatable build. Every harness built on it has the same branch lengths, clip positions and connector orientation, which is what lets it fit the vehicle or coach without stretching or excess slack. Labels, protection and fixings are added on the board, and many boards also carry test points for continuity checks.
Because a formboard is built from the customer&#8217;s drawing, a complete, dimensioned drawing shortens lead time. The harness RFQ checklist lists what to provide. APAR builds harnesses for railway coaches, locomotives, solar projects and automotive and EV platforms.

Question 27

Should wiring harness terminals be crimped or soldered?

Accepted Answer

For harnesses in vehicles, rolling stock and machinery, crimping is the standard method, and soldering is avoided wherever the harness vibrates.
The reason is solder wicking. Molten solder runs up the strands under the insulation and turns a flexible conductor into a stiff rod. Under vibration, bending concentrates where the solder stops, and the strands fatigue and break there, often hidden under intact insulation. A correct crimp compresses only the stripped conductor inside the terminal barrel, so the strands behind it stay flexible, while the insulation grip carries bending loads.
Crimps also suit controlled production. Crimp height, pull-off force and cross-sections can be measured, as described in the harness testing guide. On Indian Railways work, crimping sockets are specified to EDTS-200, EDTS-201 and ICF drawings. See APAR locomotive harnesses and automotive and EV harnesses.

Question 28

How is a covered conductor line earthed for maintenance work?

Accepted Answer

A covered conductor line must be treated as live until it is isolated, tested dead and earthed, exactly like a bare line. The covering is not rated insulation, and it does not make the line safe to touch.
The practical difference is that standard portable earth clamps cannot make contact through the XLPE covering. Covered conductor lines are therefore fitted with dedicated earthing points at planned locations, such as either side of a switch, at section poles and at work zones that crews use regularly. These provide a bare contact surface for the earth clamp while keeping the rest of the conductor covered. Stripping the covering on site for each job should be avoided, because every stripped point becomes a weak spot.
Earthing point positions are best decided at the design stage. The MVCC tender specification checklist and the covered conductor page cover line design and accessories.

Question 29

What happens if the covering on a covered conductor gets damaged?

Accepted Answer

A damaged covering does not stop the line working, but it removes protection at that point. Where the conductor is exposed or the covering is cut deep, a branch or a bird touching that spot can cause a fault just as it would on a bare line. Surface damage can also start tracking, and a cut lets water reach the conductor.
APAR&#8217;s covered conductors use a longitudinally water-tight conductor, which limits how far moisture can travel along the strands from a damaged point. Damage should still be repaired, using a repair sleeve or insulating material approved by the accessory supplier, applied with the line dead and earthed under the utility&#8217;s safety rules.
Most damage happens during stringing or when bare-line clamps are used. The MVCC accessories and installation guide covers how to avoid it, and conductor details are on the covered conductor page.

Question 30

Why are covered conductors black?

Accepted Answer

The black colour comes from carbon black added to the outer layer of the covering. Carbon black absorbs ultraviolet light and stops sunlight from breaking down the polymer, which is what lets an overhead conductor sit exposed to the sun for decades. Without it, the outer surface would gradually crack and chalk.
Only the outer layer carries the carbon black. APAR&#8217;s covered conductors have three bonded layers over the conductor: a semiconducting screen, an inner layer of pure, carbon-free XLPE that provides the insulation, and an outer XLPE layer that is UV-resistant and anti-tracking. Keeping carbon out of the inner layer protects its electrical performance, while the outer layer handles sunlight, pollution and surface tracking. Construction details are on the covered conductor product page, and the role of the outer layer is explained in why a covered conductor needs an anti-tracking layer.

Question 31

Why are high-voltage cables in electric vehicles coloured orange?

Accepted Answer

Orange marks the circuits that can seriously injure someone who touches them. In an electric vehicle, the traction battery, inverter, motor and on-board charger run at several hundred volts DC, far above the 12 V or 24 V of the conventional electrical system. Vehicle safety standards treat anything above 60 V DC or 30 V AC as high voltage, and SAE J1673 specifies orange for high-voltage cable in road vehicles. The convention is now followed across the industry.
The colour is a warning for mechanics, crash responders and recyclers: an orange cable must not be cut, pierced or disconnected until the high-voltage system has been isolated and proved dead. The colour does not change the cable&#8217;s rating. The cable still has to be designed for the system voltage, temperature and mechanical duty of its location. For EV assemblies, see harness for automotives and EV.

Question 32

What do the EN 45545-2 hazard levels HL1, HL2 and HL3 mean for rail cables?

Accepted Answer

EN 45545-2 is the European standard for the fire behaviour of materials used on railway vehicles, and it grades vehicles into three hazard levels. HL1 is the least demanding and applies where passengers can leave the vehicle quickly and easily. HL3 is the most demanding and applies where evacuation is slow or difficult. HL2 sits between them.
The level is set for the vehicle, not the component, from its operation and design categories. It rises with the time a train spends in tunnels, with driverless operation, and for double-deck and sleeping-car designs. Every material on board, cables included, is then tested for properties such as flame spread, smoke density and smoke toxicity against the limits for that hazard level. A cable accepted for an HL2 vehicle is therefore not automatically acceptable for one specified to HL3, and the hazard level should be stated in any enquiry for rolling stock cables or harnesses.

Question 33

What do GXL, TXL and SXL mean on automotive wire?

Accepted Answer

They are three insulation types defined in SAE J1128, the North American standard for low-voltage primary cable in road vehicles. All three use cross-linked polyethylene insulation and are rated for use up to 125°C, well above ordinary PVC primary wire.
The difference is wall thickness. SXL has the thickest wall and suits routes where abrasion resistance matters most. GXL has a thin wall and is the usual general choice for engine-compartment wiring. TXL has the thinnest wall, which saves weight and space in dense harnesses where the routing is well protected. Choosing between them is a trade-off between mechanical protection and harness bulk, and the harness drawing should name the type.
APAR makes e-beam cross-linked GXL, TXL and SXL wires alongside ISO 6722, JASO and DIN types. See automotive wires and cables for the range.

Question 34

Why does a covered conductor need an anti-tracking outer layer?

Accepted Answer

Tracking is slow damage caused by small leakage currents running across a wet, contaminated insulating surface. On an overhead line, rain mixed with salt spray, industrial dust or bird droppings forms a conductive film. Current creeping through that film dries it in patches, small arcs form across the dry bands, and over time they carbonise the surface into a conductive track that can end in failure.
A covered conductor sits outdoors for decades in exactly those conditions, so its outermost layer has to resist tracking as well as sunlight. APAR covered conductors use an outer XLPE layer that is UV-protected and anti-tracking, over an inner carbon-free XLPE layer and a semiconducting screen, all co-extruded and bonded. The outer layer also resists salt, humidity, fungal growth and pollution, which is why covered conductor suits coastal and industrial routes. See the covered conductor product page.

Question 35

Can covered conductor be strung on existing overhead line poles?

Accepted Answer

In most reconductoring projects, yes. Covered conductor is designed to be installed on an existing bare-conductor network without changing the poles, cross-arms or insulators in the same span, which is one reason utilities choose it over underground cable for feeders that trip on tree contact. APAR covered conductors are self-supporting on pole spans of 60 to 70 metres and need no separate messenger wire.
Two checks are still needed before ordering. First, confirm that pole strength and ground clearances suit the new conductor, because the covering adds weight compared with a bare conductor of the same size, and sag should be taken from the covered conductor&#8217;s own table. Second, plan the fittings: covered conductor needs suspension clamps, anchoring clamps and insulation piercing connectors made for the covering, to EN 50397-2. Sizes and voltage classes are on the covered conductor product page.

Question 36

What is loop-in wiring, and why do electricians use it?

Accepted Answer

Loop-in wiring is the method where the phase and neutral run from the distribution board to the first switchboard, then loop on from that switchboard to the next, and so on, with every joint made at a terminal inside a switchboard or ceiling rose rather than in a hidden connector somewhere in the conduit. Each light or fan is fed by a short branch from the nearest switchboard. The alternative, joint-box wiring, makes the connections in separate junction boxes, which is more wire and more places for a joint to loosen unseen.
Electricians prefer loop-in because every connection is behind a removable plate, so a fault can be traced and a circuit extended without opening walls. It also uses less cable. The one discipline it needs is a conductor that can be looped repeatedly without work-hardening or breaking, which is why flexible stranded house wire such as Shakti FR LSH is used rather than solid conductor. Neutral must be looped as carefully as phase; a borrowed neutral between circuits is the commonest loop-in mistake.

Question 37

How many points can one lighting circuit have in a house?

Accepted Answer

Indian wiring practice, as set out in IS 732 and the National Building Code, limits a lighting sub-circuit to ten points of lights, fans and 6 A socket outlets, with the total connected load on that circuit held to 800 W. Power sub-circuits are treated separately: each carries at most two 16 A socket outlets with a limit of 3000 W. The rule exists so that one MCB protects a load it can actually clear, and so that a single fault does not put a whole floor in the dark.
In a typical 2 BHK flat that works out to one lighting circuit per one or two rooms and a dedicated power circuit for every air conditioner, geyser and kitchen. Each lighting circuit is normally wired in 1.5 sq mm and each power circuit in 2.5 or 4 sq mm; Anushakti HR FR LSH covers both. The distribution board wiring guide shows how the circuits are grouped at the DB.

Question 38

What is the maximum pulling tension for a power cable?

Accepted Answer

There is no single figure; the limit depends on what the pulling force is applied to. When the cable is pulled by a pulling eye fixed to the conductors, the allowable tension is set per square millimetre of conductor cross-section, so a larger conductor can take a proportionally higher pull, and copper takes more than aluminium. When a basket grip (a cable stocking) is used over the outer sheath, the limit is much lower because the sheath, not the conductor, carries the load. The manufacturer&#8217;s installation data for the specific cable is the only reliable source for the figures, and it should be obtained before the pull is planned, not after.
Two other checks matter as much as the straight-line tension. At every bend the sidewall pressure, which is the tension divided by the bend radius, must stay within the limit for the cable type, and the bend itself must not go below the minimum bending radius. Heavy LV XLPE cables up to 1000 sq mm are normally pulled with a calibrated dynamometer in the line so the tension is measured rather than guessed.

Question 39

What is the semiconducting screen in an XLPE cable for?

Accepted Answer

Medium and high voltage XLPE cables carry two thin black semiconducting layers, one directly over the conductor and one directly over the insulation. Neither carries load current. Their job is to control the electric field. A stranded conductor has ridges and gaps between the wires, and if the insulation sat straight on it the field would concentrate at every ridge and slowly erode the XLPE through partial discharge. The conductor screen presents the insulation with a perfectly smooth round surface instead. The insulation screen does the same job on the outside, giving the metallic screen a smooth surface to sit on and keeping the field contained within the insulation rather than in the air gaps around it.
In APAR&#8217;s MV and HV XLPE cables up to 66 kV the conductor screen, insulation and insulation screen are extruded together in one pass, so there is no gap or contamination between the layers. Cables at 1.1 kV do not need screens because the field stress is far lower.

Question 40

What is the difference between loose-tube and tight-buffered fibre optic cable?

Accepted Answer

The difference is how each glass fibre is protected inside the cable. In a loose-tube cable the fibres lie freely inside a plastic tube, usually filled with gel, so the fibre is isolated from the pulling, bending and temperature movement of the cable around it. That construction suits outdoor duct, aerial and direct-buried routes, long lengths and high fibre counts. APAR&#8217;s multi-tube loose-tube cables run from 6 to 1152 fibres for exactly those jobs.
In a tight-buffered cable each fibre carries its own thick plastic coating, so it can be handled, stripped and connectorised directly without a fan-out kit or gel cleaning. That makes it the usual choice for indoor patching, floor distribution and equipment connections, where lengths are short and the cable is terminated often. The breakout tight-buffer range covers 1 to 24 fibres for that work. The rule is simple: loose tube for the outside plant, tight buffer for the room.

Question 41

Which cable is used for street lighting?

Accepted Answer

Street lighting is fed by underground or overhead distribution rather than ordinary building wire. The usual underground choice is an armoured LV XLPE power cable, sized both for the pole loads and for the length of the run, because lighting feeders are often long and voltage drop, not current, tends to govern the conductor size. Overhead sections use aerial bunched cable, which twists insulated cores around a bare messenger and resists tapping, abrasion and tree contact. Both are rated for outdoor and buried or aerial service, unlike indoor house wire, and the armour or messenger is earthed at the specified points. Feeder pillars, joints and pole terminations are matched to the cable. APAR supplies LV XLPE power cables and aerial bunched cables for lighting distribution.

Question 42

What does mV/A/m mean on a cable datasheet?

Accepted Answer

mV/A/m is the cable&#8217;s voltage drop in millivolts for every ampere it carries over every metre of its run. It is the figure engineers use to check voltage drop without recalculating the conductor resistance each time. The drop for a circuit is the mV/A/m value multiplied by the design current and the route length, divided by 1000 to give the answer in volts. A smaller mV/A/m means a larger conductor and a lower loss. The value rises a little with conductor temperature and differs between single-phase and three-phase, so a datasheet quotes it for each installation condition rather than as a single number. It is read straight from the manufacturer&#8217;s table, not derived by hand. APAR publishes a voltage-drop reference table for its LV cables.

Question 43

Why should an extension lead not be used while coiled up?

Accepted Answer

A coiled extension lead carries the same current as one run out flat, but it cannot shed the heat. Wound tightly on its drum, every turn warms the turns beside it, and the temperature in the middle of the coil climbs well above the rating printed for the cable in free air. Under a heavy load, a heater or a welder for example, the insulation can soften or even melt while the wall socket still reads normal. The safe practice is to unwind the lead fully before drawing a large current through it, and to check the drum&#8217;s rating, which is often quoted separately for the coiled and the uncoiled condition. APAR light-duty and flexible cables carry their rating on the sheath so the limit is always to hand.

Question 44

What size cable does a diesel generator (DG set) need?

Accepted Answer

The cable from a DG set is sized on the set&#8217;s full-load current, not on the kVA badge alone. For a three-phase set the current is kVA multiplied by 1000, divided by 1.732 times the line voltage. At 415 V that works out to about 1.39 A for every kVA, so a 250 kVA set draws roughly 348 A at full load, and the cable and breaker are chosen above that figure with room to spare. Common practice is a 3.5-core armoured cable, usually aluminium in the larger sizes, with an added margin for the high ambient inside a canopy or plant room. Where two or more cables run in parallel per phase, each one is derated for grouping. A long run to a distant panel needs a voltage-drop check as well. See APAR LV XLPE power cables.

Question 45

What is the acceptable voltage drop limit for a cable?

Accepted Answer

Voltage drop is the voltage lost along a cable as current flows through the conductor&#8217;s resistance. Left unchecked it under-supplies the load at the far end. Indian practice, following IS 732 and IEC 60364-5-52, keeps the total drop from the origin of the installation to any load point within roughly 3 per cent for lighting and about 5 per cent for power, with tighter figures for sensitive electronic loads. On short indoor circuits the drop is negligible. On long feeders it often decides the conductor size ahead of the current rating. The drop is estimated from the cable&#8217;s mV/A/m figure, the design current and the run length. Where a long run breaches the limit, the fix is a larger conductor. APAR recommends checking both current rating and voltage drop, then sizing on whichever needs the bigger conductor. See APAR LV XLPE power cables.

Question 46

Why does my MCB keep tripping?

Accepted Answer

An MCB trips to protect the circuit, so a repeat trip is telling you something, not misbehaving. The three usual causes are overload, a short circuit and, if it is actually the RCCB tripping, an earth leak. Overload is the common one at home: too many heavy appliances, an iron and a geyser and a microwave, on a circuit sized for less, so the wire would overheat and the breaker cuts in. A short circuit trips instantly and hard, often with a flash, and points to a damaged cable or a faulty appliance.
Unplug things and reset once. If it holds, you were overloading it. If it trips with nothing on, or trips the moment one appliance is plugged in, stop resetting and call an electrician: you have a fault in the wiring or that appliance, and repeated resetting only risks the very fire the breaker is trying to prevent.

Question 47

What is the difference between heat-shrink and cold-shrink cable terminations?

Accepted Answer

Both seal and insulate the end of a cable where it meets a joint or termination; they differ in how they grip. A heat-shrink termination uses tubes and boots that shrink down tight when you play a gas torch over them, so it needs an open flame on site and a steady hand to heat evenly without scorching.
A cold-shrink termination is a pre-stretched rubber sleeve held open on a removable plastic spiral. You slide it into place and pull the spiral out; the rubber contracts onto the cable on its own, no flame needed. Cold-shrink is quicker, safer in areas where you cannot use a torch, and more forgiving of a rushed installer. Heat-shrink is cheaper and copes with a wider size range per kit. Both work well when fitted to the maker&#8217;s instructions on a clean, correctly prepared cable.

Question 48

Why are single-core cables laid in trefoil formation?

Accepted Answer

Because three single-core cables each throw off a magnetic field, and how you arrange them decides whether those fields fight or cancel. Clip the three phases together in a symmetrical triangle, which is what trefoil means, and the fields very nearly cancel outside the group. That keeps induced currents and stray heating low in cable glands, armour and nearby steelwork, and it keeps the three phase impedances equal so the load shares evenly.
Lay them flat and touching instead and the field is unbalanced and the impedances unequal. Spaced-flat is sometimes used deliberately to get more air and better cooling around each cable on a heavily loaded tray, but then you re-check the current rating. Either way, cleat single-cores firmly, because a short-circuit throws them apart hard.

Question 49

How deep should an underground power cable be buried?

Accepted Answer

Deep enough that the next person to dig finds soil, not your cable. IS 1255, the Indian code for laying power cables up to 33 kV, measures depth from ground level to the top of the cable and raises it with voltage: roughly 0.75 m for LV up to 1.1 kV, about 0.9 m for 3.3 to 11 kV, and about 1.05 m for 22 to 33 kV. Under roads and crossings, go deeper and pull the cable through a pipe or duct.
Depth alone is not enough. Bed the cable in about 75 mm of clean sand, cover it with another 75 mm, lay bricks or protective tiles over that, and run marker tape at the surface. Use an armoured cable for direct burial and earth the armour.

Question 50

What is the minimum bending radius for a power cable?

Accepted Answer

It is the tightest curve the cable will take without hidden damage, and it is quoted as a multiple of the cable&#8217;s overall diameter, written as a number times D. Bend tighter than that and the insulation thins on the outside of the bend while the screen wrinkles inside, and the cable can fail there months later even though it tested fine.
As a rough guide, in line with IEC 60502-1, LV multicore cables sit around 12 &times; D, single-core LV around 15 &times; D, and screened MV and HV cables from 15 up to 20 &times; D. The diameter is the whole cable, armour and all, not the conductor. During pulling keep it gentler still and use rollers. The number printed on the drum always overrides any rule of thumb.

Question 51

Why should every wire joint be inside a junction box?

Accepted Answer

A joint is the weakest point of any wiring run — it&#8217;s where strands loosen, oxidise and heat up. A junction box does three jobs for that weak point. It contains the heat and any sparking if the joint deteriorates, keeping it away from concealed woodwork and insulation. It keeps the joint accessible, so it can be inspected or remade without breaking a wall — a taped joint buried in plaster is invisible until it fails. And it protects the joint mechanically, from tugging, dust, moisture and pests. Outdoors, the box needs to be a weatherproof one with proper cable entries, especially through the monsoon. A twist of insulation tape hidden in a conduit is the classic shortcut behind flickering lights, warm switchboards, and worse.

Question 52

Can I join two different wire sizes on the same circuit?

Accepted Answer

Avoid it. The breaker or fuse protecting a circuit is chosen to protect the wire size the circuit was designed with. If a thinner wire is added downstream — a common shortcut when extending a point — that thinner section can overheat under a load the breaker considers perfectly normal, because the breaker is still guarding the fatter wire. The thin stretch becomes the unprotected weak link, usually buried in a wall or above a false ceiling. If you&#8217;re extending a circuit, continue with the same size and grade of wire, make the joint properly inside an accessible junction box, and if the new load is heavier than the circuit was planned for, run a new circuit from the distribution board instead. It costs a little more once, and removes the risk permanently.

Question 53

What is a cable jointing kit?

Accepted Answer

A jointing kit contains everything needed to join two lengths of power cable so the joint performs like the cable itself: connectors or ferrules for the conductors, insulating layers (usually heat-shrink or cold-shrink), and materials to restore the screen, armour continuity and outer sheath. Joints exist because cable comes in finite drum lengths, and because repairs happen. They&#8217;re also the most common weak point in a buried run — a joint made in a hurry, in a wet pit, by an untrained hand, is where many cable faults begin. Good practice is simple to state: use a kit matched to the exact cable construction and voltage, follow the kit instructions to the letter, and keep the work dry. Where a route can be supplied in one drum length, no joint is the best joint.

Question 54

Why must cable armour be earthed?

Accepted Answer

The steel armour on a cable isn&#8217;t only mechanical protection — it&#8217;s a safety conductor. If the cable is damaged and a live conductor touches the armour, the armour has to carry that fault current back to earth so the protective device trips quickly. That only happens if the armour is solidly earthed, normally through the cable gland at each end, which clamps the armour wires and bonds them to the earthed gland plate or enclosure. Unearthed armour does the opposite of protecting: under a fault it can sit at a dangerous voltage along the whole cable run, waiting for someone to touch it. This is why gland selection and proper fitting deserve far more attention than they usually get on site — the gland is part of the earthing system, not just a cable clamp.

Question 55

What is water blocking in an underground cable?

Accepted Answer

Water blocking is a construction feature that stops moisture travelling along the inside of a cable. Swellable tapes or yarns are placed under the sheath, and sometimes within the conductor; the moment water reaches them they swell into a gel and seal the path. It matters because a small nick in the outer sheath of a buried cable lets water in, and without blocking, that water can migrate metres along the core and sit against the insulation, degrading it slowly until the cable fails somewhere far from the original damage — which also makes the fault harder to locate and repair. For cables that will spend their lives underground, in ducts that flood, or in high water-table areas, it&#8217;s a feature worth specifying at enquiry stage, not after laying.

Question 56

Should stranded wire ends be crimped with ferrules before terminating?

Accepted Answer

For stranded and flexible conductors going into a screw terminal, yes, and it&#8217;s worth insisting on.
A bare stranded end under a screw gets crushed unevenly. Some strands take the clamping force, others splay out sideways and carry nothing. The effective contact area shrinks, resistance rises at that point, and the joint warms up under load. Warm joints loosen, and loose joints get warmer. That cycle is behind a good share of the terminals that end up discoloured inside distribution boards.
A ferrule gathers every strand into a solid cylinder that the screw can clamp properly, and it stops stray strands wandering across to the next terminal. Use the right size for the conductor, crimp with a proper ferrule tool rather than pliers, and don&#8217;t tin the end with solder instead, because solder creeps under pressure and the joint slackens over time.

Question 57

How many spare cores should a control cable have?

Accepted Answer

Set the figure in the specification rather than leaving it to site, because a contractor asked to decide will choose zero.
The good news is that standard core counts do much of the work for you. Control cables are made at 2, 3, 4, 5, 6, 7, 10, 12, 19, 27, 37, 44, 52 and 61 cores, not at every number in between. Need eight cores and you&#8217;re buying a ten anyway, so the spares arrive free. The outside diameter barely moves either: on the 1.5 mm&#178; construction, going from 7 to 10 cores takes the cable from 14.5 mm to 18.0 mm.
Terminate the spares onto the terminal block at both ends and mark them on the drawing. A spare coiled inside a gland box is a spare nobody will find. See the core-count planning guide and the control cable range up to 61 core.

Question 58

What is a drain wire in a screened cable?

Accepted Answer

It&#8217;s a bare or tinned copper wire laid in continuous contact with the foil screen, running the full length of the cable. Foil is thin and awkward to clamp under a terminal, so the drain wire gives you something you can actually terminate. Whatever the screen intercepts travels down the drain wire to earth.
It&#8217;s tinned to slow galvanic corrosion between the copper and the aluminium in the foil, which would otherwise raise the contact resistance over time and quietly degrade the screen.
Two rules decide whether it works. Earth the drain wire at one end only, because earthing both ends creates a loop that carries current between two earth points and injects noise rather than removing it. And keep the untwisted, unscreened tail at the termination as short as you can. A screen that stops 200 mm short of the terminal has a 200 mm aerial on the end of it.

Question 59

Do I need individually screened instrumentation cable, or is an overall screen enough?

Accepted Answer

It depends on whether the pairs inside the cable can interfere with each other, not just on outside noise.
A collective screen, written CAM, is one overall screen wrapped around all the pairs. It protects the bundle from the plant around it. That&#8217;s usually sufficient when every pair in the cable carries the same kind of signal, or when the signals are digital and switched.
Individual screens on every pair plus a collective screen, written IAM/CAM or ICAT, also stops one pair talking to its neighbours. That matters when a single multipair carries several low-level analogue signals from different loops with different earth references, which is the normal situation on process plant.
If the run is analogue and shared, specify individual screens. APAR&#8217;s instrumentation cables are made in both configurations.

Question 60

What does 1P, 2P or 12P mean on an instrumentation cable?

Accepted Answer

The number is the pair count and the P stands for pair. A 12P cable carries twelve twisted pairs, so twenty-four conductors in total. You&#8217;ll also see T for triad, as in 2T, which means two sets of three twisted cores, normally used for three-wire RTDs.
Pairs are twisted rather than laid up loose for a reason. A pair running past a noisy power cable picks up induced voltage on both conductors roughly equally, and because the twist keeps swapping their positions, most of that noise cancels out at the receiving instrument.
Order by signal count, not conductor count. Each 4-20 mA loop needs one pair. Our instrumentation cable selection guide covers pair count alongside the screening and armour choices, and the screened instrumentation cable range shows what&#8217;s available.

Question 61

What is the difference between LSH and LSZH cable sheathing?

Accepted Answer

Both are fire-performance sheaths, but they are not the same claim. LSH (low smoke and halogen) means reduced smoke and reduced halogen content — the material may still be PVC-based. LSZH (low smoke zero halogen, also written LSOH or FR-LSZH) means the compound contains no halogens at all, so a fire produces no corrosive halogen acid gas. Zero-halogen grades are the stricter choice for enclosed public spaces and equipment rooms. On house wire the same split appears as FR-LSH (PVC-based) versus HFFR (halogen-free) — our guide to choosing the wire grade covers it.

Question 62

What is the difference between a covered conductor and an insulated cable?

Accepted Answer

A covered conductor has an XLPE covering bonded over an overhead line conductor — enough insulation to survive touching a branch or another phase, but not rated as full insulation, so it is still treated as a live overhead line. A cable is fully insulated and screened for its voltage and can be buried or ducted. The covering is what removes the majority of overhead-line faults: clashing in wind, vegetation contact, birds and animals. APAR covered conductors span 11 kV to 132 kV — see the MVCC selection guide.

Question 63

Can fibre optic cable survive a fire?

Accepted Answer

Standard fibre cable cannot — but fire-rated constructions can, and the distinction is measured. APAR&#8217;s Fireoproof range includes an indoor dielectric construction that keeps the circuit alive for 180 minutes at 850°C, and outside-plant designs rated 120 minutes at 750°C, tested to IEC 60331-25. They also limit smoke (IEC 61034) and acid gas (IEC 60754), which is why they are specified for metros, tunnels, airports and emergency systems where the communication link must outlive the fire. See fire resistant fibre cables.

Question 64

What is ADSS fibre optic cable?

Accepted Answer

ADSS stands for All-Dielectric Self-Supporting: an aerial fibre cable with no metal in it at all, strong enough to span between poles without a support wire. Because it is fully dielectric it can be strung safely alongside power lines — the usual route for utility fibre. APAR&#8217;s AerioFib ADSS range runs from 1F to 432F for short, medium and long spans, with an anti-tracking sheath option for high-tension corridors. Where a metallic messenger is acceptable, Fig-8 cable carries an integrated support wire instead. See ADSS and Fig 8 cables.

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