An instrumentation cable spec looks intimidating until you realise it’s answering four questions in order: how many pairs, how they’re screened, whether the cable is armoured, and which standard the buyer is holding you to. Get those four right and the rest of the line item is detail.
1. Pairs, triads and why cores are twisted
Signal cables are built as twisted pairs or triads, not as loose cores. The twist is the point. A pair that runs parallel to a noisy power cable picks up induced voltage on both conductors, but because the two conductors keep swapping position along the twist, most of that pickup appears equally on both and cancels at the receiver. Untwisted cores can’t do that.
Pair count is written as 1P, 2P, 12P and so on. Triads get 1T, 2T. A three-wire RTD wants a triad; a 4-20 mA loop wants a pair.
2. Screening: collective, individual, or both
Twisting deals with magnetic pickup. A screen deals with the electrostatic kind, and with crosstalk between pairs inside the same sheath. APAR’s screened and instrumentation cables are offered in the three configurations the standards recognise:
| Configuration | What it means | Typical use |
|---|---|---|
| CAM | Collective (overall) aluminium/polyester screen over the laid-up pairs | Digital and switched signals, or a run carrying one type of signal only |
| IAM/CAM, also written ICAT | Individual screen on every pair, plus a collective screen over the lot | Mixed low-level analogue signals sharing one cable |
| Unscreened | No screen at all | Rare in plant; occasionally used for simple contact wiring |
Individual screening costs more and eats space, so people try to avoid it. That’s usually a mistake on analogue work. If a single multipair carries several 4-20 mA loops from different loops and different earths, individual screens are what stop one pair talking to the next.
3. Armour, or not
The BS 5308 family splits neatly on this. Type 1 is unarmoured. Type 2 carries single wire armour with a bedding layer underneath it. Armour is a mechanical decision, not an electrical one: direct burial, cable trenches shared with plant traffic, anywhere a spade or a forklift can reach. Our cable armouring guide covers the wire-versus-strip choice in more depth.
4. The standard the tender is written against
This is where money gets lost. BS 5308 Part 1 covers polyethylene insulated cables; Part 2 covers PVC insulated. Each part is further split into Type 1 (unarmoured) and Type 2 (armoured). BS 5308 was superseded by the harmonised BS EN 50288-7 in 2005, and yet it’s still quoted constantly, because the harmonised version doesn’t cover the higher voltage ratings and the cable dimensions that process plants ask for. A publicly available version appeared in 2009 as PAS 5308. APAR builds to BS 5308 Part 1 and Part 2, to BS EN 50288-7, and to the US UL PLTC requirement.
So before you price anything, confirm which of those the buyer means. “BS 5308” on a purchase order is not a complete instruction.
5. Two options worth knowing about
Mica taping over the conductor is available where the circuit has to keep working during a fire. And the conductor itself is flexible copper, which matters more than people expect at the termination end, where an instrumentation cable gets bent into a marshalling cabinet in a space that was never designed for it.
Quick specification checklist
- Pairs or triads, and how many of each
- Conductor size
- Screening: CAM, or IAM/CAM (ICAT)
- Armoured (Type 2) or unarmoured (Type 1)
- Insulation: PE (Part 1) or PVC (Part 2)
- Sheath requirement, including any low-smoke grade the site demands
- Standard: BS 5308 Part and Type, BS EN 50288-7, or UL PLTC
- Fire survival needed? Ask about mica taping
For the wiring that runs alongside instrumentation in the same panel, see APAR’s LV XLPE/PVC control cables up to 61 core.