A cable voltage grade is written as two figures, U0/U. U0 is the rated voltage between the conductor and the metallic screen or earth; U is the rated voltage between conductors. A third figure, Um, is the highest system voltage the cable may be exposed to continuously. The pair matters because the insulation thickness is designed around U0, the stress from conductor to screen, and a system with an unearthed or impedance-earthed neutral can hold the full line-to-line voltage between a healthy conductor and earth for hours during a single-phase fault. Such systems need the higher grade at the same nominal voltage.
Standard voltage grades used in India
| Nominal system voltage | Cable grade U0/U | Earthing category | Indian standard |
|---|---|---|---|
| 415 V / 1.1 kV | 0.6/1 kV (1.1 kV) | Earthed and unearthed | IS 7098 Part 1 |
| 3.3 kV | 1.9/3.3 kV | Earthed | IS 7098 Part 2 |
| 3.3 kV | 3.3/3.3 kV | Unearthed | IS 7098 Part 2 |
| 6.6 kV | 3.8/6.6 kV | Earthed | IS 7098 Part 2 |
| 6.6 kV | 6.6/6.6 kV | Unearthed | IS 7098 Part 2 |
| 11 kV | 6.35/11 kV | Earthed | IS 7098 Part 2 |
| 11 kV | 11/11 kV | Unearthed | IS 7098 Part 2 |
| 22 kV | 12.7/22 kV | Earthed | IS 7098 Part 2 |
| 33 kV | 19/33 kV | Earthed | IS 7098 Part 2 |
| 66 kV | 38/66 kV | Earthed | IS 7098 Part 3 |
Earthed or unearthed: how to choose
The earthed grade is for systems whose neutral is solidly earthed, or earthed through a low impedance, so that an earth fault is cleared quickly and the healthy phases never stay at line voltage to earth for long. The unearthed grade, with U0 equal to U, is for systems with an isolated neutral, or a high-impedance or Petersen-coil earthing, where operation can continue with one phase earthed. Many older Indian industrial 3.3 kV and 6.6 kV systems and some 11 kV mine and process networks are of this kind. If the earthing arrangement is unknown, or the system may be run with a fault on it for more than a few hours, specify the unearthed grade; the extra insulation costs little compared with a cable failure.
Reading it on a datasheet
An 11 kV feeder on a solidly earthed distribution network is specified as 6.35/11 kV. The same feeder on an unearthed system is 11/11 kV, and the cable is physically thicker. Both carry a Um of 12 kV. Above 33 kV, IS 7098 Part 3 takes over and the grades follow IEC 60502-2 and IEC 60840 practice. APAR’s MV and HV XLPE power cables are made from 3.3/6.6 kV up to 38/66 kV in these grades, and the LV XLPE range covers the 1.1 kV grade. For anything not in the table, such as 1.9/3.3 kV cables asked for on a 3.3 kV unearthed system, the earthing category should be confirmed with the network owner before the order is placed. The datasheet reading guide explains the other lines that appear alongside the voltage grade.