scope:

This standard defines the method for calculating the maximum permissible earth resistance value to control touch voltages, either to BS 7671 or BS EN 50122-1, at a signalling apparatus housing comprising a functional supply point (FSP) in a Class I IT electrical system used in signalling power distribution systems.

The method detailed is also applicable for calculating the maximum permissible earth resistance value to control touch voltages to BS 7671 or BS EN 50122-1 on:

a) the armour which forms parts of armoured BS 5467 and BS 6346 cable when used as part of an IT electrical system; and

b) power sources distributing IT electrical systems, normally comprising principal supply points (PSPs) and distribution interface transformer assemblies (DITAs) in either Class I or Class II signalling power distribution systems, in accordance with NR/L2/SIGELP/27410.

The assumptions underpinning the derivation of first earth fault leakage current (I_ƒƒ) for different cable sizes and types are detailed, along with the key relationships to be utilised for deriving the total network capacitance (C_E) and the maximum permitted earth resistance value (R_Amax).

The calculation method detailed is applicable to the following signalling power distribution systems:

a) single-phase, IT-individual with 2-core unarmoured cable;

b) single-phase, IT-individual mixed with IT-group, with 2-core unarmoured cable mixed with either 2-core or 3-core armoured cable;

c) single-phase, IT-collective with 3-core armoured cable;

d) single-phase Class II signalling power distribution systems designed and installed in accordance with NR/L2/SIGELP/27410.

This standard does not set out the requirements for calculating the target earth resistance value for protection against electric shock in the event of second earth fault on an IT system, lightning protection, test and functional earths.

Purpose

This standard sets out a method for calculating the maximum target earth value at signalling apparatus housings and power supply sources to afford protection against electric shock in the event of a first earth fault in Class I and Class II signalling power distribution systems using IT electrical systems.