scope:

This specification applies to new or added safety earthing systems to apparatus housings, which include signalling apparatus housings (SAHs), location cases (LOCs), functional supply points (FSPs), relocatable equipment buildings (REBs) and relay rooms supplied by signalling power systems using an IT electrical system.

The requirements for the design and installation of earthing systems in this specification may also be applied to the following:

a) Safety earthing systems forming part of a TN or TT electrical system;

b) Safety earthing systems for the armour forming parts of armoured BS 5467 or BS 6346 cable when used as part of an IT electrical system;

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

NOTE: Three methods of fault protection can be utilised at SAHs, namely: local safety earthing, traction bonding and use of a Class II based system.

Purpose

This specification details the design, installation, construction, testing and commissioning requirements for safety earthing systems to limit touch voltage potentials on exposed conductive parts forming part of signalling power systems, to meet the requirements of BS EN 50122-1.