NS219

NS219: Telecommunications - Telephone Allocation and Isolation

This Network Standard details the ways in which the transferred-potential difference hazards as listed below should be mitigated to reduce the risk of injury to personnel and equipment.

A hazardous situation can arise whereby a telephone or modem located inside a substation is connected via conductive cable to the external environment. Such hazards include: 

• Conduction of remote earth potential to the substation, therefore presenting a potential difference between local earth and remote earth under power system fault conditions (and potentially exposing personnel to high potential difference hazards)
• Conduction of lightning strikes through the incoming conductive line, via personnel (who are using a telephone) to earth

Scope

This document outlines the requirements to be met when installing telephony equipment in a substation where connection external to the substation is via electrically conductive cable.