scope:

PURPOSE AND SCOPE

Under power system fault conditions, the substation earth (local earth) can rise to a high voltage compared to a remote earth. This elevated voltage is termed as Earth Potential Rise (EPR).

Where telecommunication services are delivered to a site the EPR can be sufficiently high to present a safety hazard both to personnel within the substation and to Public Telecommunications Operator (PTO) personnel remote from the site. A site is considered to be a 'Hot Site' where the EPR can rise above 430V rms or, above 650V rms for high reliability electricity transmission systems where high speed protection systems are employed. The higher voltage category applies to all the National Grid transmission system.

The International Telecommunications Union (ITU) has issued directives which recommend that protective safety measures are taken where services are delivered to Hot Sites. The Energy Networks Association has also agreed the process for managing 'Hot Sites' with BT (refer to ENA ER S36).

This register of Hot Sites is maintained by the National Grid and is issued to PTO companies who provide telecommunication services so that appropriate measures can be taken.

At joint National Grid / Distribution Network Operator (DNO) sites, the earthing systems are normally interconnected and therefore the EPR will be exported to the DNO sites. This register is therefore also issued to DNO companies so that they are aware of the exported EPR.