A HOT site is a high voltage installation (400kV, 132kV, 33kV, etc.) where the Earth Potential Rise (EPR) that can occur during an earth fault can exceed 430V. For installations at these sites, there is a risk that the EPR could be exported onto the HV or LV network and into a consumer’s premises, where it could cause danger to life or damage to property. Please note that all National Grid sites are generally classified as HOT.
When an earth fault occurs on a cable at a substation, a proportion of the fault current will return to the source substation through the ground.This “ground return” current will flow into the ground through the earth connection closest to the fault. The current flow through the substation earth resistance will cause the voltage of the substation earth connections to rise above that of a remote (or true) earth. This voltage is known as the earth potential rise or EPR.
On LV installations located within designated HOT sites, one common method of protection against the effects of a high Earth Potential Rise is to design the LV installation as a TT supply and install a 1:1 ratio isolation transformer with RCD protection on the incoming and outgoing, as shown above. The incorporation of an isolation transformer ensures the LV installation cannot be affected by an Earth Potential Rise.