In the recent case, the inspectorate said 240 volts of alternating current was measured on the plug pins of the disconnected UPS mains supply cable while the UPS was powering a load.

“Subsequent testing of other units on site revealed 75 per cent of all models and brands tested had backfeed voltages of 55Vac or more,” the inspectorate said.

The identified cause was a lack of backfeed protection with the UPS system under existing regulations.

“Testing several units revealed varying backfeed voltages from identical units, as well as backfeed on a unit from another manufacturer,” the inspectorate said.

“These units were not fitted with protective devices to limit or shut down supply if a person received a backfeed shock.”

Mines were recommended to ensure UPOS units and systems met existing regulations.

They were also advised that checking backfeed protection circuitry formed part of routine maintenance procedures and for immediate testing to take place.