Faulty ACB

28th February 2025

We were recently called in the early hours by a contracted customer who had a loss of supply from one of their substations. Our incident manager immediately headed to site to assess the situation and put a rectification plan into action.

They discovered that the issue was not HV but caused by one of the main incoming LV ACB’s tripping. There was no fault indicated on the electronic trip unit as being the cause of the trip, and as there was no sign of a fault or damaged equipment, it was decided that the ACB would be closed and then reassessed. The ACB was able to be charged but would not close. As the ACB was withdrawable, we withdrew the unit from the LV panel and re-tried it. The ACB closed straight away, so we knew it wasn’t a mechanical issue and was, instead, likely a contact that was giving a signal to open the ACB.

At this stage, we called on our LV specialist partner, who could send an engineer to site within a few hours. They discovered that the cause of the open signal was a fault phase failure relay. The fault relay was removed from the system, and the ACB was then closed, restoring supplies to the site, much to our customer's delight.

2No. replacement phase failure relays were ordered, one for the known issue and one for an identical ACB on site to prevent the same situation from occurring. A planned shutdown was organised, and both relays were replaced.

EME Power Systems Electrical Engineer James Grigg commented:

The main incoming LV device is an integral part of any electrical system, but its maintenance is often overlooked. EME can provide assistance and support with these devices as well as 11kV systems.

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