LV fault tripping HV ring main unit

Daveash123 · Mar 16, 2023

Hi all, I have been on a site repairing damage caused by travellers collecting copper for the last week. We have found and replaced cabling to get the site up and running. Power has been on for a few days but today the 11KV ring main unit tripped. This feeds a transformer attached to the main switchboard. We called in an HV engineer to assist. We carried out testing and pinpointed the cause of the trip to a single phase 63A type D MCB in a remote DB. The circuit is not marked so it was disconnected until we can investigate tomorrow. We restored the power and left site for the night.
Both myself and the HV guy have never come acrosss this before and was wondering if anyone on the forum has any ideas. The fault could be a cut cable somewhere and its been disturbed or got wet today. The fault is bypassing a 63A MCB, 200A MCCB on a section board, 400A HRC fuse in the main switchboard and the 3200A ACB before tripping the ring main unit.
Obviously we will be carrying out full testing tomorrow to identify this circuit though the site is a former manufacturing plant with cables everywhere, both redundant and in use and the marking of the boards has never been updated
Any advice appreciated

pc1966 · Mar 19, 2023

Personally I find it hard to see how the RMU could be tripped by a MCB fault. At, say, 10kA MCB breaking limit (as I assume the related panel has a lower PFC...) that is just 3x the ACB's rating so it ought to be well under the HV side protection threshold.

So either the HV side is broken or very badly configured (which you would think would have been noticed when previously in use), or that MCB's circuit is somehow upsetting it via a control signal or somehow tricking it in to thinking there is an earth-fault or similar.

pc1966 · Mar 19, 2023

Just to add this is way outside of my area so believe anything @Julie. tells you before what I have to say!

Daveash123 · Mar 21, 2023

Update on this, we have found out that there is a small panel within the switchroom that is connected to the RMU, no one on site knew about this and the guy that attended the initial fault and reset the RMU didn`t know either. It looks like a power supply with batteries but it is not labelled and is at the opposite end of the switchroom from the RMU. We have assumed that the feed to this has been cut and when the batteries discharged the trip occurred. However, it has been off for a while now and not tripped since. We have installed a new supply to the panel but leaving it off until the specialist can confirm what it does. He was due yesterday but didn`t turn up. The 63A MCB was causing the trip but only because it supplied the DB that supplied the panel in the switchroom.
Thanks for all comments and advice