Hi all,


I am working on the LV side of a scheme with a private HV transformer (>1MVA) and have been told by the HV contractor that since it is fed / protected by a fuse switch, it is not appropriate to make use of the transformer overtemp / tank pressure / oil level relay to disconnect at HV, reason being that it can't break a fault and that's what the fuse is for. I do get the principle that RMU switches are not fault-breaking devices and you shouldn't, for example, use an external overcurrent relay on the HV terminals to operate one, but should these functions not be considered to prevent the fault developing sufficiently to fire the fuses in the first place? For example if the oil level alarm triggers via SCADA, or the oil temp trip triggers despite LV load shedding, is there really time to wander up to site and manually operate?


Also I would have thought that you'd be pretty unlucky for the fault to strike at precisely the same time as the transformer relay, even if both happen in a single event, due to thermal inertia etc?


Please correct me before I make a fool of myself questioning it in public!


Ultimately it's not our scope but we'll be picking up the multicore wiring one way or the other; also for future reference.


TIA,

Jam