Unrestricted earth fault protection

[Ukconsult]

Hi there, new to the forum and I need a little help.

I have recently started a new job and have been put in charge of approving a contractors proposal for a main LV switchboard at a hospital in the UK. There is also a new 1MVA supply authority transformer going in under a seperate contract.

The company spec asks for unrestricted earth fault protection to the main incommer of the board, the manufacturer is advising me that it is probably not needed on the job (There is a tendency to copy specifications from another job in here), but he will supply it if I want it.

I have never specified this before, and don't really understand what it is, other than it is like a large ELCB/RCD device.

 

[Sanditech]

In basic terms Unrestricted earth fault protection detects all earth fault current downstream of the device. In the case of a main Incomer breaker it provides back-up protection to the feeder breakers on the switchboard as well as the protection for the bus bar system on the switchboard.
I would expect to see this protection device on a UK circuit breaker In order to meet the IEE regs on disconnect times of earth fault current (5secs for static plant ie: the Switchboard enclosure).
I would also expect to see Restricted earth fault protection on the main LV breakers fed from the transformer.

 

[ScottyUK]

I agree with Sanditech.

The E/F protection at the incomer provides a valuable backup to that fitted to the indvidual outgoing ways, plus it protects the board itself from an internal earth fault and thus allows the installation to conform to the disconnection time required by BS7671. Without the E/F protection at the incomer the upstream protection is on the HV side of the transformer, and this is normally intended to protect the transformer itself, not the LV cables and board. Furthermore, and assuming a normal Dy??n transformer, from the HV side it is difficult to distinguish LV load imbalance from an LV earth fault other than by magnitude. The protection afforded from the HV side is of very little use in protecting against anything other than a bolted short circuit on the LV side. Your supplier might argue that any earth fault is likely to result in an overcurrent which will trip your incoming LV breaker on instantaneous O/C, but in my opinion that is a risky proposition which potentially leaves some awkward questions for the designer in the event that it does not clear the fault. E/F protection is not a costly addition to the tripping modules fitted to most large ACBs.

A word of caution: when meeting the 5 second disconnect requirement, bear in mind that few boards will withstand fault current for 5 seconds. 1 second or 3 seconds are typical ratings. Clear the fault as quickly as possible while maintaining coordination with downstream breakers.



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[mc5w]

U.S. rules REQUIRE feeder ground fault protection immediately downstream of the service in a hospital. There must be a restraint wire running from the feeder circuit breakers to the service circuit breakers to prevent the service from tripping while a feeder circuit breaker detects a ground fault.

Is would not be a bad thing to copy this idea.

 

[Ukconsult]

I did have an idea that I would get a good response guys, thanks a lot!

I do however have a little reservation on applying earth fault protection to a main incommer. Doesn't that mean that any imbalance in the WHOLE system downstream will trip the main breaker. i.e. Joe Bloggs puts a drill through a final circuit cable which suppies a fixed piece of equipment. I know that in theory (and practice), the device protecting the final circuit will most probably trip and the installation will remain operational upstream of this, but that is not always a guarantee. Just a bit nervous, thats all!!

 

[DiscoP]

The earth fault protection on the incomer should be set wth a delay to allow protection downstream to operate first.

You should allow a margin of 300-400 milliseconds betwenn the operating time of the feeder protection and the protection of the incomer.