Earth leakage CT on neutral Vs 4-pole breaker

[marko333]

Hello,

If you have a setup where there is a 11kv/415V transformer feeding into a main circuit breaker, then on to a switchboard, what is the advantage of having an earth leakage CT on the neutral at the main circuit breaker point. The CT operates the shunt trip of the main circuit breaker.

Is there a situation where this CT would be an advantage to have?

The load of the switchboard will be primarily motors. A power factor correction unit is installed to compensate.

There IS a chance of 'back-feeding' this switchboard as it is linked to a second transformer at the low-voltage end.
Is it better to use a 4-pole breaker as the main circuit breaker rather than use a CT on the neutral? Or will the two be doing a different thing?

thanks,

marko

 

[rbulsara]

Firstly, neutral CT and 4-pole breaker are not a substitute for each other. Both have distinct functions.

Refer to NEC section 250 and IEEE green book for references.

If your transformer secondary is wye connected, which it appears to be, its neutral must be grounded.

Once you have a grounded system you should have a earth (ground)fault protection. (NEC requires EFP (GFP) for all feeders 1000A or greater at 480V, in the USA. Your jusrisdictio may have similiar or slightly different requirements).

If ALL of your loads are 3-wire (not most or primarily) then you may not need a neutral CT for EFP. But if any of the circuits uses a neutral , then you do.

Now coming to your backfeed ( I presume it is a main-tie main type of setup?) or tie connection: The four pole breakers definitely make the system simple (although at some cost and space) for EFP protection scheme. 4-pole breakers will permit gorunding both systems at their respective end and prevents interconncetion of their neutrals. You do need the neutral CT's to make your EFP work at both sources. (only exception being all 3 wire loads).

If you use 3-pole breakers for mains and tie, and bioth sources are grounded, there is a scheme that connects the netural CTs such that it will cancel out earth fault currents from the opposing neutral CT and make the EFP work correctly. Contact the switchboard manufacturers or someone here may be able to post a diagram.

Other alterantive is to use the other source as 'not seprately' derived system and only ground one of the system and use 3-pole breakers.

 

[CJFlatters]

Contrary to the above, I assume that you are UK based (ie 11kV & 415 are UK system voltages), therefore the NEC is irrelevant as far as legislation is concerned, however the green book is a good source of reference (no doubt about it).

THE GEC PRAG (Protective Relay Application Guide is a sueful reference text on the subject of transformer protection). Are you simply referring to a single CT on the Neutral, or do you have three phase CT's also ?

BS 7671 does not actually require you to use a 4 pole ACB and it is not really necessary.

You ought to look at inter-tripping between the LV and MV breaker (if the MV breaker is yours).

_______________________________________
Regards -

Colin J Flatters
Consulting Engineer & Project Manager

 

[rbulsara]

Just a note...NEC does not contradict IEEE or principles of engineering. It is another matter that it may have some specific requirements for safety over and above basic engineering and it may or may not have been adopted as Code elsewhere.

As far as this particular matter go..NEC is very much in agreement with IEEE.

And not grounding the system neutal at more than one point is an 'engineering' requirement in order to EFP to function properly, unless you have some other 'engineered' means as stated above.