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Topic Title: Metering / protection CT circuits
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Created On: 07 February 2013 05:24 PM
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 07 February 2013 05:24 PM
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Posts: 1
Joined: 08 May 2012

Good day folks,

I am looking for a little back up regarding a metering / protection CT issue.

Generally, when we design switchboards we would use a metering type CT for connection to panel mounted ammeter / transducers for power meters etc and use protection type CT for connection into a protection relay.

I have stumbled upon a switchboard (designed by a third party) which has a 5P20 CT used for input to a protection relay AND has ammeters installed between the CT and protection relay input.

I have been scouring the internet to find a rule or regulation which this design may be breaching with no return. As we know, ammeters are not designed to have large magnitudes of fault current pass through them, and any fault or open circuit in the ammeter would firstly remove the input to the protection relay and secondly leave the CT secondary winding open circuit!

Does anyone know or a regulation which may indicate meters should not be controlled by protection class transformers?

Best regards,
 08 February 2013 12:29 AM
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Joined: 27 December 2005

It is fairly usual for panel ammeters / measurement transducers to share the same CTs as overcurrent protection. A good quality ammeter / transducer should be able to survive the current flow for the duration of the faults. If I discovered metering class CTs purely to drive panel ammeters, I would wonder why my company's money was being wasted on what I would consider as unnecessary CTs.


 09 February 2013 09:37 AM
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Joined: 28 September 2007

As per Alan above, all of the panels I have seen will have protection class CTs feeding protection relays and transducers / ammeters in the same circuit.

The only time I have seen measurement CTs feeding just instrumentation is when there are power meters involved for fiscal metering of electricity useage.

The only argument you may have is if the protection relays and CTs form part of a SIL rated system and the ammeters are not SIL rated.

Kind regards

Donald Lane

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