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Topic Title: Current transformers and energy meters
Topic Summary: star point earth wire
Created On: 07 August 2013 11:16 AM
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 07 August 2013 11:16 AM
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Rozek

Posts: 2
Joined: 21 January 2010

Hi,
I am confused about the necessity of earth wiring Current Transformers and Three phase energy meters. Must the CTs and/or 3 phase meters be earth wire or not.
I cannot find a straight answer for it. The manuals for energy meters hardly ever mention about earth wiring the meter, however (i have been told) the good practice is to earth wire either ct at S2 or at energy meters or at both devices.
Is there a British Standard, which would definitely answer and specify why and where the ct and energy meters must be earth wired?
 07 August 2013 12:50 PM
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AdrianWint

Posts: 261
Joined: 25 May 2006

It is usual practice to earth the star point of the CT secondary wiring. This is done to reference the CTs to earth & help prevent insulation problems. Without this the secondaries could/would float off to some undefined potential with respect to true earth.

As to a BS... I can't help you there... I've never seen a manufacturers instruction that doesnt demand that the earth is fitted... however, that said, it will make no operational difference whether the secondaries are earthed or not.

Adrian
 07 August 2013 02:50 PM
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Rozek

Posts: 2
Joined: 21 January 2010

Thank You Adrian,

As You say "it is usual practice to earth star point" , " however not having it done it will make no operational difference whether the secondaries are earthed or not". Explanation: A contractor has fitted in a fair amount of energy meters in our new building and none of the three phase meters or cts' has been earth star point wired. I have been trying to put the case forward for remedial works, due to lack of earth, however without hard evidence as the earth is required i need to rest my case.
 11 August 2013 06:33 PM
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sreeraasi94

Posts: 5
Joined: 11 August 2013

Rozek: It is agreed that it will not make any opeartional difference whether you earth the CT Star poibts or not. But, you must appreciate that a CT Secondary is a source isolated from the primary. So, when you earth the neutral of this 'source', you get all the benefits of an earthed neutral system. That's why it is advised to earth the neutral point of the CT secondaries.
 12 August 2013 12:32 AM
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alancapon

Posts: 5746
Joined: 27 December 2005

Originally posted by: Rozek
. . . As You say "it is usual practice to earth star point" , " however not having it done it will make no operational difference whether the secondaries are earthed or not". . .

If we are talking LV, then it doesn't really matter and in my opinion will not contribute to the safety of a CT circuit - there are far more dangerous things you can do, such as open-circuiting it!

If we are talking HV, then the earth connection is your last line of defence to a fault in the CT chamber raising the potential of the CT wiring towards the phase voltage, with all the equipment / personnel damage that is likely to cause.

Regards,

Alan.
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