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giri
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Measurement CT secondary is earthed why?

10/07/2009 4:13 AM

I have seen many motor feeders where the secondary of the measurement CT used for current indication in the field is earthed.WHY? Pls shed some light on this.........

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electricalexpert65
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#1

Re: Measurement CT secondary is earthed why?

10/07/2009 10:53 AM

Why only measurement CTs? It is a good practice to earth one of the secondary terminals of each CT Secondary Winding. The reason is to reduce the insulation costing. When you earth one secondary terminal, you can insulate the CT to phase volatge or else you have to insulate for line voltage which might increase the cost as well as the size of the CT. But, it is essential that all the three phase CT secondaries are earthed. Also, same secondary winding treminal needs to be earthed uniformly in all the three phases. (i.e.) if it is S1 it should be S1 in all the three phases and so on.
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giri
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#2
In reply to #1

Re: Measurement CT secondary is earthed why?

10/07/2009 12:57 PM

Hi .....can u provide more insight into this?Thanks in advance......

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electricalexpert65
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#3
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Re: Measurement CT secondary is earthed why?

10/08/2009 5:58 AM

For the same reason, why do you earth your power system neutral! For the secondary side loads, the CT is a power system and hence it has to be treated the same way as you would treat a power system.

The prime reason for power system earthing is to have good control over voltage trasinets during an earth fault. Same is the reason for CT Secondary side earthing too.
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giri
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#4
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Re: Measurement CT secondary is earthed why?

10/08/2009 10:13 AM

Thnak u electricalexpert65!......but here we should be very careful in earthing S2 terminal(the neutral wahtever it is)...beautiful reply by you...

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electricalexpert65
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#5
In reply to #4

Re: Measurement CT secondary is earthed why?

10/08/2009 10:20 AM

It need not specifically be S2. It could also be S1 (but, S1 in all the three phase CTs); I have commissioned many schemes with S1 earthings too.
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giri
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#6
In reply to #5

Re: Measurement CT secondary is earthed why?

10/08/2009 1:48 PM

How come this possible ......that must be the neutral then.........we cant earth the phases!

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