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dvmdsc
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L1 AND L2 Together Through the Same CT What

09/06/2011 10:53 PM

Happens?

In a single phase 120 VAC- N- 120 VAC residential net.

Your answers much appreciated. D.

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Tornado
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#1

Re: L1 AND L2 together through the same CT what

09/06/2011 11:54 PM

This scheme can detect a ground (earth) fault. See also RCD (residual current device), ELCB (earth leakage circuit breaker), and other similar arrangements.

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kvsridhar
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#2
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Re: L1 AND L2 together through the same CT what

09/07/2011 12:53 AM

Since i am not familiar with the US system, maybe a stupid question ... wouldn't one need to run the neutral also through the CT for sensing earth leakage ?

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dvmdsc
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#3
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Re: L1 AND L2 together through the same CT what

09/07/2011 1:05 AM

This is the principle as I experience on Doubloon Road (Bahamas), similar (I hope) with the US of A.

It will be not free of criticism, referring to earlier posts.

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Tornado
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Re: L1 AND L2 together through the same CT what

09/07/2011 1:10 AM

In the U.S., 120V circuits with L1 and N are sometimes described with L1 as "hot" and L2 as neutral. In this case, a difference in current (as detected by a CT surrounding both) would indicate a ground fault.

I may have misread dvmdsc's system. In his case, a CT around L1 and L2 (both "hot") would normally equal any unbalanced current in N. A CT around all three (L1, L2, N) would indicate a ground fault.

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dvmdsc
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#5
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Re: L1 AND L2 together through the same CT what

09/07/2011 1:28 AM

Hi Tornado,

I found a kWh meter panel that works with 1 CT (90- 260 Volts AC/50-60 Hz) to measure a normal L + N net configuration. (uses a mini 70/5 A CT)

In the provided schematic, should I experiment with the 2 similar CT's parallel and phase reversed to measure the total of kWh used? Should I order it for testing or is it a lost case? Thank you.

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Tornado
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#6
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Re: L1 AND L2 together through the same CT what

09/07/2011 2:34 AM

It seems that we are trying to combine the discussion of two threads, in which total power consumption and ground fault detection are separate objectives.

For measuring power consumption in an L1/N/L2 (+120/N/-120) network, I think you need CT's on L1 and L2, adding their absolute values together.

It would be nice to find a way to capture the total load with just one CT, but I don't think that is possible.

For ground fault detection, either you are subtracting infeed from returning current, or adding currents vectorially (some of which are negative), such that the nominal sum should be zero. Other than zero = ground fault.

These two scenarios--total power versus ground fault--are different, and require different computation.

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dvmdsc
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#7
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Re: L1 AND L2 together through the same CT what

09/09/2011 1:02 AM

Thank you for the clear explanation. GA

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