Disconnecting current transformer leads from a meter.

[zog]

Have you ever worked with shorting blocks? The blocks are used to Short the 2 terminals of a particular CT. When the 2 terminals are shorted, example CT-1 for Phase 1, the current read on the meter for phase 1 will read 0

Becasue it is a parallel circuit, and the resistance of the shorted branch is much less than the meter.

 

[rcwilson]

An old relay electrician showed me this trick.

After installing the shorting screws and verifying no current is flowing, carefully back off but don't remove the current wire's screw terminal on the meter. Carefully wiggle the wire while watching for any arcing. If it is a ring tongue terminal on a screw, slide the ring across the threads. If the CT is not correctly shorted, you will see some arcing. Tighten the connection back up and find the problem. If no arcing is detected, proceed to remove the wire, carefully.

 

[GMc]

Becasue it is a parallel circuit, and the resistance of the shorted branch is much less than the meter.

So are you saying I'm wrong also?

 

[SG-1]

So are you saying I'm wrong also?

GMc

I did not mean to imply you were wrong. I should have chosen my words more carefully. I do apologize for that. I may well be wrong, possibly my blondeness asserting itself, yet again . I will not conceed until I am able to try this on some modern instruments. This weekend or next week I should get an opportunity. The point I was trying to make was the reading might not be absolute zero. The meter might read some small amount. Looking back I see where you could have taken my post not as it was intended.

On electro-mechanical instrument like a KA-241 panel meter I am in complete agreement that the current on the meter will drop to zero. I am not sure it will be zero on a modern electronic meter. I think the reading will approach zero. I will short some blocks while doing some routine tests & report my readings back here. Stay tuned to see how blonde I am !

Also while posting I was thinking about an FT-1 test switch that 30 years ago could bring the current reading to zero by lowering the shorting knife switch. That test switch is now lucky to drop the reading to 40%, because the burden is so low in a modern meter or relay.

 

[zog]

So are you saying I'm wrong also?

As SG-1 mentioned, the meter will read 0 and there is close to 0A flowing through that branch of the circuit when the shorting blocks are used. Theer still is current in the circuit, but most of it is flowing through the shorted leg of the circuit. You have a parallel circuit, the shorting blocks are one leg, with a very low resistance, the meter is the other, with a much higher resistance.

 

[GMc]

GMc

I did not mean to imply you were wrong. I should have chosen my words more carefully. I do apologize for that. I may well be wrong, possibly my blondeness asserting itself, yet again . I will not conceed until I am able to try this on some modern instruments. This weekend or next week I should get an opportunity. The point I was trying to make was the reading might not be absolute zero. The meter might read some small amount. Looking back I see where you could have taken my post not as it was intended.

On electro-mechanical instrument like a KA-241 panel meter I am in complete agreement that the current on the meter will drop to zero. I am not sure it will be zero on a modern electronic meter. I think the reading will approach zero. I will short some blocks while doing some routine tests & report my readings back here. Stay tuned to see how blonde I am !

Also while posting I was thinking about an FT-1 test switch that 30 years ago could bring the current reading to zero by lowering the shorting knife switch. That test switch is now lucky to drop the reading to 40%, because the burden is so low in a modern meter or relay.

Thanks for taking the time to clear that up. My experience has been with a meter similar to the KA-241 also. I personally don't see the electronic meter reading anything other than zero. I'm anxiously waiting for your results. Thanks again for your response.

As SG-1 mentioned, the meter will read 0 and there is close to 0A flowing through that branch of the circuit when the shorting blocks are used. Theer still is current in the circuit, but most of it is flowing through the shorted leg of the circuit. You have a parallel circuit, the shorting blocks are one leg, with a very low resistance, the meter is the other, with a much higher resistance.

I believe I said the meter would read zero. Originally Posted by GMc

You don't have to shut the power off to the meter first, in fact you can verify the ct was shorted if you leave the meter powered and watch the current drop to 0.

Thanks for the response

 

[kwired]

After reading the last few posts arguing about where the current is flowing, isn't the important thing that is being overlooked is the fact that the reason for shorting blocks is not necessarily to remove all current from the metering device, but to prevent open circuit of the CT secondary circuit when the metering device is removed.

If you install shorting device, you created a parallel path, if you observed a significant change in the meter when installing the shorting device this is your verification that current is flowing in another path. It is then safe to remove the meter without causing an open circuit in the CT secondary circuit.

 

[GMc]

After reading the last few posts arguing about where the current is flowing, isn't the important thing that is being overlooked is the fact that the reason for shorting blocks is not necessarily to remove all current from the metering device, but to prevent open circuit of the CT secondary circuit when the metering device is removed.

If you install shorting device, you created a parallel path, if you observed a significant change in the meter when installing the shorting device this is your verification that current is flowing in another path. It is then safe to remove the meter without causing an open circuit in the CT secondary circuit.

If you read all the post's and not just the last few I think you will see nothing was overlooked. I would not call it arguing either, just trying to understand what each other is saying..

 

[SG-1]

Ok, the results are in, mostly. I got lucky a KA-241 ammeter is installed on phase A of each circuit tested. The other device is a GE Multi-Lin SR750 relay. You can see the impact of that analog meter in the readings.

I have a couple of pictures, but cannot upload to the forum since the change. I will upload to photobucket & post a link later.


Circuit 1 has approx. 5 amperes applied & the relay is reading a little over 1000A on each phase. The leads from the CT drivers are connected to the CT side of the shorting block.

Circuit 1 with shorting screws applied to the neutral & phase A.
A-Phase 113
B-Phase 1018
C-Phase 1006

Circuit 1 with shorting screws applied to neutral & phase B.
A-Phase 1013
B-Phase 158
C-Phase 1020

Circuit 1 with shorting screws applied to neutral & phase C.
A-Phase 1013
B-Phase 1032
C-Phase 132

The next two circuits had simular results. With only one phase shorted at a time the readings looked like this:

A-Phase Shorted to neutral 133
B & C around 1000

B-Phase shorted to neutral 170
A & C around 1000

C-Phase shorted to neutral 175
A & B around 1000

Circuit 3

A-Phase shorted to neutral 78
B & C around 1000

B-Phase shorted to neutral 170
A & C around 1000

C-Phase shorted to neutral 192
A & C phase around 1000

So much for the single phase tests. When all 4 shorting screws are applied to the block the readings go to zero, absolute zero.

I think I can get zero with 3 screws. What do you think? I will try that later today.
With all 4 screws in place what happens if I single phase the circuit ?
Will it still read 0 ?

 

[SG-1]

Here is the link to a picture of the shorting block test connections & a picture of the SR750 display. Should be pictures 1 & 2 of 8. The other pictures are of some antique electrical devices not related to this thread.

http://s1204.photobucket.com/albums/bb408/Finduilas1/

 

[SG-1]

I think I can get zero with 3 screws. What do you think? I will try that later today.
With all 4 screws in place what happens if I single phase the circuit ?
Will it still read 0 ?

The meter will dance on zero, but never settle out with 3 shorting screws on the phases, no screw on the neutral. I think this is only because of a slight inbalence. If the phases were perfectly balenced I believe they would zero. I would never recommend doing this while changing out an instrument.

With all four screws in place, I know some of you think by this time I surely have lost at least one or more screws, no combination of single phasing ( like 1-N, 1-2, etc...) will produce anything but, zero.

 

[SG-1]

After reading the last few posts arguing about where the current is flowing, isn't the important thing that is being overlooked is the fact that the reason for shorting blocks is not necessarily to remove all current from the metering device, but to prevent open circuit of the CT secondary circuit when the metering device is removed.

If you install shorting device, you created a parallel path, if you observed a significant change in the meter when installing the shorting device this is your verification that current is flowing in another path. It is then safe to remove the meter without causing an open circuit in the CT secondary circuit.

You have provided a very good summary. There is an old indian saying that goes something like this; when you discover that you are riding a dead horse you should immediately dismount. The saying does not prohibit an autopsy.

 

[GMc]

SG-1

I had some time so I tried the test on some newly added Square D PM800 meters and with every screw the meter went to zero, just like the analog meters.