Is it safe to disconnect CT leads if there is voltage but no current?

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shockking

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engineer
We have two transformers, a main transformer and a backup transformer. Each transformer has it's own utility connection on the primary side. Both transformers have CT's for metering on both the primary and secondary sides, so four sets of CT's total. One set, the CT's on the primary side of the backup transformer, are backwards.

The facility is operating under normal conditions. All power is coming through the main transformer. An isolating switch on the secondary side of the backup transformer is open. The backup transformer is on an open circuit and sees zero current. However, there is still voltage from the utility on the primary side.

In this case, is it safe to disconnect and flip-flop the CT leads at the meter?
 
221101-1618 EDT

shockking:

If there is truly no current flowing thru a current transformer primary, then the secondary could see an open circuit load and no voltage would be on the secondary side. But why take a chance? It is simple enough to short the secondary, standard practice is to short the secondary.

Note: even though the primary wire thru the current transformer core has no current that does not mean that no stray magnetic field can not link with the current transformer. This is not likely, but not impossible.

A further point is that the primary side of a transformer will produce some primary current if voltage is applied.

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I agree with Gar.

A CT is essentially a stiff current source, trying to make its output current flow. The proper way to 'shut off' a current source is to short it.

In theory, in the situation you describe, there should be no output. But why take the chance.

Jon
 
221102-1423 EDT

jaggedben:

Clearly with a jumper wire. You don't create current transformer circuits where it is not feasible to provide the needed jumper wire.

You should have had this training way back in high school physics class.

I also might point out that when running physics tests on DC shunt wound motors with current excitation to field coils you always had one person responsible to remove excitation to the armature under loss of field excitation.


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shockking said:
However, there is still voltage from the utility on the primary side.
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Then there is some CT circuit current due to primary energizing current.

When normally connected in the CT/meter circuit, there is virtually no voltage across the CT secondary terminals, so it's safe to install shunts while energized.
 
It depends on the CTs.

If the CT just has lead wires for output, and those leads are screwed down to a metering terminal, then I don't see a practical way to short them.

But if the CT has exposed output screws, then they can easily be shorted while the lead wires are disconnected and swapped.

Many metering CTs have built in shorting bars right on the terminal block, specifically to allow disconnecting them while in service.

Jon
 
winnie said:
It depends on the CTs.

If the CT just has lead wires for output, and those leads are screwed down to a metering terminal, then I don't see a practical way to short them.
...

Jon
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This. The CTs I typically deal with are for lower amp (<400) and for monitoring for customer owned equipment, not the utility. There are no exposed leads or shorting bar in the equipment for attaching a jumper wire. It seems perfectly safe to switch leads without shorting them (except that with enough load the CTs vibrate during the process) but there are these warnings in the manual.
 
gar said:
221102-1423 EDT

jaggedben:

Clearly with a jumper wire. You don't create current transformer circuits where it is not feasible to provide the needed jumper wire.

You should have had this training way back in high school physics class.

I also might point out that when running physics tests on DC shunt wound motors with current excitation to field coils you always had one person responsible to remove excitation to the armature under loss of field excitation.


.
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The question wasn't conceptual, it was about what nuts and bolts (literally) you need to look for or bring with you to make it happen.

(Also I think you underestimate how special your high school physics class was. Anyway I took chemistry.)
 
221102-10-28 EDT

jaggedben:

I did not consider my high school physics class to be special. However, we did do one experiment that most would never experience. This was an experiment on reaction time, automotive breaking time, and driver reaction time. This included determining deceleration. Each student had the opportunity to be the driver for one trial. 1947-48 was when I had high school physics.

Our physics class covered mechanics, electrical, optics, and a little nuclear. And one night our physics teacher had us over to his home, look thru his reflective telescope that he had made, and for which he had ground the optics.

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winnie said:
It depends on the CTs.

If the CT just has lead wires for output, and those leads are screwed down to a metering terminal, then I don't see a practical way to short them.

But if the CT has exposed output screws, then they can easily be shorted while the lead wires are disconnected and swapped.

Many metering CTs have built in shorting bars right on the terminal block, specifically to allow disconnecting them while in service.
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This was very helpful. Thanks all for the feedback.

For any future readers: google "ct shorting terminal block"
 
gar said:
221102-1423 EDT

You should have had this training way back in high school physics class.
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Man Michigan is WAY ahead of North Carolina. I didn't see that in my HS or university physics; indeed, not even in my BSEE at NCState. I had no idea there was such a thing at a CT or PT until I worked in a POCO generating station as my first "real" job.
 
jaggedben said:
In practice, how does one normally short the leads when the leads are what you need to disconnect?
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Could just strip a small portion of the secondary leads and twist both leads together under a wirenut. Disconnect and reinsulate when finished.
 
xformer said:
Could just strip a small portion of the secondary leads and twist both leads together under a wirenut. Disconnect and reinsulate when finished.
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Interesting. Probably don't need the wire nut actually.
Honestly I had to deal with this yesterday when we energized a system and it turned out one of the CT leads wasn't properly terminated. These are low current high ratio CTs and I don't think there's a genuine safety issue? I just held the loose lead against the other terminal for a second when I needed it out of the way, then swiftly put it back.
 
shockking said:
We have two transformers, a main transformer and a backup transformer. Each transformer has it's own utility connection on the primary side. Both transformers have CT's for metering on both the primary and secondary sides, so four sets of CT's total. One set, the CT's on the primary side of the backup transformer, are backwards.

The facility is operating under normal conditions. All power is coming through the main transformer. An isolating switch on the secondary side of the backup transformer is open. The backup transformer is on an open circuit and sees zero current. However, there is still voltage from the utility on the primary side.

In this case, is it safe to disconnect and flip-flop the CT leads at the meter?
Click to expand...
We have before, no issues.

Just make sure there truly isn't any current flow. Your isolating switch would take care of that.

Im curious… why metering on primary and secondary? You checking system losses?
 
Hv&Lv said:
We have before, no issues.

Just make sure there truly isn't any current flow. Your isolating switch would take care of that.

Im curious… why metering on primary and secondary? You checking system losses?
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This is the 115kV input to a 50 MW substation that feeds our entire campus, around 1,000 buildings and 500 distribution transformers. Our distribution transformers do not have much metering but these two being so large and critical, they do. There's a pair of 50MVA transformers in the substation, a primary and backup, and both have both primary and secondary metering.

I'm new to this company but I suppose it should have been obvious, the CT's are terminated on shunting terminal blocks, so it is trivially easy to shunt the CT's and switch the polarity of the jumpers between the terminal block and meter.
 
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