# 5kv ct



## Apelectric (Dec 6, 2017)

Anyone else see a ct melt like this before?


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## Southeast Power (Jan 18, 2009)

Apelectric said:


> Anyone else see a ct melt like this before?


Looks like someone didn't close the output.


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## Apelectric (Dec 6, 2017)

How long would the output have to be open for the ct to start melting like that? Would a loose connection on the secondary side of the ct be able to cause that if it had enough resistance? We recently installed a scada system, the ct output could have been lifted inadvertently during that work.


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## Zog (Apr 15, 2009)

Apelectric said:


> Anyone else see a ct melt like this before?


Like this?


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## Southeast Power (Jan 18, 2009)

Apelectric said:


> How long would the output have to be open for the ct to start melting like that? Would a loose connection on the secondary side of the ct be able to cause that if it had enough resistance? We recently installed a scada system, the ct output could have been lifted inadvertently during that work.


I dont recall how long it takes to actually burn but I'm panicky whenever we open one for a few seconds.


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## EB Electric (Feb 8, 2013)

Southeast Power said:


> I dont recall how long it takes to actually burn but I'm panicky whenever we open one for a few seconds.


With no burden on the ct/open circuited I would think the more primary current flowing the quicker the voltage produced in the ct will climb to levels the ct insulation cannot handle. Up here all our ct's are wired up to shorting blocks and/or flexitest switches for isolating equipment from ct's for testing and maintenance. I am curious when would you knowingly open circuit a ct with primary current flowing, even for a few seconds?


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## brian john (Mar 11, 2007)

Yep I have seen this a few times, was in the room one time when we closed in a 4000 amp BPS, one utility CT was left open, we got a high pitch whine that hurt my ears, that old fashion whoops something is burning all in a matter of seconds before we opened the main.
\

Even if you do not smoke the CT you can destroy the accuracy, read that in an IEEE Mag some years ago.


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## brian john (Mar 11, 2007)

Two GFPE CTs that were left open, the 3rd picture I included as I happened to run across this looking for the 1st two Pictures. A little inductance.


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## Southeast Power (Jan 18, 2009)

EB Electric said:


> With no burden on the ct/open circuited I would think the more primary current flowing the quicker the voltage produced in the ct will climb to levels the ct insulation cannot handle. Up here all our ct's are wired up to shorting blocks and/or flexitest switches for isolating equipment from ct's for testing and maintenance. I am curious when would you knowingly open circuit a ct with primary current flowing, even for a few seconds?


I was QC on a project adding a CTs or swapping CTs, on a neutral buss, don't exactly recall but, during the pre job brief I reinforced the hold I wrote in for an inspection hold where we could see the terminations before it went over the bus bar.
It was more for the sake of good practice as the load wasn't that much and it could be done by determinating one or two cables at a time while slipping the CT over them.
The big concern for the installation crew was to maintain the directional flow. I was overly concerned they would pinch the CT output wires or pull them out of the TB.
The risk started escalating after the first couple of installations. The crew was getting overconfident, the foreman was being a typical worm trying to push his guys and always looking for a shortcut.


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## Southeast Power (Jan 18, 2009)

brian john said:


> Two GFPE CTs that were left open, the 3rd picture I included as I happened to run across this looking for the 1st two Pictures. A little inductance.


I don't guess that was an aluminum cabinet.


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## paulengr (Oct 8, 2017)

CT's fail very quickly if you leave the secondary open. It's pretty easy to figure out why. Let's say we have a 50:5 CT (a pretty low ratio) mounted on a 480 V line. Normally with just a few ohms at most on the output side the CT output voltage on the secondary side (we're basically short circuiting it) is only a few volts using basic Ohm's law. So for instance if we have a 10 ohm resistance in the secondary side (a pretty high amount of burden) and 100 A of current on the primary side, we get 10 A on the secondary side x 10 ohms = 100 V. This is pretty typical for CT circuits...they rarely get above 100 V except with extremely large burdens like some of the old electromechanical relay days. With today's digital relaying burdens are comparatively tiny, so the voltages are comparatively small. Most CT circuits are wired up with XHHW-2.

BUT let's say we open circuit it so that the resistance on the secondary side is nearly infinite. If we just use Ohm's law as before, the math "blows up". That's not what happens in the real world but it's not far from it. In this case we look at the CT like a power transformer. Remember that in a power transformer as current goes down, voltage goes up. So knowing that the current is being reduced by a ratio of 50:5 or 10:1, the voltage is being raised by the same ratio so the output voltage is now 480 x 10 = 4800 Volts. That's with a very small ratio 50:5 CT, not the more typical 100:5, 300:5, 600:5, or higher ratios.

It's almost always opening the secondary circuit that destroys CT's, almost every time. It's to the point where when I go on a troubleshooting call and I find the CT is destroyed, it pretty quickly becomes obvious that either the local electrician didn't know or is dancing all around the subject trying to avoid it, knowing full well what happened, and just hoping and praying that I just happen to have a spare CT or two on the truck.

This should be a hint why the shorting block is there and why CT's are typically wired up with ring terminals...we want to do anything we can to avoid open circuiting a CT at all costs.


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