Open CT

[gfdona]

I recently found arcing/melting on shorting terminal block for a set of 2000:5 ct for some 480v gear.The terminal block is melting and it would not be possible to tighten the screws. Obviously, the CT's are open and I don't want to touch it. Unfortunately I have to shut the entire plant down to fix this - very, very painful. What will happen if the CT goes fully open? Could this cause a fault in the gear?

 

[scottf]

If the CT is open circuit, then a very high voltage is possible across the CTs secondary terminals. The magnitude of the voltage is dependant on the primary current through the CT and the size of the CT (normally, the bigger the core, the higher the voltage). Assuming these are protection class CTs, then the voltage is probably in the range of 30 kV peak or so. Note that the waveform is non-sinusoidal.

If the CT terminals are open, then there will probably be some flashovers between the terminals, depending on how close the are. If they are not sparking, then there is probably some internal flashing, and eventually there will be a short in the secondary winding. This causes high current flow and potentially high temperatures.

It's hard to say if the gear will fail or not, without more detail on the installation.

My advise.....take the outage. Besides equipment damage, it is a definate safety hazard!

 

[busbar]

Discuss the financial effects and safety aspects of a forced/unscheduled outage with your managers.

 

[DanDel]

An open CT will simply burn itself out, usually inside it's winding. The problem is, you don't know how long this will take or how much damage this process will cause.
Can you verify which phase CT conection is shorting?
I would expect that if you said that the shorting block 'is melting' that you have a continuing problem(i.e. it hasn't burned itself closed or open). You might be able to verify this with an IR scan.
Bottom line, schedule a shutdown ASAP, but make sure you have replacement CTs(and a shorting block) on hand.

 

[busbar]

A problem may be the internal-CT activity externally propagating to melt larger pieces of copper.

 

[scottf]

busbar-

I don't follow your last comment. Can you please offer more detail?

Unless there is enough primary current in the CT to drive the secondary current higher, the secondary current should not be at a level to melt any suitable copper connection. If the primary current is high enough to drive high currents through the secondary, then he's got a bigger problem!

 

[jjsacco]

Arcing and melting at the terminal block generally indicates a loose connection. In the case of a CT circuit, it is a self correcting problem. Either the terminal gets hot enough to weld itself closed (measure the current in the CT secondary wires to determine if the CT is still operating), or the secondary voltage was high enough to cause the CT to fail. An immediate shutdown is probably not necessary if you are in a stable condition (i.e. the CT circuit is still functioning properly, or the current in the secondary wiring is zero). CT failure is either benign, or catastrophic and generally occurs fairly quickly (on the order of minutes or perhaps hours, not days). That said, it would still be advisable to correct the situation ASAP to avoid further problems.

 

[busbar]

Had an x000:5 C200 CT "drool" and track across a Glastic bus support, faulting at 277V and tripping a unit-sub hi-side (12kV) 50/51. About a dozen local ACBs had working 'LSIG' protection, but they were downstream of the failed CT. CT "singing" had been noticed, but its meaning was not understood, and didn't get reported in time.

IIRC, the gear builder submitted 2-element watt and watthour meters in as-builts, but 2½- or 3-element components were installed.

 

[jbartos]

Suggestion: Visit

http://www.insulatedtools.com/

for high voltage insulated tools and cloth. The CT secondary voltage against ground should be measured first.

 

[jbartos]

Suggestion: Visit

http://www.ee.uidaho.edu/ee/power/EE525/lectures/L16-17/plots.pdf

for Open CT Secondary Voltage plot, ~40000VAC