Open CT detection?

[crthompson]

Does anyone know of a device or circuit that can detect an open CT? I have found devices that will detect an open CT while current is flowing through it, but I need something that will detect an open CT WITHOUT current flow. The application I am designing this for is a 3 phase 1000V system.

 

[cranky108]

Have you tried an ohm meter?

 

[davidbeach]

But, the ohm meter would put a current through the CT if it isn't open and that doesn't meet the spec. ;-)

 

[JG2828]

You would need to open the CT circuit in order to test it with an ohm meter. I think he is looking for something that can be left in the circuit.

 

[crthompson]

I am designing a vacuum circuit breaker and MSHA requires that the breaker open in the event of an open CT... If current is flowing this is no problem but they require you to trip your breaker even if there is no current flow and you have an open CT. These CTs are used for overcurrent detection.

 

[waross]

If this is a doughnut type you may be able to put a wire with a test current through the window. Put a test current that is small in relation to the trip current through the CT and trip on loss of the test current.
Cancel the test current out with appropriate protection settings.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crthompson]

Thanks waross! That is the only thing I could think of... I think I may have a little trouble canceling out the test current with the protection settings. My test current is going to be single phase and the load I am measuring is 3phase... This being the case, wouldn't they add(or subtract) vectorily? How would I get a protection relay to eliminate the single phase current?

 

[davidbeach]

CTs on both sides of the breaker and a breaker differential. You'll trip any time the two currents don't measure the same.

 

[electricpete]

I’m not sure I can come help with good solution, but maybe you can clarify:
Does a given CT enclose one phase of your three phase system or all three-phases?
Do you have one CT per phase?
What type of protection does this CT feed into ?


If it’s a differential, then as David as mentioned, it’s pretty good at detecting open already, but does rely on primary current flowing. If this is not acceptable for you (assuming it is a differential), you’d want to add your test current flowing through CT’s on both sides of the zone which accomplishes same thing as system current... will trip if one of CT’s opens.


=====================================
(2B)+(2B)' ?

 

[electricpete]

Although thinking some more, providing a continous source of ac test current doesn't sound particularly reliable or practical or conventional. You hate to design some kluge of a system...I've got to think some standard approach had been developed for satisfying this requirement.

=====================================
(2B)+(2B)' ?

 

[JG2828]

crthompson,
You could easily detect an open CT circuit by measuring the voltage across the secondary terminals of the CT and tripping the breaker at some voltage set point. Obviously this would only work if there is current flowing in the primary.
Are you sure this is a requirement and not just someone's interpretation of said requirement?

 

[crthompson]

Thanks for the responses guys! Pete, I have one CT installed per phase, they are tied in a WYE, and feeding into a Multilin MIF II Relay. I have tried searching the internet for a device that does this but I haven't found one that detects an open CT when there is no current present. JG2828, I went to MSHA's office personally and asked for clarification on this requirement and they told me that it had to detect an open CT even if there was no load... I have worked on a few fire supression systems recently and they have indications for open circuits such as the bells and strobes. Each device has to have a terminating resistor located at the device. I think they might be using a diode and reversing the polarity to achieve this but I can't put a diode on my CT output...

 

[cranky108]

Who said you had to use the same frequency on the test current as with the primary current? If you are measuring 60 Hz, then you can use a 400 Hz test current. Many digital relays will filter this out for determining over current quanties, and a different device can filter out the 60 Hz to determine the presence of the test current.

 

[crthompson]

I will look into that cranky108... Thanks!