zero sequence CT

Status
Not open for further replies.
M

mull982

Senior Member
It is my understanding that a zero sequence CT is typically used for ground fault sensing and can be used to detect a ground fault on a circuit by summing the current on the three phase legs. These currents under normal operation will vectorally sum to zero, however with a ground fault, since some current will go to ground and not return on the other phases the CT will see this imbalance and send a current signal to a relay. Is this correct?

Understaning this, my question comes in as to weather or not a zero sequecne will sense an unbalanced load? Can a zero sequence trip a relay due to an unbalanced load or strictly from a ground fault? I have been told that with an unbalanced load the total current going into the CT is coming back on the other phases so it should still some to zero? Is this correct? Can someone ellaborate any further?

Thanks
 
mull982 said:
It is my understanding that a zero sequence CT is typically used for ground fault sensing and can be used to detect a ground fault on a circuit by summing the current on the three phase legs. These currents under normal operation will vectorally sum to zero, however with a ground fault, since some current will go to ground and not return on the other phases the CT will see this imbalance and send a current signal to a relay. Is this correct?

Understaning this, my question comes in as to weather or not a zero sequecne will sense an unbalanced load? Can a zero sequence trip a relay due to an unbalanced load or strictly from a ground fault? I have been told that with an unbalanced load the total current going into the CT is coming back on the other phases so it should still some to zero? Is this correct? Can someone ellaborate any further?
Click to expand...

You are correct, in the basic way a zero sequence CT operates. If all current carrying paths pass through the CT (what goes out comes back in) then an unbalance will not cause a trip.

There are other ground fault schemes that consist of multiple separate CTs that have their outputs summed together rather than a single CT surrounding all conductors. If there are sufficiently large enough performance differences between the separate CTs it may be theoretically possible for an unbalance to cause a trip. I have seen some improperly installed neutral current sensors that I have advised would be prone to improper tripping on unbalanced currents.
 
Last edited:
CT backwards, CT in the wrong location, Open CT, Improperly wired CTs. Some of these issues depend on the type of GFP Zero Sequence, Residual or Ground Return.
 
Last edited:
CHWflorida said:
What would be an example of an improper neutral current sensor installation?
Click to expand...

The past two that I dealt with (in one week) involved grounding conductors downstream of the neutral current sensor but still within the service entrance equipment. While both contractors and both inspectors had no problem with this situation and there is no warning in the manufacturer's instruction, it clearly is against NEMA installation procedures.
 
jim, if you have time could you provide a little more information ? how does the grounding cause a problem. also, is this why some poco's don't want the bonding wires in the service equipment?
 
nakulak said:
jim, if you have time could you provide a little more information ? how does the grounding cause a problem. also, is this why some poco's don't want the bonding wires in the service equipment?
Click to expand...

In the cases I was just involved with the location of the grounding conductors and the ground-neutral bonding jumper caused a parallel path with the neutral sensor/CT. This parallel path meant that some current that should have flowed through the sensor did not and so the "total current" sensed would not sum to zero. Also, currents flowing on the grounding conductors could return to the neutral sensor meaning that a downstream ground fault might not be acted upon. Both of these result in an improper tripping condition.
 
The past two that I dealt with (in one week) involved grounding conductors downstream of the neutral current sensor but still within the service entrance equipment. While both contractors and both inspectors had no problem with this situation and there is no warning in the manufacturer's instruction, it clearly is against NEMA installation procedures
Click to expand...

Jim I recently was called to a site to test a GFP the inspector had made them install a bond? jumper around the Neutral Sensor CT. Now I questioned the electrician regarding because I refused to believe the inspector requested this. Few week later same jurisdiction different contractor same thing, same explanation.

I had trouble getting the contractor to change the set up, finally called the manufacture and had them tell the contractor this was wrong. Waiting to hear from the inspector.

One of the most common issues is a neutral to ground connection downstream from the main neutral to ground bond connection, this can be accidental or intentional. There are several issues with this, one is ground current which if sufficient enough can trip the main line switch. During a ground fault can desensitize the operation of the GFP. Ground current results in EMF and issues related with this. Should there ever be an open neutral for what ever reason this point of unknown resistance can become the sole source neutral current carrying connection.

A primary reason to megger neutrals to ground with the feeder neutral lifted, panel by panel, back to the main service
 
Last edited:
Status
Not open for further replies.
Top