Neutral CT connection (N) vs Ground CT (G)

[kookypedia]

Guys, my understanding is if we have one CT around all 3 phases (to measure the residue), ANSI calls this as Zero sequence or G(round) CT and if we have one CT around neutral wire, ANSI calls it N(eutral) CT.
If the above statement is correct, what are the applications of each?

 

[mbk2k3]

my limited understanding:

a zero sequence CT would be used around the 3 phases in a feeder circuit.
a neutral CT would be used on the neutral only, for example the neutral leg of a transformer, or a generator.

hopefully others can shed more light on this?

 

[waross]

For residual ground current, when there are line to neutral loads, the CT must be around four conductors, the three phase conductors and the neutral conductor. The CT must be located so that only neutral current and not ground current passes through the CT.
When there are line to neutral loads, you don't want normal neutral current tripping your ground fault protection.

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

 

[kookypedia]

Bill,
Considering any imbalance current component (either from imbalance load or fault) goes to neutral wire, how putting 4 wire inside the CT will work?

http://www.kookypedia.com

 

[waross]

The ground CT may around the ground conductor and measure ground current directly or it may be around ALL the load conductors and measure the current that does not return through the load conductors.
You may be confusing the neutral conductor with the grounding conductor, Although these are generally connected at the transformer or elsewhere, they serve different functions and carry current under different conditions.

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

 

[jghrist]

Considering any imbalance current component (either from imbalance load or fault) goes to neutral wire, how putting 4 wire inside the CT will work?

It works because you don't want to measure imbalance load current that would flow through the neutral, only ground fault current not flowing in the neutral.

 

[bikenut44]

Typically the zero sequence CT or (BYZ CT) has a low ratio (50:5)and is utilized on medium voltage motors because it cane be set at very low trip settings (10-20A). Neutral ct's typically have much higher ratios to allow for downstream coordination.

 

[dpc]

To my knowledge, there is no ANSI standard for when to use "G" vs "N". In fact, two of the largest manufacturers of protective relays do it exactly opposite of each other. Or at least they used to.

 

[davidbeach]

Gets really awkward when a certain relay has both 67N and 21G elements, both working off calculated residual values. In subsequent relays from the same manufacturer they went with 67G to avoid having 21N. C37.2 says they made the wrong choice and that it should be 21N, not 21G. Maybe if we called it Earth Distance rather than Ground Distance it would be easier to designate it as 21N instead of 21G.