How do you coordinate a ground relay with those upstream?

[bdn2004]

And why is it that a separate ground relay is to be installed on a solidly grounded system when all the circuit breakers are equipped with LSIG protection anyway? What's the purpose of this? And when you get a ground fault does this relay always trip?

 

[davidbeach]

There's no where near enough information to begin to formulate an answer. Could be lots of different factors, some that drive the answers one way and some that drive it another way. A better description of the system in question could be useful.

 

[bdn2004]

Ok here is a portion of this system. At three places ground fault detection is installed.

 

[bdn2004]

Or take for example this motor starter circuit. The phase relay will trip the breaker on a fault, so why do we install ground protection?

 

[JuanBC]

Hi,

Here is my opinion based on my -not much- experience with ground faults:

And why is it that a separate ground relay is to be installed on a solidly grounded system when all the circuit breakers are equipped with LSIG protection anyway?

Just an idea.. That's maybe to get a more sensitive fault detection. Relays are usually more accurate than Circuit Breakers (CB) ground protection (G). (So, easier to coordinate each other and leave CB ground protection as a "back up" ground fault trip)

Or take for example this motor starter circuit. The phase relay will trip the breaker on a fault, so why do we install ground protection?

Even in solid-grounded systems there can be ground currents that can not be detected with an overcurrent protective device fast enough (safety issues)

Let's wait for other members point of view

Best regards,

JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"

 

[GroovyGuy]

Hi bdn2004,
1) Do you not have a NGR on the secondary of your (480V) transformer T101A?
This would be extremely common for any industrial facility (assuming you have few, if any, L-N loads (as might be the case in a commercial facility.)
2) Your second attachment above is for a 5kV motor controller, yet there is no 5kV included in the single-line.
3) Have you considered the impact of arc-flash? (ie a 2.5MVA 480V transformer is going to have a lot of let-thru energy).
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[bdn2004]

I think you're right JuanBC....it's just a lower sensitivity to what the overall breaker would provide. Attached is a simple explanation.
Now the question remains what kind of sensitivity are we looking for? And why at that threshold? And how are they coordinated with a other breakers?

And GroovyGuy...no, on these transformers there is no resistance in the ground to earth. This is not an industrial plant, more like heavy commercial. Warehouse type lighting uses 277V through lighting panels directly off these transformers. I was just giving an example on the 4160V motor, that's another substation. And yes the arc flash is considered - and is an issue. This is an existing installation. These were put in long before arc flash became the main topic of concern.

 

[waross]

Phase to ground may be a higher impedance than phase to phase.
An arcing ground fault may not draw enough current to trip the supply breaker.
For circuits protected at or over 2000 Amps at 150 Volts to ground or less and,
For circuits protected at or over 1000 Amps at over 150 Volts to ground;
Ground fault detection is a code requirement in Canada.

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