Relationship between System Earthing & circuit protection

[mithko]

Is there any particular relationship between the type of system earthing and the protection of the circuit?

What are the typical examples.

Regards,
Mithko

 

[rbulsara]

Yes, without the earthing, there will be no return path for an earth fault to compete the circuit and no overcurrent device would operate when a line faults to earth.

Draw a simple circuit yourself and study.

Rafiq Bulsara

http://www.srengineersct.com

 

[PHovnanian]

What Rafiq said.

In addition, different earthing schemes (resistance, zig-zag transformer, etc.) will affect the magnitude of the ground fault current. This must be taken into account when designing protection systems and selecting settings.

 

[wcarter]

The only disagreement I have is assuming you have a wye-delta transformer supplying a three phase load, you can place a zero sequence CT on the load side of a breaker, BP/bolt-loc switch, etc with a ground fault relay. The phase-to-ground fault current will return via the ground connection on the wye side of the transformer causing a unbalance in the CT. This will cause a trip when the set-point is reached.

 

[rbulsara]

wcareter:
With all due respect, your above statement is incorrect. In two winding transformers, secondary current will not return to primary winding. The current will only "return" to its source winding. Ungrounded delta has no return ground path. (Not withstanding capacitive coupling for large systems or systems with very long cables. That is a separate discussion).

ZCT is not for detecting ground (or earth) faults in ungrounded delta systems. There are other means of detecting first GF in ungrounded systems.

You may want red flag your post and post a revised one if you wish.

Rafiq Bulsara

http://www.srengineersct.com

 

[wcarter]

rbulsara,

You state this is incorrect, but the priciple is applied currently by multiple manufacturers including Siemans and Schneider Electric, with three phase, three wire systems. And I assure you, they do work. If you wish, I can find one of their drawings for this system utilizing a zero-sequence CT. Though it might be early next week when I can post a file of it (I do not have it with me here in the field).

 

[wcarter]

I noticed I missed a caviat, that made you correct rbulsara. Note to self, don't attempt to think without your coffee in teh morning! The zero sequence is for three phase, four wire systems where they are supplying a three phase, three wire load. The ungrounded delta requiring a hRg, lRg, or coupled capacitance monitor.