modified differential ground fault detection system

[jcole]

Good day guys and gals.

The 480V switchgear (located in a substation) we have that supplies power to our MCCs in our buildings is fed from 2 sources and has a tie that can connect each side if need be. There is also a generator that can take the place of one of the transformers in case we loose power by the switching of an ATS. There is a MDGF detection system installed in case of ground faults. I am not familiar with these systems. I am told they are important in designs like the one we have because of the multiple sources we have. Our Main Breakers (800A) that are fed from these two sources have ground fault detection on them. Why wouldn't the ground fault detection on the breakers be sufficient for tripping of breakers in case of ground faults? Why is the MDGF detection system even needed? How do these MDGF systems work different than the ground fault detection on the breaker?

Thanks for the replies.

 

[jim dungar]

Good day guys and gals.

The 480V switchgear (located in a substation) we have that supplies power to our MCCs in our buildings is fed from 2 sources and has a tie that can connect each side if need be. There is also a generator that can take the place of one of the transformers in case we loose power by the switching of an ATS. There is a MDGF detection system installed in case of ground faults. I am not familiar with these systems. I am told they are important in designs like the one we have because of the multiple sources we have. Our Main Breakers (800A) that are fed from these two sources have ground fault detection on them. Why wouldn't the ground fault detection on the breakers be sufficient for tripping of breakers in case of ground faults? Why is the MDGF detection system even needed? How do these MDGF systems work different than the ground fault detection on the breaker?

Thanks for the replies.

When you have multiple sources, with their own neutral-ground bonding, you have the potential for current to flow over different ground paths and yet not be seen as a GF condition. There is also the possibility of normal current being interpreted as a fault resulting in unnecessary tripping, particularly when GF protection is included in the Tie breaker.

It is important to look all of the grounding and neutral paths on your one-line drawing.

For example:
Fault current on a panel fed from source A could flow over several parallel ground paths until it finds the neutral to ground bond at source B. The source A GF device may not see any problem because current is returning via the neutral as expected.

 

[jcole]

Thanks for the reply dungar.

Is the MDGF system required by code for a dual supply and tie system like we have? Or are the basic requirements for grounding and bonding of systems the only requirement by code? Does code specify anything about this type of installation?

Thanks again.

 

[jim dungar]

Thanks for the reply dungar.

Is the MDGF system required by code for a dual supply and tie system like we have? Or are the basic requirements for grounding and bonding of systems the only requirement by code? Does code specify anything about this type of installation?

Thanks again.

An MDGF is not required by code, it is a design choice.

Code does require you to install a proper GF system. In the 2014 edition of the NEC, section 230.95(C) requires you to performance test the system, not just the individual components. Informational Note 3 provides a warning that a simple GF system may not work when multiple sources are interconnected.