Help With Ground Fault Feeder Protection

[Mike01]

On systems the have a unit substation with a medium voltage primary and secondary voltages @ 480Y/277V-3PH-3W secondaries the main GFI protection they would utilize the N for the GFI correct? If GFI protection was also required on the feeders but all the feeders were 480V only utilizing step down transformers at the location of panelboards how would they provide GFI on the secondary breakers? How would this operate if there is no neutral to sum it to? Would it act like a delta and not operate on the first line to ground fault but trip on the second? Or would I have to provide a neutral to make the GFI operate?

 

[brian john]

If it is a grounded system you do not need a neutral to operate a GFP system, you could use zero sequence, residual or ground return method.

This could be inherent in the CBs or added as long as the CBs or bolted pressure switches had a UV or Shunt trip.

 

[jim dungar]

On systems the have a unit substation with a medium voltage primary and secondary voltages @ 480Y/277V-3PH-3W secondaries the main GFI protection they would utilize the N for the GFI correct? If GFI protection was also required on the feeders but all the feeders were 480V only utilizing step down transformers at the location of panelboards how would they provide GFI on the secondary breakers? How would this operate if there is no neutral to sum it to? Would it act like a delta and not operate on the first line to ground fault but trip on the second? Or would I have to provide a neutral to make the GFI operate?

The NEC requirement is 277V to ground, 1000A or greater. Everything else is a design consideration

Most GF protection sums the currents of each normal current carrying conductor (zero sequence). Even if the circuit does not use a neutral this method still applies.

 

[Mike01]

Thanks for the responses this happens to be a health care facility where an additional layer of ground fault is required, and I was not sure how it would be achieved it sounds like the breakers need to be equipped gfi that uses ?zero sequence? where could I find additional information about zero sequence and how it works? Thanks again.

 

[zog]

On systems the have a unit substation with a medium voltage primary and secondary voltages @ 480Y/277V-3PH-3W secondaries the main GFI protection they would utilize the N for the GFI correct? If GFI protection was also required on the feeders but all the feeders were 480V only utilizing step down transformers at the location of panelboards how would they provide GFI on the secondary breakers? How would this operate if there is no neutral to sum it to? Would it act like a delta and not operate on the first line to ground fault but trip on the second? Or would I have to provide a neutral to make the GFI operate?

I am a little confused about how your system is set up, is your 480V system grounded or ungrounded? Delta or Wye? What is the breaker type that you are asking about and what type of trip units do you have? It sounds like you already have everything you need equipment wise but have questions on how it operates, is that right? Some trip units compare currents on all 3 phases to determine if a ground fault is present, I can give you some better answers when you provide some more details.

 

[jim dungar]

Thanks for the responses this happens to be a health care facility where an additional layer of ground fault is required, and I was not sure how it would be achieved it sounds like the breakers need to be equipped gfi that uses ?zero sequence? where could I find additional information about zero sequence and how it works? Thanks again.

Zero sequence is the most common method for GF. It mathematically sums all of the circuit currents, either by having all conductors pass through a single sensor, or by "combining" multiple sensor outputs into a single value.

3-phase electronic protective devices usually perform an internal summation of the individual phase currents, but have an option to connect an external neutral sensor for when a neutral conductor is present.

 

[Mike01]

Description

Description

A single end unit substation (1500Kva ∆-Y) however a lager amount of the loads are 3?-3W loads either distribution boards form motor loads or for xfmr. Primary feeders that serve normal / critical / life safety panelboards so there would be no neutrals installed in the feeders they would be 3W+grd. To the distribution board or xfmr. Just curious as to how the c.b sum?s the current for ground fault tripping, looks like molded case circuit breakers with solid state electronic trip units.

 

[zog]

A single end unit substation (1500Kva ∆-Y) however a lager amount of the loads are 3?-3W loads either distribution boards form motor loads or for xfmr. Primary feeders that serve normal / critical / life safety panelboards so there would be no neutrals installed in the feeders they would be 3W+grd. To the distribution board or xfmr. Just curious as to how the c.b sum?s the current for ground fault tripping, looks like molded case circuit breakers with solid state electronic trip units.

Cant tell you more without specific breaker types and trip unt types.

 

[jimday]

Since it is for a hospital, it needs to comply with 517.17(A-D). Pay particular attention to 517.17(B) for feeders....

 

[anbm]

On systems the have a unit substation with a medium voltage primary and secondary voltages @ 480Y/277V-3PH-3W secondaries the main GFI protection they would utilize the N for the GFI correct? If GFI protection was also required on the feeders but all the feeders were 480V only utilizing step down transformers at the location of panelboards how would they provide GFI on the secondary breakers? How would this operate if there is no neutral to sum it to? Would it act like a delta and not operate on the first line to ground fault but trip on the second? Or would I have to provide a neutral to make the GFI operate?

NEC 517- two level of ground fault protections

1. If you main switchoard (480V) has a main CB and its size equal or more than 1,000A. Then just speficy this CB + first level downstreamm breakers to be electronic breaker with ground fault feature.

2. If your main SWBD does not have main CB and if any of (6) main CBs is larger than 1,000A, then do similar as #1 above with those CB >= 1000A CB.


3. If one of the (6) Mains is more than 1,000A and feeds a transformer, then main CB of panel downstream of XFMR need to be ground fault type, I guess.

4. If you specify your swbd with fuses vs. CBs, then adding a ground fault relay to your fuse then. I never tried this option before but I heard it can be done.

5. If you main CB or (6) main CBs are less than 1,000A, the rule can be omited.