Ground fault took down whole building 3

[MNMaint]

In a building I manage I experienced a situation where a motor failed and tripped the GFI protection on the main switchgear taking down the whole building. I'm not an electrician or an electrical engineer and I want to know if it is common practice to provide GFI protection in a switchgear with no similar protection downstream (except at bathrom receptacles). It seems to me that a ground fault of any sort will take down the service to this whole building.

Am I missing something?

Is there a relatively simple solution to prevent this from happening again?
Thanks

 

[rbulsara]

Typical case of improper (low and sensitive) setting of the GF on the main, lack of coordination or bad design or a combination of the above.

Judging from your use of the term GFI, you appear to be in NEC world. In that case GFP is a code requirement for a main rated 1000A at 480V. (not on 208V).

Between the Code and UL, you can set GF pick up to 1200A and 0.5 seciond of time delay. This is allowed to coordinate with "most" of the downstream breakers.

However, a breaker with GFP will not always coordinate with the downstream breaker without GF. So if selectivity is very important you need to provide second layer of GFP too, that in the feeders, so as to limit the outage to a feeder level and so on.

Have an experienced electrical engineer review your system.





Rafiq Bulsara

http://www.srengineersct.com

 

[Zogzog]

Your situation is fairly common because someone decided to save money by not having GFP on your feeders, so there is nothing to coordinate with. As Rafiq mentioned have an EE evaluate your system and settings. You can always retrofit your feeders with trip devices that use GFP so any GF's will only effect the feeders when properly coordinated.

 

[MNMaint]

Thanks a lot, that was what I had thought but wanted confirmation. The motor in question is protected by time delay fuses (picture attached). Does that change anything?

 

[rbulsara]

Not really, but again someone has to review the system.

Rafiq Bulsara

http://www.srengineersct.com

 

[rcw retired EE]

This is a common occurrence for "cost-effective" installations. Code required ground fault protection is left at minimum settings either to avoid liability or because that is how the unit was shipped. The contractor or owner or architect reduce costs by not having an engineer determine coordinated settings. The end result is a "safe" system that creates operational problems.

I did a lot of testing and commissioning of ground fault systems for commercial and industrial buildings. Our policy was to leave the settings at minimum unless settings were provided by an engineer. Our test reports recommended a coordination study be done to optimize the tradeoff between fault damage and coordinated protection.

But even if your GF unit was set at maximum there may be faults that will trip the building main before the local fuse or circuit breaker trips.