LV Ground Fault Coordination

[philly]

Does anyone have any good references discussing the theory for setting ground fault settings for breakers on LV systems.

I have heard some rules of thumbs in the past but have never really seen any good documentation discussing it.

I've usually seen the mains set to 1200A pickups with .5s delay and then any feeder set to 50% of breaker rating with a 0.3s delay. This is one of the rules of thumbs I've seen in the past.

When looking at LV GF settings should you take the downstream breakers "phase" curves into consideration when coordinating?

 

[topgone]

With ground faults, we are using definite time elements and the only coordination we do is grading the trip times taking into consideration the breaker speed and inherent relay delays. The idea is that a ground fault sensed downstrean could also be seen by the protection upstream. The breaker nearest the fault point should trip ahead to localize the interruption.

 

[Tony S]

http://www2.schneider-electric.com/documents/designers/top-downloads/090197c6800a84fd_.pdf

 

[ATSman]

Unless you have a PE in EE as part of your title I would be careful in taking on the liability of providing settings for GF Relays in a distribution system. This is usually the job of a Consulting Engineer who can do a proper power systems/ coordination study by lining up all the breaker characteristic curves, determining the settings and PE stamping the report. Can you imagine the consequences like say in a hospital environment whereby areas far from the fault are tripped offline because of improper coordination causing disruption to critical areas (surgery, life support, etc) and maybe even deaths.
We as testers, in performing startup & commissioning, maintenance testing, are very careful in not making any setting changes to breakers, protective relays, etc unless authorized by the EOR or CE on the project.
Can any of the PE's on this forum please comment on the importance of the work you do and why it should be offlimits to anyone who has not been properly trained in this area.
Thank you.

 

[Bugman1400]

It used to be just the potential for causing a mis-coordination. Nowadays, adjusting pickups and timedial settings has the potential for affecting arc flash PPE class levels. So, if your initial AF study results in a PPE Class of 1 or 0 and you make a change to a protective device that results in a higher PPE Class level, you can potentially cause a safety issue for your co-workers.
The rules of thumb that you stated may be good places to start but, you must finish by performing the coordination with the other upstream and downstream relays and then see how that affects the arc flash levels.