Tripping Main?????

Status
Not open for further replies.

nathan

Member
I am currently working on a highrise apartment buildingwith 273 units. Each unit has a 100 amp 22k A.I.c rated main breaker located in a central electrical rm on each of the floors. The farthest panel is 125ft the closest 25ft. They are all run in #2 aluminum with 208v(3 phase is main service). We are currently trimming and trouble shooting so the load on the feeders is pretty low. All equipment is series rated. Problem is that a 15 or 20 amp circuit phase to ground will bypass the branch circuit breaker that is rated at 10k aic and knock out the main for the unit. I have tried to research this myself and found that it is common for breakers to trip out "together" but only my main does. All I can think it is the fact that there is no load on the building itself...but that gets a little hazy when I try to run it through in my head. Any takers Im all ears. :confused: :confused:
 

ron

Senior Member
Re: Tripping Main?????

Low load will not affect your problem.
Some series rated breaker combinations will operate together to accomodate the higher AIC rating. Does this happen on one panel only, or is thei similar for multiple arrangements of the same breaker combinations.
Unfortunately it is imposible to coordinate the instantaneous region of the trip characteristics.
 

nathan

Member
Re: Tripping Main?????

It happens on all of them. I havent done a whole lot of residential ...mostly commercial and Ive never seen a branch circuit drop out a main so this concerns me. Are you telling me that it is possible that eveytime a tennent grounds something out that they will kill their main...man I hope not. If that is the case do you know of a better way to do this in the future??
 

nathan

Member
Re: Tripping Main?????

Thanks for the reply Ron ...I was hopeing you might explain the two breakers accomidateing the higher AIC...is that the 800 amp breaker ahead of the main...also the branch circuit (15 or 20) never trips just the 100amp main
 

bennie

Esteemed Member
Re: Tripping Main?????

The 100 amp has a faster instant trip, than the 20 amp breaker.

Higher AIC ratings equal faster opening.

Both are looking at the same fault current.

Breakers above 150 amp have adjustable instant trip settings.
 

gwz2

Senior Member
Location
Indiana
Re: Tripping Main?????

Not addressing your 22Kair tripping and not the 10Kair , which does happen sometimes and the BEAST of a series system, I am surprised that the 22Kair is not protected by a larger Kair OCPD upstream.

273 units would seem to require a substantial transformer with a high Available Fault current.
 

bennie

Esteemed Member
Re: Tripping Main?????

This appears to be about 24 floors. (wild guess)

There is probably a buss duct system supplying primary power to each floor. Each floor may have a 150 KVA transformer.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Re: Tripping Main?????

KAIR has nothing to do with speed of operation, coordination or tripping time. KAIR is simply the ability of the OCPD to not fail too violently under a specific maximum fault.

Series ratings are only a concern in high fault levels where the OCPDs both operate (typically over 1000A). KAIR again has no affect on the speed at which the individual series rated devices open.

Based on the size of your building, and even though your system is 208V, it is possible that your main breaker was supplied with Ground Fault Protection (GFP)?
 
Status
Not open for further replies.
Top