Are circuit breakers rated to open more than one short circuit?

[natfuelbill]

Are molded case circuit breakers rated to open more than one short circuit?
Assume the breaker is properly specified, installed and maintained.

What code or standard defines this ability (or lack of)?

 

[natfuelbill]

Square D factory response:

"A circuit breaker should be able to safely clear two short circuits at its maximum rating. It may need replacement after the second short circuit."

Do you agree?

 

[jim dungar]

Are molded case circuit breakers rated to open more than one short circuit?
Assume the breaker is properly specified, installed and maintained.

What code or standard defines this ability (or lack of)?

It depends on the amount of short circuit current.

UL and NEMA standards require the circuit breaker to open a rated maximum short circuit - be able to be closed on to the short circuit - and then open the short circuit again (this is often called a O-C-O test). The breaker is not required to be tested for a third opening.

 

[infinity]

Would this also apply to GFCI breakers? I've seen temp panels on big jobs where the CB's tripped several times a day and seemingly worked fine.

 

[480sparky]

Huge difference between guys plugging in too many things and a short circuit. And a GFI breaker may not be opening due to either, it may be a bona-fide ground fault.

 

[jim dungar]

Huge difference between guys plugging in too many things and a short circuit. And a GFI breaker may not be opening due to either, it may be a bona-fide ground fault.

I should have been clearer, the UL standard is concerned with maximum bolted fault current not "all" shorts.

 

[templdl]

I haven't done this in a while but this is supposed to link to typical UL489 molded case circuit breaker tests that I have on file.
These breaker go through more tests than you can imagine and must survive them and still work.

 

[natfuelbill]

The real question is,

Should a breaker be replaced after it has succesfully opened two short circuits if the current was at the maximum breaker short circuit rating?

 

[brian john]

Depends on the magitude of the fault as noted, the CB can be tested, Micro-ohmed/ductored, meggered and on large frame CB visually inspected.

 

[natfuelbill]

For "commodidty" type of breakers like Square D QO, FA etc. what is most common in the workplace regarding replacing it, or not??

If a breaker has successfully O_C_O on maximum short circuit condition then is it simply unknown how the breaker will react?

If a breaker O_C_O on less than maximum short circuit events, then how many times before the breaker becomes in question? Is there a database that defines breaker operations for less than maximum? It might show something like

500 times for 25% short circuit values
400 times for 40% short circuit values
100 times for 80% short circuit vslues
15 times for 95% short circuit values
2 times for 100% short circuit values?

 

[davidr43229]

Should a breaker be replaced after it has succesfully opened two short circuits if the current was at the maximum breaker short circuit rating

IEEE Std 1015-1997 - Blue Book
3.35 circuit breaker useful Life - It is prudent to replace any MCCB that has interrupted , at most, two faults at rated maximum current
IEEE Std 902-1998 - Yellow Book
Protective Device Testing
6.3.2 Low-Voltage Circuit Breakers
Covers Power Circuit Breakers, Molded Case Circuit Breakers and Insulated Case Circuit Breakers
Power circuit breaker have mechanical adjustments and inspections that should be periodically checked.
Tests should confirm devices perform within time-current specifications provided by manufacturer and are calibrated to conform to the coordination study.
NEMA AB 4-1996 For Molded Case Circuit Breakers
Recommended Tests
Visual Inspection for overheating
Mechanical Operation
Insulation Resistance Test
Individual Pole Resistance Test
Inverse Time Overcurrent Trip Test
Instantaneous Overcurrent Trip Test
Rated Hold-In
Just my $.02

 

[rbalex]

There are multiple problems:

• It is often difficlut to determine whether a molded case circuit breaker tripped on simple overload or shortcircuit.
• If it is was a fault, it is usually difficult to determine the fault level.
• As the IEEE Blue Book states: It is prudent to replace any MCCB that has interrupted , at most, two faults at rated maximum current.
• If the NEMA AB-4 recommended tests are performed, the Instantaneous Overcurrent Trip Test IS THE SECOND FAULT.

In my opinion, once a molded case breaker has tripped, unless it is clearly a simple overload, it should be replaced.

Edit :corrected a bit of grammer

 

[LarryFine]

In my opinion, once a molded case breaker has tripped, unless it is clearly a simple overload, it should be replaced.

In that case, we might as well have stayed with fuses.

 

[rbalex]

In that case, we might as well have stayed with fuses.

If you want to move the results from unknown to known - that's correct.

Edit: MAybe I should say lower to higher probability - nothing is absolutely certain of course.

 

[brian john]

it should be replaced

That is why test equipment was made.

On another note I have been trying to locate some pics of CBs that were damaged during the clearing of faults, in one case the electrician closed the CB without testing and sustained 2 degree burns to hands and face.

IMO at a minimum (at least) service electricians should understand proper testing procedures. A megger should be a common as a wiggy or multi meter

 

[jim dungar]

The standards recommend replacing a breaker that has twice interrupted its maximum bolted fault current. These types of faults rarely clear themselves so standard troubleshooting usually determines they existed.

Most short circuits are of the arcing type so rarely approach the maximum current rating of the breaker. These types of faults usually burn themselevs clear so they are harder to identify as the cause of the breaker trip.

Due to the lack of actual available system fault current as well as the impedance of the circuit conductors, very few short circuits are at the maximum rated fault current of properly applied breakers.

Testing breakers makes more sense than blindly tossing out every breaker that has experience two "unexplained" trips.

 

[wptski]

What would be the procedure for testing a CB with a megger(insulation tester) and milliohm meter? What's good?

 

[mdshunk]

IMO at a minimum (at least) service electricians should understand proper testing procedures. A megger should be a common as a wiggy or multi meter

Amen, brother.

As the unwitting recipient of shrapnel from a Square D PAF breaker that was closed during a bolted fault condtion by another in the early 90's, I couldn't agree more.

 

[rbalex]

The problem is there are no standards for returning an MCCB to service that has had less than the maximum fault. After the first fault, the unit is still suspect if it has been fully retested by the NEMA standards. And NEMA knows this.

In the late 80s/early 90s I was a member of the IEEE/IAS/PCIC Task Force that attempted to create a performance standard for MCCBs, similar to IEEE 841 for motors, because large industrial users were finding MCCBs are far less robust than expected.

The Task Force became dominated by NEMA MCCB representatives. (IEEE doesn't require "balance" like NFPA does) Eventually all progress came to a standstill. I finally quit. I can't remember the Project number. I think the Task Force still exists though - it just meets once a year and agrees to do something by the next year.

 

[mdshunk]

TAfter the first fault, the unit is still suspect if it has been fully retested by the NEMA standards. And NEMA knows this.

You said "...if it has been fully retested...". Did you really mean to say "...until it has been fully retested..."?

The reason I ask is because if the unit is still suspect, even though it has been retested, it's basically disposable.... just like a fuse.