Danger of resetting a 400 amp 480 volt breaker with a dead short

[tom baker]

A fellow electrician had a service call for no power. Found the 400 Amp 480V breaker at the service (ten feet from the 175 KVA transformer) tripped.
He reset it.
Shortly there after it tripped again.
Another electrician megged the conductors, a dead short. A high pot was then used, dead short. Fault was determined later to be an electrician on a nearby job had cut partially thru the 4" feeder conduit that the 400 A breaker fed and nicked the wire, before it was energized.
The fault blew a hole in the UG PVC
Question: What hazard was there to the electrician who reset the tripped 400 A CB?
Recommended Policy/Procedures?

 

[iaov]

Exploding breaker, arc flash, badly messed pants!!

 

[Karl H]

Closing a breaker in a fault is what You Tube videos and lawsuits are made of.
Not a wise practice.

 

[Mule]

ALWAYS, diagnose as to why a breaker of that size tripped, before reseting.......ALWAYS!!!!!

 

[charlietuna]

I know of one instance that a chief engineer for a large office building found a 400 amp - 480 volt breakers tripped! This breaker fed and elevator machine room and was located in the main switchgear -- the molded case breaker was mounted inside the gear with a mechanical operating linkage. The engineer closed the breaker via the assembly linkage and the breaker tripped again, he reset the breaker and reclosed it again -- the breaker actually exploded causing the heavy cover plate to blow off the switchgear-shearing three closure screws and two heavy hinges !!! This is about 30 inches high and 12 inches wide! The original problem was a cable failure due to a loose connection on the line side of a disconnect switch causing the breaker to trip, the last trip caused a phase to phase fault on the load side of the 400 amp breaker. The cover plate broke the engineer's wrist, luckily he was standing off to the side of the bucket.

 

[mxslick]

Try this VIDEO on for size..looks like about the size of breaker in question.

And I second what Mule posted:

ALWAYS, diagnose as to why a breaker of that size tripped, before reseting.......ALWAYS!!!!!

 

[brother]

so do you all recommend wearing an arc suit just to reset a breaker?? or meggering the the wires on these big amp breakers?

I know if i was at a job I wouldve done the same thing, tried to reset the breaker. If it tripped then i wouldve done meggering.

 

[Mule]

so do you all recommend wearing an arc suit just to reset a breaker?? or meggering the the wires on these big amp breakers?

I know if i was at a job I wouldve done the same thing, tried to reset the breaker. If it tripped then i wouldve done meggering.

How long does that take, five minutes? If the breaker tripped there was a reason. Your playing with alot of calories.....

 

[mxslick]

You're kidding, right?

You're kidding, right?

so do you all recommend wearing an arc suit just to reset a breaker?? or meggering the the wires on these big amp breakers?

I know if i was at a job I wouldve done the same thing, tried to reset the breaker. If it tripped then i wouldve done meggering.

Are your questions in jest? I hope so. Did you watch that video? Yeah you may think it's wasteful to take the time for PPE or to actually megger before resetting, but it's better than time in a burn ward, or morgue.

I have personally seen too much carnage in my time, both working with EC's and in my current line of work, to even consider resetting a breaker of that class without first trying to find out why it tripped, or at the least wearing full PPE.

Some years back I was at a cinema doing some sound system tuneups when their 480 to 208/120 dry transformer went into meltdown mode. The manager wanted ME to go shut off the breaker feeding it.. 600amp, 480v, I told him to call 911 and get the fire dept. rolling, get the building emptied and then call their electrician. As we were finally leaving the booth the breaker did trip, with a bang so loud it made us all jump.

After the FD left their electrician found out that the breaker had suffered extensive damage, with evidence of flash on the deadfront. When I told that EC what the manager wanted me to do, he read that manager the riot act.

Don't take unnecessary chances!!

 

[brother]

Are your questions in jest? I hope so. Did you watch that video? Yeah you may think it's wasteful to take the time for PPE or to actually megger before resetting, but it's better than time in a burn ward, or morgue.

I have personally seen too much carnage in my time, both working with EC's and in my current line of work, to even consider resetting a breaker of that class without first trying to find out why it tripped, or at the least wearing full PPE.

Some years back I was at a cinema doing some sound system tuneups when their 480 to 208/120 dry transformer went into meltdown mode. The manager wanted ME to go shut off the breaker feeding it.. 600amp, 480v, I told him to call 911 and get the fire dept. rolling, get the building emptied and then call their electrician. As we were finally leaving the booth the breaker did trip, with a bang so loud it made us all jump.

After the FD left their electrician found out that the breaker had suffered extensive damage, with evidence of flash on the deadfront. When I told that EC what the manager wanted me to do, he read that manager the riot act.

Don't take unnecessary chances!!

I was NOT asking in jest, I was just getting some clarity. I have been on other jobs where i see other electricians resetting (high amperage) breakers that have malfunctioned. Fortunately, there wasnt a 'bang' and no one got hurt.

The most recent incident was several months ago, there was this 800 amp 'power breaker'( the one you have to charge with handle and then close) that when it was opened to kill power to the disco we were working on, would NOT reclose at first. Most of this was because there havent been proper pm's done on this power breaker. I stood back and watch my co worker, (without ppe) take a few cranks at it to get charge so it can be closed.

I was going to leave it in that condition until the next day so the boss can get someone in to repair it, but he decided to go head and see if he can get to close, and it did. Fortunately no 'bangs' or booms.

Just trying to spread safety awareness and increase/better my own safety. I think the springs in these power breakers had jammed or whatever.

By the way, what is the 'RIOT ACT'.

 

[Mule]

A fellow electrician had a service call for no power. Found the 400 Amp 480V breaker at the service (ten feet from the 175 KVA transformer) tripped.
He reset it.
Shortly there after it tripped again.
Another electrician megged the conductors, a dead short. A high pot was then used, dead short. Fault was determined later to be an electrician on a nearby job had cut partially thru the 4" feeder conduit that the 400 A breaker fed and nicked the wire, before it was energized.
The fault blew a hole in the UG PVC
Question: What hazard was there to the electrician who reset the tripped 400 A CB?
Recommended Policy/Procedures?

IMO if he had took the time to meg the feeder before the reset, he would have seen the fault and in good conscious would have refused to reset it. I know I would not......

Putting PPE on and reseting a fault (closing into a fault), is like jumping off a cliff, with knee pads and a helmet....its still go'na hurt....IMO

 

[mxslick]

I was NOT asking in jest, I was just getting some clarity. I have been on other jobs where i see other electricians resetting (high amperage) breakers that have malfunctioned. Fortunately, there wasnt a 'bang' and no one got hurt.

The most recent incident was several months ago, there was this 800 amp 'power breaker'( the one you have to charge with handle and then close) that when it was opened to kill power to the disco we were working on, would NOT reclose at first. Most of this was because there havent been proper pm's done on this power breaker. I stood back and watch my co worker, (without ppe) take a few cranks at it to get charge so it can be closed.

I was going to leave it in that condition until the next day so the boss can get someone in to repair it, but he decided to go head and see if he can get to close, and it did. Fortunately no 'bangs' or booms.

Just trying to spread safety awareness and increase/better my own safety. I think the springs in these power breakers had jammed or whatever.

By the way, what is the 'RIOT ACT'.

No problem, I just had to do a double take when you asked.

Those other guys you witnessed have been very lucky, it's only a matter of time before their luck runs out though. I do bristle when I hear of people being so careless around electricity.

One thing everyone needs to consider: In MANY cases, as the POCO's add capacity to the grid to accomodate increased usage, the available short circuit current is far above what the original switchgear designers built for....so thus even the simple resetting of a breaker into a shorted load may exceed it's rated interrupting ability.

The "riot act" is a euphemism for getting one's rear end chewed for doing something stupid.

 

[brian john]

IMO if he had took the time to meg the feeder before the reset,

Megger the feeder and CIRCUIT BREAKER, Line to load across all poles with the CB open., line to ground, load to ground, pole to pole.

 

[don_resqcapt19]

And note that there are OSHA rules that prohibit the resetting the breaker after a trip unless you have checked the circuit first.

 

[rbalex]

Since Don already mentioned OSHA, I'll add NEMA AB-4 and IEEE 1015. Both are ANSI documents. The NEMA document is available as a free download at their site. Unfortunately, the IEEE document is only available by purchase (or descrete quotes like that below). It is one of the "Color" series - the IEEE Recommended Practice for Applying Low-Voltage Circuit Breakers Used in Industrial and Commercial Power Systems (Blue Book).

In the NEMA AB-4 Introduction, third paragraph, it mentions the rigorous UL testing the circuit breakers go through. Still, it ends with the statement: “Thus circuit breakers have an extensive but finite interrupting capability, and breakers that experience multiple high short-circuit-current faults should receive a thorough inspection with replacement if necessary.

The IEEE document considers two (2) “high short-circuit-current faults” sufficient to warrant replacement:

3.35 Circuit breaker useful life

It is prudent to replace any MCCB that has interrupted at most, two faults at rated maximum current. This is so because the MCCB short-circuit proof test consists of an “O-t-CO” sequence, which means that in proof testing of the circuit-breaker design and in periodic follow-up testing thereafter, the circuit breaker is required to open a fault from an initially closed position (corresponding to the “O’ operation), then after a period of time (t) to reset is allowed, to be closed into a maximum fault and trip open for a second time (corresponding to the “CO” operation). This demonstrates a circuit breaker’s ability to perform at least two maximum level fault interruptions with the second at least a little worse than the first. No maintenance of the circuit breaker on test is permitted between interruptions.

The trips your colleague experienced were probably not rated maximum level faults as indicated by the time delay in tripping. The problem is that he no longer has any idea what service life is left or that it will safely interrupt another fault - and neither does NEMA nor UL.

 

[zog]

And note that there are OSHA rules that prohibit the resetting the breaker after a trip unless you have checked the circuit first.

More specifically 1910.334(b)(2) "Reclosing circuits after protective device operation." After a circuit is deenergized by a circuit protective device, the circuit protective device, the circuit may not be manually reenergized until it has been determined that the equipment and circuit can be safely energized. The repetitive manual reclosing of circuit breakers or reenergizing circuits through replaced fuses is prohibited.

Note: When it can be determined from the design of the circuit and the overcurrent devices involved that the automatic operation of a device was caused by an overload rather than a fault condition, no examination of the circuit or connected equipment is needed before the circuit is reenergized.

 

[charlietuna]

Lets consider a 2000 amp main installed in a high rise office building. The breakers feeds the upper half of the building via bus duct. The breaker has ground fault protection. It's 9:00am on Monday. Whats your call?????

 

[zog]

The IEEE document considers two (2) “high short-circuit-current faults” sufficient to warrant replacement:

To add to that (Excellent post) the manufactures operation and maintenance manual states the number of normal operations and the number of fault interuptions the breaker is rated for and if exceeded the manufacture recommends the breaker is refurbished.

The trips your colleague experienced were probably not rated maximum level faults as indicated by the time delay in tripping. The problem is that he no longer has any idea what service life is left or that it will safely interrupt another fault - and neither does NEMA nor UL.

Exactly, but proper testing can determine the condition of the breaker, which can help determine if the breaker is still fit for service.

 

[augie47]

Lets consider a 2000 amp main installed in a high rise office building. The breakers feeds the upper half of the building via bus duct. The breaker has ground fault protection. It's 9:00am on Monday. Whats your call?????

at age 65 with my experience (and attitude).... take the time to check...
when I was younger (and faced those situations)...a long stick

 

[zog]

Lets consider a 2000 amp main installed in a high rise office building. The breakers feeds the upper half of the building via bus duct. The breaker has ground fault protection. It's 9:00am on Monday. Whats your call?????

What caused the trip?