Intentionally creating branch circuit fault

[K8MHZ]

I have seen 15 amp breakers that did not trip at 70 amps. Things can get pretty warm at that level. Specialy when holding a couple #14s in your #$@ little hands.

I saw a 15 amp CH breaker that did not trip at 115.3 amps. Not volts, amps. We did a service upgrade and when I flicked on that breaker all the lights in the basement went real dim. I shut it off and thought there may be a bad connection somewhere. I figured that I would see a very low current reading due to a higher resistance. Not so. Instead I read 115.3 amps. There was a bolted short (hot and neutral wire nutted together by whoever removed an outside light) on the other side of the house.

I know an inspector that has a knife that the end (about 1/2 inch) was melted off when he shorted out a couple wires 15 feet from a 15 amp CH breaker. The breaker never tripped. (That was before he was an inspector).

 

[don_resqcapt19]

I saw a 15 amp CH breaker that did not trip at 115.3 amps. Not volts, amps. We did a service upgrade and when I flicked on that breaker all the lights in the basement went real dim. I shut it off and thought there may be a bad connection somewhere. I figured that I would see a very low current reading due to a higher resistance. Not so. Instead I read 115.3 amps. There was a bolted short (hot and neutral wire nutted together by whoever removed an outside light) on the other side of the house.

How long did you leave the breaker on? The 115 amps may be below the instantaneous trip point for that breaker.

I know an inspector that has a knife that the end (about 1/2 inch) was melted off when he shorted out a couple wires 15 feet from a 15 amp CH breaker. The breaker never tripped. (That was before he was an inspector).

That would not really be a bolted fault and you have no way of knowing how much current was flowing. The fact that you melted metal does not always mean you had excessive current.

There is no real way to know in the field if these breakers were operating within their designed trip range or if they were defective. It could be either.

 

[templdl]

How long did you leave the breaker on? The 115 amps may be below the instantaneous trip point for that breaker.


That would not really be a bolted fault and you have no way of knowing how much current was flowing. The fact that you melted metal does not always mean you had excessive current.

There is no real way to know in the field if these breakers were operating within their designed trip range or if they were defective. It could be either.

NEMA AB4 is an excellent reference: Guidelines for Inspection and Preventive Maintenance of Molded-Case Circuit Breakers Used in Commercial and Industrial Applications
It includes a field testing procedure for MCCBs. It is not intended to be a calibration test but just assures that the MCB operates within an acceptable parameter.
I have an older copy but it's missing in action having filed somewhere where I wouldn't loose it.
But here's the on line link I found to the PDF:
http://www.nema.org/download.cfm?docId=4431&filename=/AB-4.pdf

NEMA AB# in another good publication: Molded-Case Circuit Breakers and Their Application PDF:
http://www.nema.org/download.cfm?docId=4430&filename=/AB3.pdf

 

[K8MHZ]

How long did you leave the breaker on? The 115 amps may be below the instantaneous trip point for that breaker.

I would say 3 or 4 seconds. Long enough to take a reading.

 

[templdl]

The NEMA AB4 procedure for field testing a 15at MCCB (up to 30at) is to apply 300% of the breaker rating (45a) in a 25degC environment using the test setup and described the beaker should trip in no more that 50sec. Doing an instantaneous (magnetic) field test is very likely going to damage the breaker and therefore not recommended. (31-50AT 80sec, 51-100AT 140sec, 101-150AT 140sec etc.)
NEMA AB4 is only a 30 page document which is very informative.
But then again it's a lot more fun to go off on you own using you own theoretical test standards as to how a breaker should respond to a field test with no real test standard to reference.

 

[mivey]

I never used a switch but have intentionally grounded 15-20 amp 120 volt circuits before to cause a trip. :ashamed1: I am wiser now and have a circuit tracer. :thumbsup:

 

[acrwc10]

If you have a Higher than normal resistance on the ground/grounded path (bad connection, what ever) you may have a fire before a tripped breaker.
I tripped a 200amp main once trying to "short out" a 20a circuit breaker. The Sq D main had a faster instantainious setting than the Bryant branch circuit breaker downstream. Opps.

 

[ptonsparky]

My help accidently connected two phase wires together at a motor. 15 amp 480v bus plug. Shut down the whole SW section of a factory when it tripped the 200 amp feeder first. Sure got quiet in there.

 

[Jraef]

Not that I would have thought about it back when, but it doesn't quite sound like the way to check for a GF. Do we have pictures other than the one burned into your memory? Ground Bar or Earth?

I was told to build it, I didn't actually have to do it because the Sr. EE stood up to the boss.

 

[don_resqcapt19]

The NEMA AB4 procedure for field testing a 15at MCCB (up to 30at) is to apply 300% of the breaker rating (45a) in a 25degC environment using the test setup and described the beaker should trip in no more that 50sec. Doing an instantaneous (magnetic) field test is very likely going to damage the breaker and therefore not recommended. (31-50AT 80sec, 51-100AT 140sec, 101-150AT 140sec etc.)
NEMA AB4 is only a 30 page document which is very informative.
But then again it's a lot more fun to go off on you own using you own theoretical test standards as to how a breaker should respond to a field test with no real test standard to reference.

We are talking about breakers of the type used in dwelling units. I can't imagine anyone doing any of that testing for a breaker that has a cost of less than $10.

 

[templdl]

We are talking about breakers of the type used in dwelling units. I can't imagine anyone doing any of that testing for a breaker that has a cost of less than $10.

I agree Don that it is impractical on small breakers. But there are obviously a number of those on this forum that do these things for some reason, they want to "test" them rather than purchase a replacement for less than $10, (maybe $6) because of their curiosity. It is apparent by what they are trying to do with their home grown testing already. I personally like to see them do it if it is done in a safe manor as there could be much learned in the process getting a much better understanding of the products that they are installing.
But, never the less NEMA does have a field testing procedure for them.
If they are going to do it then NEMA AB4 is the procedure that should be used if one wanted to get some reasonable results.

 

[jeremysterling]

What are the dangers to the electrical system from this practice?

Thanks

Chris, you have asked an excellent question and I am surprised at the responses thus far.

Hazards exists for personnel using this method of breaker "testing" but you did not ask that.

As far as the electrical system:

I can surmise the possibility of arcing (and therefore pitting) at the breaker contacts when it opens.

If the single pole switch is cord and plug connected, some damage could occur to the receptacle's internal jaws as the fault plays out. The jaws will see intense heat or arcing and the jaw spring will lose its temper and strength.

Have you ever heard THHN rattle and shake in EMT when the conductors are heavily loaded from either a motor start or short circuit? Imagine the romex straining against its staples as huge Electromagnetic fields pull it back and forth. Insulation stress could occur as the romex attempts to yank its staples out.

I hope this describes the damage technically enough so that electrician's will go to Home Depot and buy a $40.00 "Circuit Breaker Finder" from Ideal. I bought this tool a couple years ago and use it at least once a week. The tracer is worth every penny.

 

[templdl]

Chris, you have asked an excellent question and I am surprised at the responses thus far.

Hazards exists for personnel using this method of breaker "testing" but you did not ask that.

As far as the electrical system:

I can surmise the possibility of arcing (and therefore pitting) at the breaker contacts when it opens.

I hope this describes the damage technically enough so that electrician's will go to Home Depot and buy a $40.00 "Circuit Breaker Finder" from Ideal. I bought this tool a couple years ago and use it at least once a week. The tracer is worth every penny.

Not to worry about the breaker as they do go through some very rigorous test per UL 489, must worse than they will see in a residential setting.
It's that switch that I would be concerned about which would be closing into a dead fault which they aren't designed for. The circuit tracer as you suggested is the way to go.

 

[powerplay]

I was told from a supplier that breakers aren't really meant to trip more than once under warranty. I suppose the contacts that open under load incur some minor damage from arcing, and the in line breaker would eventually not function properly? Why not get a circuit tracker? I connect and tape over wires to an receptacle that I splice into a circuit, then plug in the two bladed tracker that sends the signal to the panel that gets identified by the "receiver" wand.... then you can see before turning the circuit off what might be on the circuit.

 

[templdl]

I was told from a supplier that breakers aren't really meant to trip more than once under warranty. I suppose the contacts that open under load incur some minor damage from arcing, and the in line breaker would eventually not function properly? Why not get a circuit tracker? I connect and tape over wires to an receptacle that I splice into a circuit, then plug in the two bladed tracker that sends the signal to the panel that gets identified by the "receiver" wand.... then you can see before turning the circuit off what might be on the circuit.

Typical supplier doesn't know squat.
This is UL489 and the testing that is required.

 

[rcwilson]

Besides the safety issues in creating a short circuit in your hands, I would worry about the possibility of a fire due to a breaker not clearing fast enough and some part of the circuit overheating. Compare a fire caused by a "normal" accidental short with one caused by someone shorting a circuit intentionally. Which one do you think the lawyers are going to get rich on?

A Bell Telephone (AT&T) Central Office near Wall Street in Manhattan was partially gutted by a fire about 1968-70. Losses were in the millions with phone service interrupted to many brokers and lawyers. Cause was traced to a 20 or 30 Amp breaker that did not trip to clear a fault. The main breaker may have been a bad also. That incident led to a Bell System maintenance practice of periodic full current injection testing of all breakers over 100 Amps at all US facilities. I was told some managers even wanted to test as small as 30 Amp. That policy was some good money for testing companies for many years.

Moral of the story is it could happen. A 120V short could lead to a fire. Why tempt fate?

 

[templdl]

Besides the safety issues in creating a short circuit in your hands, I would worry about the possibility of a fire due to a breaker not clearing fast enough and some part of the circuit overheating. Compare a fire caused by a "normal" accidental short with one caused by someone shorting a circuit intentionally. Which one do you think the lawyers are going to get rich on?
Moral of the story is it could happen. A 120V short could lead to a fire. Why tempt fate?

Great advice.
As I said previously you NEVER rely on either a fuse or breaker should you intentionally overload or cause a fault to occur.
One thing that I have to say in defense of breakers and for that matter fuses is that they don't have eyes to see what is occurring down stream from them which can result in a fire. All that they will respond to is current Vs time of the current that is going through them. It is not that uncommon to have an arcing fault that will never draw enough current to trip a breaker of blow a fuse because it doesn't fall within the trip curve. Even a bolted fault could be dynamic enough to have the same results. That is one good reason for both GFCI and AFCI devices which can detect a L-G fault and the AFCI also has the ability to detect arcing keeping in ming that are those who don't belieive AFCIs really work.
I've seen a duplex outlet kicked in by a student that started to arc and the breaker didn't trip. It turned out that the breaker was perfectly good but didn't have the eyes to see the fault like we do. If the current doesn?t fall within the trip curve it isn't going to trip.

 

[kwired]

When I was in college our class took a tour of Square D plant where they make QO circuit breakers.

I don't know details of any tests being performed, but do remember a demonstration for us in the testing lab. They had a three pole breaker (standard 10kAic) I believe and a supply capable of delivering 10kA. They tied all three load side terminals together and shut a big heavy door enclosing everything. Operator started the test current - made a pretty loud boom and there was some smoke rolling out of even the heavy enclosure the tested equipment was inside of.

I remember they opened it up and that breaker looked like you would never want to use it again - but it did reset and was checked for proper operation and passed the test. Of course they only test so many and do not sell the ones that were subjected to this testing.

Would be even more interesting to see that again and ask more questions about things that I have a better understanding of at this point in time. That plant tour was nearly 25 years ago and I was only 19 or 20 at the time.

 

[templdl]

Would be even more interesting to see that again and ask more questions about things that I have a better understanding of at this point in time. That plant tour was nearly 25 years ago and I was only 19 or 20 at the time.

Yes, I had the opportunity to tour numerous Westinghouse facilities before I could really understand what I was seeing. Boy, if I could do it all over again today I would get much, much more out of it. When you first see the stuff you can be awed but don't have the slightest clue about what your are seing.

 

[cadpoint]

there are circuit locaters on the market. if this is residential just take the extra half hour to flip some breakers bro . helper and a walkie talkie works just as well this brought to you by No Shorts Electric:thumbsup:

I took a radio along with me with it on off station, I could hear the wood head through the speaker of the radio, for those one horse shows.



Note all the components that are available for this product, this is only limited to 120V applications.

Other than trying to explain it Sperry .