Repetitive resetting

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tallguy

Senior Member
This is a situation I came across at my church...

We put on community dinners from time to time, and I wandered in one night recently to find a folding chair next to the main panel... of course this is only there because the panel violates the 6'7" rule (by about 18 inches).

So, I pop into the kitchen as one of the ladies heads out the door to reset the breaker, again. "What's going on?"... "Oh, it happens all the time"... "Well, it's telling you something here"... "It can't be helped. We can't put the electric warmer doohickey anywhere else and the coffee maker is on the other side of the wall." (we're talking one of those behemoth restaurant style ones)

Apparently they were going back and forth (a short distance, so this was not too terribly inconvenient) and resetting the durn CB every 2 or 3 minutes until I prevailed upon them to unplug the warmer since the dinner was almost over anyway.

Here's the question --- at what point (if any) does repetitive resetting of a CB and continued use under overload conditions become a hazard, either to the wire/devices or to the CB itself??
 
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brian john

Senior Member
Location
Leesburg, VA
At the price of a 20 amp MCCB, replace it, but this tripping is at low currents as compared to a fault. There is a standard out there for MCCB's somewhere and someone will post it long before I can look it up.
 

cschmid

Senior Member
I would bet if you took the panel face off and looked the insulation is already damaged..the copper will be discolored. why not donate couple of hours to your church and help them nice old lay's out and fix that so they can have a seperate circuit for their hot plate. I would bet they would love you for it..
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
The basic Standard is NEMA AB4. Its copyrighted, so I can't post it; however it is a free download from NEMA's Standards site.

I recommend reading the Introduction on Page iii, especially the third paragraph.
 

iwire

Moderator
Staff member
Location
Massachusetts
What about the branch circuit conductor insulation in regards to the temps due to overload?

I would bet if you took the panel face off and looked the insulation is already damaged..the copper will be discolored.

As part of any repair we would perform a visual inspection and meg the conductors.

Can you guys help me wrap my head around this?

I am assuming we are talking about a properly wired 15 or 20 amp circuit with reasonably modern materials.

Now you guys are suggesting that the NECs requirements do not protect the conductors insulation due to overload(s)?

I find that odd, especially with 15 and 20 amp circuits considering the minimum ampacity of these conductors are 20 and 25.

The entire time the conductors are getting hotter the breaker and it's neighbors are getting hotter resulting in faster trip times, try to do this enough times and the breaker will not latch at all.
 
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iwire said:
Can you guys help me wrap my head around this?

I am assuming we are talking about a properly wired 15 or 20 amp circuit with reasonably modern materials.

Now you guys are suggesting that the NECs requirements do not protect the conductors insulation due to overload(s)?

I find that odd, especially with 15 and 20 amp circuits considering the minimum ampacity of these conductors are 20 and 25.

The entire time the conductors are getting hotter the breaker and it's neighbors are getting hotter resulting in faster trip times, try to do this enough times and the breaker will not latch at all.




I had a legitimate question, asking others what their experiences have been. It is all a learning experience. I have seen overloaded circuits that have reduced the integrity of conductor insulation, and I want to know what others think/see/understand/ are willing to share. Isn't that what this site is really about?

What about the 10C temperature rise leading to the lifespan being reduced by 50%? I am not sure if that is for conductors as well???
 

iwire

Moderator
Staff member
Location
Massachusetts
It was a legitimate question, as was mine.

Of course if the wiring system is not modern, or shows other signs of problems I might exercise more caution.
 

tallguy

Senior Member
iwire said:
It was a legitimate question, as was mine.

Of course if the wiring system is not modern, or shows other signs of problems I might exercise more caution.

Now play nice boys... :roll:

Intuitively, what Bob is saying makes sense. The whole system is arranged such that the CB is supposed to be the weak link in the chain in order to avoid conductor/device damage. My question stems from wondering what happens to the conductors, etc. when someone misuses that weak link (MCCB) repeatedly.

As for NEMA, it appears that this circumstance (close to the low end of the trip curve) is not taxing the CB all that much. We would have a different circumstance if we were talking about "multiple high short-circuit-current faults".

Perhaps we have a loose connection, and perhaps we have a system which is not "modern" (I removed a switch recently which I would estimate is 40-50 years old anyway), but that is not why it is tripping. It's tripping because it is overloaded, and it is working properly.

As for those who suggest I just run another circuit... I'd love to, but I'm not licensed to do so on my own. Maybe we'll put on another dinner as a fundraiser to hire someone. :grin:
 

cschmid

Senior Member
ahh it is late and I am tire after working for 12 hrs straight, long day... I have seen this type of thing happen frequently. I suppose there are many factors involved when wire damage is involved. Yes the CB is the weak link but on the average trip wire has a cool down period. In the incident discribe I would think the ambient temp of the wire increase and is held there for extended periods of time. this then cause the insulation to start to deteriorate. after repeated incidents you have things like threads on CB screws that have streched and become loose, which causes the wire to heat because of poor connection and increases the deterioration process. Now this can occure on all connection of the wire as well. I do not believe this is an over night problem but an over the years problem. If the lady's have got the process honed like what was discribe this is an over the period problem. I hope this is a reasonable discription with out having to write a paper on the procees..
 

dalesql

Member
the first one up against the wall, will be the food vendors.

the first one up against the wall, will be the food vendors.

Ran into a similar situation at an outdoor festival a couple years ago. this was all portable cordage, but we had the church ladies in their portable food sales booth mounted on a trailer. Inside it was full of fridges, electric skillets and so on. Whomever had built it initially had done a good job, and it had fifty feet or so of 8/4 SO cable with a 14-50 plug on it to supply power to everything. Well, that person was nowhere to be found, and the nice church ladies had no idea that they needed to mention ahead of time that they needed a 120/240 volt 50 amp feed.

We had provided a bunch of edison outlets for the row of food vendors to argue amongst themselves over, and went on to other tasks. This particular food booth was all the way over at the far end of the site as well, so we didn't happen to get back there before we got called about the power going out there.

turns out someone else had cobbled together an adapter to allow the thing to run. it was a male edison plug to a female 14-50 receptacle, with the hot wired to both hots of the 14-50. This, in turn was fed by their rather beat up skinny 50 foot extension cord that was plugged into our panel.

The church ladies were frying up sausages in their electric skillets and whenever the breaker tripped, just walked down and turned it back on. Repeat for a couple of hours, until one of the other food vendors got nervous upon seeing their extension cord starting to melt and called us.

We got there, When we went to unplug their extension cord from our panel, it was hot enough that it burned my fingers. We followed the melty cord back, and found their *wonderful* adapter. After we picked our collective jaws back up from the ground, we went and had a little discussion with the church ladies about electrical power and overloading.

We directed them to the hardware store, conveniently located across the street, to purchase some 12/3 extension cords, and figured out their loading so they could run everything they needed.

Post festival inspection of the panel showed a rather badly discolored and partly melted outlet in our panel, and the insulation of the wires to that outlet were kinda toasty looking. replaced the wires and outlet. CB seemed to be working fine, so we left it alone.
 

pfalcon

Senior Member
Location
Indiana
tallguy said:
Here's the question --- at what point (if any) does repetitive resetting of a CB and continued use under overload conditions become a hazard, either to the wire/devices or to the CB itself??

Immediately!

There is a technical answer to this involving degradation curves, times between resets, etc. The pragmatic answer is the client will always understate the problem. By the time you become aware of the CB issue it is time to replace it and inspect the wiring.
 
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