What Caused This?

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Little Bill

Little Bill

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Tennessee NEC:2017
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Semi-Retired Electrician
I believe I know what caused this but would like other's opinions also.
It is a 100A main breaker from an old ITE panel.


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Loose connection? Maybe.

By comparison, I had a Square D breaker fail in a similar manner, after decades of service. Oddly enough, my follow-up visits showed that the new 100-A breaker was running for extended periods with 115 amps running through it.

That sets the stage for gradual degradation of everything, until something fails.
 
renosteinke said:
Loose connection? Maybe.

By comparison, I had a Square D breaker fail in a similar manner, after decades of service. Oddly enough, my follow-up visits showed that the new 100-A breaker was running for extended periods with 115 amps running through it.

That sets the stage for gradual degradation of everything, until something fails.
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You have to quantify "extended periods of time" because 115 amps through a 100 amp breaker isn't "bad". It's only "bad" for high values of quantification ...
 
"Extended periods" was a gentle way of saying any time all the dryers were in use .... like, say, half of Saturday and all of Sunday.

It only became a problem when the operator replaces all of his (single) dryers with double-stack units. I estimate he was able to operate for three years before the breaker died.
 
Excuse me, renosteinke, but you seem to mistake a switch for a breaker for any healthy breaker is bound to trip sooner or later if its operating current exceeds its setting. Is it not so?
 
Haji said:
Excuse me, renosteinke, but you seem to mistake a switch for a breaker for any healthy breaker is bound to trip sooner or later if its operating current exceeds its setting. Is it not so?
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Yes it will trip sooner or later, if load remains continuous.

If high load is short enough in duration, it may not trip, yet has had some effect on integrity of connections, contacts, etc. hasn't it? May take years before it has a total failure, but will likely fail sooner than same unit that only sees 50% of its rating on a regular basis.
 
Haji said:
... any healthy breaker is bound to trip sooner or later if its operating current exceeds its setting. Is it not so?
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No.

You need to look at the breaker trip curve and also make adjustments for the ambient temperatures.
For example, in a 1986 edition of the NEMA AB 1 standard for breakers I find a table that says; breakers >50A, need to carry 100% of their rating for 1 hour then they need to carry 135% for not more than 2 hours
 
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Thank you, Jim.

It was a real eye-opener for me, that overloaded breaker.

I've had Square D reps insist otherwise - claiming the breakers were rated for only 80%, that they would always trip right at the handle value, etc .... but here I had real-world proof that such was not true.

135%? That 100-A breaker would still meet NEMA specs if it let 135 amps flow for two hours? Wow. Seems my concerns over the 115-a measured load were exaggerated. Small wonder the replacement breaker has survived hundreds of Sundays since I put it in.
 
renosteinke said:
I've had Square D reps insist otherwise - claiming the breakers were rated for only 80%, that they would always trip right at the handle value, etc .... but here I had real-world proof that such was not true.

135%? That 100-A breaker would still meet NEMA specs if it let 135 amps flow for two hours? Wow. Seems my concerns over the 115-a measured load were exaggerated. Small wonder the replacement breaker has survived hundreds of Sundays since I put it in.
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100% and 80% ratings have very little to do with when a breaker trips. The rated loading is important when sizing protection per the NEC.
Except for special overcurrent relays, nothing trips at exactly 100%. There is always a tolerance band, partly because real world conditions are usually different than those in the testing lab.

Testing involves a device in 'open air' not in an enclosure. All protective devices (breakers and fuses) are tested as 100% devices 'in open air', but as soon as they are installed into an enclosure, they 'magically' become 80% devices (unless the entire assembly has been specifically labeled as 100%).

Yes, while the 135% seems high, it is true for all devices, both breaker and fuses. And remember the 2 hours is an upper limit and the ambient temperature needs to be dealt with.

Prolonged operation above the 80% enclosed rating will likely cause eventual connection failure due to overheating.
 
Little Bill said:
I'm sure it was a bad connection but I don't think it was in the lug that holds the wire. IOW, I don't think the wire was loose. I think the lug its self was loose inside the breaker.
Probably age and heat related.
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There would not be excessive heat to cause problems without a loose connection or overloaded circuit breakers.
 
Haji said:
Excuse me, renosteinke, but you seem to mistake a switch for a breaker for any healthy breaker is bound to trip sooner or later if its operating current exceeds its setting. Is it not so?
Click to expand...

WRONG, WE see thermally damaged circuit breakers all the time that are not tripped or tripping. Heat on the on the line side can be excessive and not affect the thermal on the load of a circuit breaker.
 
jim dungar said:
100% and 80% ratings have very little to do with when a breaker trips. The rated loading is important when sizing protection per the NEC.
Except for special overcurrent relays, nothing trips at exactly 100%. There is always a tolerance band, partly because real world conditions are usually different than those in the testing lab.

Testing involves a device in 'open air' not in an enclosure. All protective devices (breakers and fuses) are tested as 100% devices 'in open air', but as soon as they are installed into an enclosure, they 'magically' become 80% devices (unless the entire assembly has been specifically labeled as 100%).

Yes, while the 135% seems high, it is true for all devices, both breaker and fuses. And remember the 2 hours is an upper limit and the ambient temperature needs to be dealt with.

Prolonged operation above the 80% enclosed rating will likely cause eventual connection failure due to overheating.
Click to expand...

I have seen cases where a fused switch was in need of replacing, but to get by the owners left the door open and placed a fan in front of it to keep it cooler and this resulted in less frequent tripping, so air flow does make some difference.
 
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