OCPD - do they really work ?

[Stevenfyeager]

Overcurrent Protection Device. Do they really protect from overcurrent ? On a job, a customer's 20 amp breaker was tripping. It turns out it was overloaded. But with my amp meter, it was supplying and holding at 30 amps and getting very warm but wouldn't trip when I was there. I took some loads off of it and that fixed the problem. But why allow and hold at 30 amps when it says 20 ? That year, I asked my instructor about it, and he said " breakers are not supposed to limit current but only protect in case of a short to ground." I'm confused. I've been wondering about that for years.
Years later, this same small vacation house customer had me replace a very outdated meter (Last week The POCO gave him an ultimatim) I noticed that his 60 main breaker panel has a # 6 Cu wire feeding it from the meter.
I also noticed the wires inside the old meter had been hot, and melted insulation. Does this mean the 60 main breaker is not limiting current to 60 amps? Thank you

 

[gar]

181029-1107 EDT

Stevenfyeager:

Look up a Sq-D QO trip time curves. Also note thermal breakers are temperature dependent. And in addition there are tolerances on the spefcifications.

.

 

[JPinVA]

Breakers have a Time/Current Trip value. This is a function of the breaker rating and in some cases can be adjusted. Basically, the setting is designed to allow a circuit to be temporarily overloaded (e.g., during a motor surge) without tripping, provided it eventually drops down to a normal operating value.

To see if the breaker in question is operating properly, one would need to obtain the Time/Current trip graph. Load it to 30A and see how long it takes to trip. If the trip occurs within the spec zone for the breaker, it is operating properly. If it trips beyond the time identified on the chart, it is operating out of spec and should be replaced.

 

[mbrooke]

Overcurrent Protection Device. Do they really protect from overcurrent ? On a job, a customer's 20 amp breaker was tripping. It turns out it was overloaded. But with my amp meter, it was supplying and holding at 30 amps and getting very warm but wouldn't trip when I was there. I took some loads off of it and that fixed the problem. But why allow and hold at 30 amps when it says 20 ? That year, I asked my instructor about it, and he said " breakers are not supposed to limit current but only protect in case of a short to ground." I'm confused. I've been wondering about that for years.
Years later, this same small vacation house customer had me replace a very outdated meter (Last week The POCO gave him an ultimatim) I noticed that his 60 main breaker panel has a # 6 Cu wire feeding it from the meter.
I also noticed the wires inside the old meter had been hot, and melted insulation. Does this mean the 60 main breaker is not limiting current to 60 amps? Thank you

Breakers have a time current curve, meaning they will not trip the second a 20amp breaker sees 20.001 amps. Rather 24 amps may never trip it, 25 amps may take hours, 40amps may take a minute, 100amps a few seconds and 400 amps a few cycles. Reason being is because of inrush in things like motors which have to be accounted for. Even an incandescent bulb will draw more power for a fraction of a second when switched on because the cold filament has a lower resistance and as such more current will pass. In fact under UL489 standards a breaker is not even required to trip at 134%, must trip in 1 hour or less at 135% and must trip in 2 minutes or less at 200% over current.

Another reason is that ambient temps effect the trip threshold (due to breakers using a temperature dependent bi-metal to sense over current) and as such the trip threshold actually starts higher then 20amps. If breakers started at 100%, its possible that in a warm environment the breaker will trip at a level below its listing. Thus as such the curve is set to start above 100%, typically 125% as done by most manufactures. This way in all anticipated environments (ie panel outdoors with the sun beating on it in a heat wave) you will have a guarantee that the breaker will never trip below its listed rating.


The wire sizes are designed to handle these low level overloads without issue.


Although in your case 30 amps continuous on a 20 is pushing it but then again you only measured 30amps at a brief snapshot in time. If the loads were cycling, and current was varying between 20 and 30amps because of loads switching on and off / drawing varying power, then I could see the breaker being able to handle that without tripping. 30amps would push the bimetal toward tripping, but as the load drops down toward 20amps it cools off a bit, current goes up to 30 it gets hotter, then current drops and its cools back down... ie it sits in limbo never actually being pushed off the edge of mountain despite being just centimeters away from the edge at times.


In terms of your meter base, 60amps are fine on #6 wire according to the code. Most likely it was a loose connection heating the wire, something that a breaker will NOT catch.

 

[kwired]

Breakers have a time current curve, meaning they will not trip the second a 20amp breaker sees 20.001 amps. Rather 24 amps may never trip it, 25 amps may take hours, 40amps may take a minute, 100amps a few seconds and 400 amps a few cycles. Reason being is because of inrush in things like motors which have to be accounted for. Even an incandescent bulb will draw more power for a fraction of a second when switched on because the cold filament has a lower resistance and as such more current will pass. In fact under UL489 standards a breaker is not even required to trip at 134%, must trip in 1 hour or less at 135% and must trip in 2 minutes or less at 200% over current.

Another reason is that ambient temps effect the trip threshold (due to breakers using a temperature dependent bi-metal to sense over current) and as such the trip threshold actually starts higher then 20amps. If breakers started at 100%, its possible that in a warm environment the breaker will trip at a level below its listing. Thus as such the curve is set to start above 100%, typically 125% as done by most manufactures. This way in all anticipated environments (ie panel outdoors with the sun beating on it in a heat wave) you will have a guarantee that the breaker will never trip below its listed rating.


The wire sizes are designed to handle these low level overloads without issue.


Although in your case 30 amps continuous on a 20 is pushing it but then again you only measured 30amps at a brief snapshot in time. If the loads were cycling, and current was varying between 20 and 30amps because of loads switching on and off / drawing varying power, then I could see the breaker being able to handle that without tripping. 30amps would push the bimetal toward tripping, but as the load drops down toward 20amps it cools off a bit, current goes up to 30 it gets hotter, then current drops and its cools back down... ie it sits in limbo never actually being pushed off the edge of mountain despite being just centimeters away from the edge at times.


In terms of your meter base, 60amps are fine on #6 wire according to the code. Most likely it was a loose connection heating the wire, something that a breaker will NOT catch.

I wouldn't say will not catch, but often they don't. Bad connection on a breaker terminal or bus to breaker connection may sink enough heat into the breaker to trip on thermal action even if load current level is below trip curve.

 

[JFletcher]

A 60 amp main would probably hold 66 to 72 amps close to indefinitely depending on trip curves, ambient temperature, intermittent loading, Etc.

Think about it, the time a breaker in a residence would be the most loaded would probably be in the dead of winter with numerous space heaters running. The low ambient temperature could definitely affect its factory trip curves, and a 60 amp breaker in say Barrow Alaska might hold twice its rated current indefinitely.

I would say as a matter of statistics, that ocpd work.
If none of them worked, houses and businesses would be burning to the ground everyday and numbers far greater than we see.

I have seen a single pole 15 amp Federal Pacific stab Lok trip under a 13 amp load, it probably tripped 500 times in its lifespan. Also seen 2 20 amp Cutler-Hammer tanhandle single pole hold under a bolted fault and the wires exploded in the junction box, breakers never tripped.

Ocpd work, and I would rather be in a world slam full of FPE or Zinsco panels than nothing at all.

 

[kwired]

A 60 amp main would probably hold 66 to 72 amps close to indefinitely depending on trip curves, ambient temperature, intermittent loading, Etc.

Think about it, the time a breaker in a residence would be the most loaded would probably be in the dead of winter with numerous space heaters running. The low ambient temperature could definitely affect its factory trip curves, and a 60 amp breaker in say Barrow Alaska might hold twice its rated current indefinitely.

I would say as a matter of statistics, that ocpd work.
If none of them worked, houses and businesses would be burning to the ground everyday and numbers far greater than we see.

I have seen a single pole 15 amp Federal Pacific stab Lok trip under a 13 amp load, it probably tripped 500 times in its lifespan. Also seen 2 20 amp Cutler-Hammer tanhandle single pole hold under a bolted fault and the wires exploded in the junction box, breakers never tripped.

Ocpd work, and I would rather be in a world slam full of FPE or Zinsco panels than nothing at all.

I agree but do see a conflict with wording when you say 66-72 amps indefinitely and then use "intermittent" in the same sentence

I will also say I have seen RK-5 fuses not blow on a line to line fault (not bolted but loose contact of free conductor ends on 480/277 volts) because of current limiting effects of a ~1400 foot run of conductors.