Breaker adventures..

[danTt]

Hello Braintrust, encountered a strange breaker issue over the past few days I'd love some opinions on as I continue to research the answer..

I've inherited an annual installation that runs for multiple hours each night during the month of February. This is my first year dealing with it, though it has happened in the same arrangement and setup at this venue for multiple years now.

The installation is ~45,000 watts of parcans powered by a portable dimmer rack. Math Suggests this draws ~375 amps total, or in a balanced world 125 amps/leg. The dimmer rack is (and has been) powered by a 100amp 3phase company switch. Did the setup as per usual, and amp-clamped the legs at ~140, 145, and 80 amps (the installation fades continuously, so the power varies--but the highest legs remained above 100 amps.) If this was a brief spike I could see the breaker not catching it, but this runs for hours above the breaker spec without tripping. It's a union company switch with a 100amp 3-pole breaker protected upstream by a 100-amp 3 pole breaker, so even with a bad breaker in the company switch itself I'd think that the breaker upstream would trip.

Now, obviously this is not good (tm), and power has been redirected/rerouted/rebalanced to fit within what the breaker should trip at, but I'm concerned that it worked fine for as long as it did. I'm also concerned that there are another dozen or so company switches in the building that are the same model and protected in a similar fashion.

I suppose my questions for the brain trust are as follows:

1) As far as I can tell, the breaker in the company switch is not indicated as rated for continuous duty, and I can almost guarantee the breaker feeding it isn't. How do two breakers not fail when loaded so far over their duty rating for much longer than the 80% rule should allow..?
2) Suggests on safe ways of testing the remaining company switches in the building short of rolling around enough watts of light to trip them?
3) War stories related to similar experiences? (I'm looking at you, @RonHebbard )

Thanks in advance--will be looping in electrical engineer and local electricians as well, but wanted to inquire here as well.

 

[JD]

They probably are not defective. At 100%, you would expect the breaker to trip in less than 3 hours. As you climb above that number the time gets shorter and shorter. A dead short would trip the magnetics (and if that were to happen, it would be considered to be damaged and should be replaced.) Many of us old-timers can remember running shows on super-undersized breakers back in the 80's and actually getting away with it.
I suspect the full load is not left on for extended periods of time. Don't have the graphs handy, but a 100 amp breaker passing 140 amps may take a lot longer than you would think to trip.
If you have real doubts that the breakers are safe, I would have them changed. Overload testing is destructive to breakers.

This video covers smaller breakers, but the concept is the same:

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[danTt]

Thanks for the video--it's definitely helpful, I'll have to see if I can pull the trip curves for these breakers.

I will say that what you say certainly makes sense in general, and matches what I'd expect to see. You say "At 100% you'd expect to see the breaker trip in under 3 hours" and yes, that makes sense to me. However, this scenario has a loads that fluctuate from a low of 105 to a high of 140 amps for a six hour period, which is where things seemed weird to me. That being said, if the trip curves on the 100A breaker are anything similar to the ones in that video, it looks like the 100Amp breaker could run for an undefined length of time at 140 amps and it may or may not trip. This seems surprisingly high to me (and maybe when the breaker sizes get larger the tolerance is much tighter--I'll see what I find tomorrow.)

 

[TJCornish]

Thanks for the video--it's definitely helpful, I'll have to see if I can pull the trip curves for these breakers.

I will say that what you say certainly makes sense in general, and matches what I'd expect to see. You say "At 100% you'd expect to see the breaker trip in under 3 hours" and yes, that makes sense to me. However, this scenario has a loads that fluctuate from a low of 105 to a high of 140 amps for a six hour period, which is where things seemed weird to me. That being said, if the trip curves on the 100A breaker are anything similar to the ones in that video, it looks like the 100Amp breaker could run for an undefined length of time at 140 amps and it may or may not trip. This seems surprisingly high to me (and maybe when the breaker sizes get larger the tolerance is much tighter--I'll see what I find tomorrow.)

Thermal breakers are significantly affected by ambient temperature (usually in the DErating direction due to panel heat); magnetics not so much. Some breakers have adjustable trip points.

Can you post exactly the model of breaker in question? 140 amps sustained (longer than 3 hours) does seem a little high, but I agree with JD, the answer will likely be that this falls within the accepted trip curve of the breaker.

 

[SteveB]

Just as thought, are you using an amp meter that measures true RMS ?.

If not you could be getting skewed readings due the SCR dimmers.

 

[RonHebbard]

Hello Braintrust, encountered a strange breaker issue over the past few days I'd love some opinions on as I continue to research the answer..

I've inherited an annual installation that runs for multiple hours each night during the month of February. This is my first year dealing with it, though it has happened in the same arrangement and setup at this venue for multiple years now.

The installation is ~45,000 watts of parcans powered by a portable dimmer rack. Math Suggests this draws ~375 amps total, or in a balanced world 125 amps/leg. The dimmer rack is (and has been) powered by a 100amp 3phase company switch. Did the setup as per usual, and amp-clamped the legs at ~140, 145, and 80 amps (the installation fades continuously, so the power varies--but the highest legs remained above 100 amps.) If this was a brief spike I could see the breaker not catching it, but this runs for hours above the breaker spec without tripping. It's a union company switch with a 100amp 3-pole breaker protected upstream by a 100-amp 3 pole breaker, so even with a bad breaker in the company switch itself I'd think that the breaker upstream would trip.

Now, obviously this is not good (tm), and power has been redirected/rerouted/rebalanced to fit within what the breaker should trip at, but I'm concerned that it worked fine for as long as it did. I'm also concerned that there are another dozen or so company switches in the building that are the same model and protected in a similar fashion.

I suppose my questions for the brain trust are as follows:

1) As far as I can tell, the breaker in the company switch is not indicated as rated for continuous duty, and I can almost guarantee the breaker feeding it isn't. How do two breakers not fail when loaded so far over their duty rating for much longer than the 80% rule should allow..?
2) Suggests on safe ways of testing the remaining company switches in the building short of rolling around enough watts of light to trip them?
3) War stories related to similar experiences? (I'm looking at you, @RonHebbard )

Thanks in advance--will be looping in electrical engineer and local electricians as well, but wanted to inquire here as well.

@danTt I'm not ignoring you Dan, only following my policy of learning more with my eyes and ears open and my mouth (and fingers) closed. Personally, I've ALWAYS went with limiting my connected loads to 80% of the name plate rating of the breaker. In my early days as an assistant electrician in an IATSE venue and IBEW apprentice, we'd have all manner of road shows rolling in with all manner of shoddy, friction tape encrusted, conglomerations of 4/0 feeders (of all manners of insulation types and ratings) with bizarre combinations of paralleling and splitting Camlock T's buried within their taped together "portable assemblies" handing us five tails and decreeing "Tie us in to a 3 phase 400 amp breaker and let me check your connections before you power it up." At that point my boss, the IA head electrician, who'd been a telephone installer in his earlier years, would nod at me and say "Ron. Do what the man is asking and let him inspect it BEFORE you re-lock the access cover." At that point, we did as were instructed and literally stood by with two fire extinguishers nearby keeping our eyes and noses peeled for any signs of smoking or overheating paying particular attention to cams that arrived under layers of friction tape which we could not personally confirm were fully rotated and / or clean and snug. If anything was especially visually irksome, I'd often check the production's feeders with my gloved, then bare, hand for signs of overheating. Did this from 1973 through 1977 prior to moving on to IA head of sound in the Stratford Festival's main stage and leaving others to concern themselves with the terminating and powering of shoddy road feeders. Today "Company Switches" are MUCH better than what passed for touring road power in the late 60's and early seventies. Thanks for the inquiry @danTt and THANKS for the interesting and informative posts @JD & @TJCornish
Toodleoo!
Ron

 

[JD]

What @SteveB brought up is a significant factor. Dimmer waveforms are ANYTHING BUT sine wave! Only a true RMS meter is going to give you a reasonable reading. Because of the sawtooth waveform, a standard magnetic clamp-on meter is going to misread the sharp edge of the waveform. Modern breakers are mainly thermal but usually have a fast acting coil for extremely fast response to massive overloads. You can take consultation that general wire gauge guidelines are based on the characteristics of how a circuit breaker trips.

 

[BillConnerFASTC]

Do you think the SCRs distort the sine wave of the input enough to make a significant difference? I know they do the output, and I know they can affect the feed, but I thought it was a relatively small amount, and very dependent on a lot of feeder side conditions - like how much wire between load and transformer and so on. Someone else can espouse on harmonics, triplen harmonics, and harmonic mitigating transformers - which I always ask for but are always way to expensive according to the engineer. I suspect asking for feeds for dimmers that would support a nearly fully loaded rack helped, considering dimmers in a dimmer per circuit system are probably not loaded to 50% on average.

I worked on a renovation (the black box at the University of New Mexico if anyone is familiar) and powers that be made the decision to not upgrades - all 2.4kw (e the feeder for the dimmers - 225 amps IIRC. We put in two 96 dimmer KW - CD80 - and I had asked for 600 amps. It would have required a major upgrade of the service and finding real estate for a new transformer, so not an unreasonable fiat. So facility reopens and the LD wanted to test it so he hung all he has and ran 1-192 @ FULL. Again, 25+ years ago, but IIRC 350 amps if balanced. Popped around 40 minutes. Conclusion was the stress test was far more than a reasonable load in normal use, so OK. I never heard if it presented a problem and I can only guess now, in the age of LEDs and green, its more than enough.

 

[STEVETERRY]

Hello Braintrust, encountered a strange breaker issue over the past few days I'd love some opinions on as I continue to research the answer..

I've inherited an annual installation that runs for multiple hours each night during the month of February. This is my first year dealing with it, though it has happened in the same arrangement and setup at this venue for multiple years now.

The installation is ~45,000 watts of parcans powered by a portable dimmer rack. Math Suggests this draws ~375 amps total, or in a balanced world 125 amps/leg. The dimmer rack is (and has been) powered by a 100amp 3phase company switch. Did the setup as per usual, and amp-clamped the legs at ~140, 145, and 80 amps (the installation fades continuously, so the power varies--but the highest legs remained above 100 amps.) If this was a brief spike I could see the breaker not catching it, but this runs for hours above the breaker spec without tripping. It's a union company switch with a 100amp 3-pole breaker protected upstream by a 100-amp 3 pole breaker, so even with a bad breaker in the company switch itself I'd think that the breaker upstream would trip.

Now, obviously this is not good (tm), and power has been redirected/rerouted/rebalanced to fit within what the breaker should trip at, but I'm concerned that it worked fine for as long as it did. I'm also concerned that there are another dozen or so company switches in the building that are the same model and protected in a similar fashion.

I suppose my questions for the brain trust are as follows:

1) As far as I can tell, the breaker in the company switch is not indicated as rated for continuous duty, and I can almost guarantee the breaker feeding it isn't. How do two breakers not fail when loaded so far over their duty rating for much longer than the 80% rule should allow..?
2) Suggests on safe ways of testing the remaining company switches in the building short of rolling around enough watts of light to trip them?
3) War stories related to similar experiences? (I'm looking at you, @RonHebbard )

Thanks in advance--will be looping in electrical engineer and local electricians as well, but wanted to inquire here as well.

What is the ambient temperature?

ST

 

[danTt]

In response to True RMS -- Great point. (Un)fortunately I'm using a Fluke 323 True RMS clamp meter. And mathematically 45,000 watts of light certainly is going to exceed 100 amps/leg, so reality matches the math, unfortunately.

In response to temperature--"room temperature"--Probably between 68-72 degrees at the disconnect, and similar at the other end of the feeder.

I pulled the trip curve on the breaker today, it's an eaton 100amp molded case circuit breaker, I don't recall the exact number any more but the curve did make it look like tolerance would allow for 140 amps at the high end of tolerance for not tripping, but still within parameters after 10,000 seconds--2.7 hours (the upper end of the graph provided.).

I was a bit confused by the graph ending at 2.7 hours though, as I always was under the impression that for "continuous usage" of > 3 hours the breaker had to be derated further to 80% of capacity. Shouldn't that be reflected in the trip graph? I wouldn't expect a hard left turn at 3 hours, but I would expect that if derating happened by code it would be supported by trip curves on the breaker, and reflected in documenatation. I'd also think that the longer past 3 hours the load continued the higher the likelyhood of a trip from > 100amps, although it looks to become fairly static in the graphs.

 

[JD]

Not all breakers will trip at or after 3 hours at 100%, but it you are designing a system then the 80% derate is required, especially if you want to make sure everything stays on
People often think of breakers (and fuses) as avalanche devices that trip at a specific point. They aren't. They are fire/damage prevention devices that safeguard a system from dangerous events. Many factors affect the trip point. Room temperature is one. The temperature inside the panel is another. The temperature inside the breaker is a third- for example, a breaker that is hot from running near it's load trip point will trip much sooner than one that has not had any power running through it and is basically at ambient temperature.

 

[danTt]

Not all breakers will trip at or after 3 hours at 100%, but it you are designing a system then the 80% derate is required, especially if you want to make sure everything stays on
People often think of breakers (and fuses) as avalanche devices that trip at a specific point. They aren't. They are fire/damage prevention devices that safeguard a system from dangerous events. Many factors affect the trip point. Room temperature is one. The temperature inside the panel is another. The temperature inside the breaker is a third- for example, a breaker that is hot from running near it's load trip point will trip much sooner than one that has not had any power running through it and is basically at ambient temperature.

Definitely agree.

That being said, shouldn't the trip curves reflect that as you continue past 3 hours the trip point of the breaker should continue to lower? Not looking for any hard lines, and maybe I'm interpretting the 80% rule too literally, but it would seem that breakers not rated for continuous duty and thus more likely to trip after 3 hours of heavy usage would reflect that in their trip curves, and I do not see that in any of the literature (In fact most eaton curves top out at 10,000 seconds). Am I missing something obvious?

 

[RonHebbard]

In response to True RMS -- Great point. (Un)fortunately I'm using a Fluke 323 True RMS clamp meter. And mathematically 45,000 watts of light certainly is going to exceed 100 amps/leg, so reality matches the math, unfortunately.

In response to temperature--"room temperature"--Probably between 68-72 degrees at the disconnect, and similar at the other end of the feeder.

I pulled the trip curve on the breaker today, it's an eaton 100amp molded case circuit breaker, I don't recall the exact number any more but the curve did make it look like tolerance would allow for 140 amps at the high end of tolerance for not tripping, but still within parameters after 10,000 seconds--2.7 hours (the upper end of the graph provided.).

I was a bit confused by the graph ending at 2.7 hours though, as I always was under the impression that for "continuous usage" of > 3 hours the breaker had to be derated further to 80% of capacity. Shouldn't that be reflected in the trip graph? I wouldn't expect a hard left turn at 3 hours, but I would expect that if derating happened by code it would be supported by trip curves on the breaker, and reflected in documenatation. I'd also think that the longer past 3 hours the load continued the higher the likelyhood of a trip from > 100amps, although it looks to become fairly static in the graphs.

@danTt @JD @StB Allow me to swerve this thread a little further in the direction of ill-thought installations and folks assuming why their breaker is tripping. I'm sure I've posted of this installation before.
Massive renovation and expansion of the hospital in Burlington, Ontario, Canada. 1968 and 1969 I believe. Two new wings were being added from scratch; foundations, basements plus 6 floors per each wing. One existing basement and three floor wing was being extended, one of these floors was operating rooms with another being obstetrics. The OR's and obstetrics needed to remain fully operational while their floors were being extended. Last phase of the project was to gut all three or four floors of the original building, retain the exterior walls, replace all floor slabs and increase the height to six floors. To make it interesting, the main substation was in the basement of this original building, and the hospital is on the shore of Lake Ontario and subject to adjacent water table issues along with frigid winter winds off the lake.
Enough bacground, taking this back to breakers and well meaning, though misguided, installation practices.
The main sub was fed 3 phase Delta 13K8.
One of its intermediate step-down panels provided WYE 347 / 600 Volts, a common commercial distribution voltage for us in my area of Canada. There was physical space and electrical capacity in this panel to add a 3 pole 800 Amp breaker to feed a new smaller sub being built in the basement of of one of the wings. The Electrical P. Eng. had decreed the feed from the 800 Amp 3 pole breaker to be routed via 5 runs of single conductor Core-flex (Sp?) to a Delta / Wye step-down transformer in the new small sub. One of the two sub-foremen on the project had his electricians install the 800 Amp 3 pole breaker in the last remaining space then punch 5 holes in a vertical column in the side of the panel locating 3 of the holes such that they'd line up perfectly with the load lugs of the 800 Amp 3 pole breaker while the other two holes would conveniently route to the neutral and ground buses.
The installation proceeded, eventually everything was powered and in operation, literally, pardon the pun on "operation".
Loads had been checked and all currents were well within all ratings.
The 800 Amp feed tripped suddenly while its sub-sub station was powering at least one operation in progress.
Unlike in plumbing, excrement rolled up hill at an alarming rate as the operating staff complained to their superiors who complained to their bosses who called Comstock Canada's electrical VP and then excrement rolled down hill to our project foreman, his two sub foremen, then our journeymen and finally to the lowly likes of moi.
When you punch holes in a metallic panel and route single conductor Core-flex (Sp?) through the holes and then power the Core-flex (Sp?), you have to remember the magnetic field of the Core-flex (Sp?) will produce circulating eddy currents in the metal surrounding each conductor. The circulating currents generated heat. The copper conductor conducted not only the electrical current but the heat as well directly into the nearby load lugs of the breaker. The breaker then did exactly what it was designed to do and tripped. NOT due to excessive current, not even close, but due to overheating.
If our journeymen and minor minions thought we'd seen excrement fly before, we were treated to a PHD level course at close range with bonus courses on creative buck-passing being thrown in for good measure.
Two large pedestal fans were commandeered. One directed at the face of the breaker and the second at three of the Core-flexes where they entered the panel. The game was to keep the breaker on and cool until the operation in progress could be completed.
Next steps:
- Switch off the breaker.
- Wait for everything to cool, including flaring tempers.
- Disconnect and withdraw the Core-flex.
- Destroy a quantity of metal cutting blades in cutting a vertical rectangular opening in the side of a live main substation.
- File the edges of the hole real pretty.
- Source and fabricate an appropriate piece of phenolic. [Forgive me. I can't pull the correct name for a particular brand of mechanically stable laminated and compressed insulation to mind]
- Neatly drill, de-burr and bolt the phenolic into place where the metal was removed.
- Clean, burnish and re-install the Core-flex.
- Re-power everything, apply the load and pat ourselves on our perspiring backs.
We could have cut slots in the metal panel to interrupt the paths of the circulating eddy currents but while the effluent and excrement were freely flowing, someone well above my station opted for the 'Buy once / Cry once option.
So be it and we all lived happily ever after.
There you go @danTt I hope that'll suffice for this chapter and you're feeling neither ignored nor short changed.
EDIT: Sorry. Missed an 'S'.
Toodleoo!
Ron Hebbard.

 

[JD]

^^^ Proof that even when things all look good on paper, the lights may still not stay on!
...and as for dealing with temperatures outside of the breaker and of the human type, you should have been around that day my bi-line got created! (read below the name)

 

[SteveB]

When our new service entrance panel was installed to provide for a new HVAC chiller unit, we were able to get them to add an 800 amp 3 phase for stage lighting.

The install electrician laid in a single 500 mcm wire (roughly 320 amp capacity wire) per phase for this extra panel, from SE to the Con Edison vault. I caught it and asked if maybe he should be doubling that and he replied with “Nah, from here out it’s Con Ed’s problem”. It stayed for 30 years before they finally fixed it.

 

[JD]

200 amp service entrance, 4/0 Al feed to the meter and the box. PECO loops a #6 Al from the pole to the SE head. I call it "the fuse."

 

[chausman]

We found the best way to keep water out of our underground conduit was to run a fully utilized 400 amp disconnect feeding a very full load center with what I believe was 4/0 Cu from the transformer. Keeps that conduit run nice and warm.There's not a lot of snow that stays on that particular part of the ground either.

 

[FMEng]

It seems utility companies get away with things mere mortals cannot. We were increasing the service capacity at a tower site to lease space for police and fire trunking radios. In taliking to the power company it was discovered that their transformer was rated at about half the actual load of the existing FM broadcast transmitters. One transmitter ran full time and the other was a backup for another station, so the overload wasn't full time. Still, the transformers endured running many days severely overloaded. Apparently, they are rated pretty conservatively, or were 35 years ago. In fact, I had to really convince the power company to upgrade the transformer to support the public safety equipment. I guess all is well until it explodes.

 

[RonHebbard]

200 amp service entrance, 4/0 Al feed to the meter and the box. PECO loops a #6 Al from the pole to the SE head. I call it "the fuse."

@JD Welcome to the world of "Free air" ratings.
Toodleoo!
Ron Hebbard.

 

[RonHebbard]

We found the best way to keep water out of our underground conduit was to run a fully utilized 400 amp disconnect feeding a very full load center with what I believe was 4/0 Cu from the transformer. Keeps that conduit run nice and warm.There's not a lot of snow that stays on that particular part of the ground either.

@chausman There's a lot to be said in favor of underground "steam heating".
Toodleoo!
Ron Hebbard.