Thanks for another great article. The one comment I have is to me it looks like your example trip curve is off. Pushed left as it was. According to your curve a breaker at 1x its rating will trip somewhere from 1-15 minutes. I think more telling is that at 80% load (hard to read log scale) the breaker can still trip in 3-4 minutes.
We've all camped at 30A sites and have draw close to 30A for extended periods of time. Of course that doesn't prove anything. But all trip curves I've seen put 1x load at close to never tripping. I don't know if this is a reputable site, but just one example:
This article brought up memory of a camp host gig we had in Arizona just a few years back during the winter. First week there had a camper tell me his camper plug, 30amp, wouldn't stay in the receptacle. Already knowing what the problem was I went to the shed and grabbed a new receptacle. After killing the power and verifying same I open the box and wow, talk about weakened contact springs!! When I replaced that one I had to cut off about 2 inches of wire as they had been really HOT. Such that the sheath was dark brown on the ends and hard as a rock. I ultimately found 11 more that way, weak contacts and cooked, and replaced them. This campground is right on the Colorado River and summer use is minimal but the A/C's are on max 24/7. I wish I would have taken a few photos. Those wire ends were crazy cooked. Would have been a good presentation of how hot the wiring and breaker can get and still function. I have a bad/good habit of telling campers about cleaning plug contacts and how to check for good connection with the receptacle. Thank goodness we have 50amp!!
TT-30 receptacles are notorious for loose contacts that overheat and burn the wires. I’m beginning to teach classes for campground pedestal maintenance this fall.
Thanks again Mike for a very informative article. I’m really surprised it takes so long for a breaker to trip at its rated current (>>10 sec according to your chart). I always knew there had to be a delay but I thought it was much shorter than it actually is. In another example, my AC compressor has a 30A double pole (240V) breaker. The LRA rating is 79 Amps. According to your chart it would trip in something like 2 seconds which is fine since the start-up transient lasts for less than 1 sec. It all work out quite well in practice. Thanks also for your very well done high speed pictures of transient start ups. I struggle making sense of inrush measurements made with clamp on Amp meters but there is nothing like a real graph of current vs time! Nice work Mike!
Thanks very much. I had been working with all sorts of electric motors for decades before I began gathering data and making high speed graphs of what the various inrush currents actually looked like. That’s when I really began to understand the relationship between inrush current and circuit breakers tripping.
Another thing I often hear is when there is a fire people will say "the circuit breaker should have tripped and prevented the fire" and they blame the breaker. There are plenty of ways an electrical fire can start without tripping a circuit breaker.
Thanks for yet another informative article.
Thanks for another great article. The one comment I have is to me it looks like your example trip curve is off. Pushed left as it was. According to your curve a breaker at 1x its rating will trip somewhere from 1-15 minutes. I think more telling is that at 80% load (hard to read log scale) the breaker can still trip in 3-4 minutes.
We've all camped at 30A sites and have draw close to 30A for extended periods of time. Of course that doesn't prove anything. But all trip curves I've seen put 1x load at close to never tripping. I don't know if this is a reputable site, but just one example:
https://www.electricaltechnology.org/2021/07/tripping-curves-circuit-breaker.html
This article brought up memory of a camp host gig we had in Arizona just a few years back during the winter. First week there had a camper tell me his camper plug, 30amp, wouldn't stay in the receptacle. Already knowing what the problem was I went to the shed and grabbed a new receptacle. After killing the power and verifying same I open the box and wow, talk about weakened contact springs!! When I replaced that one I had to cut off about 2 inches of wire as they had been really HOT. Such that the sheath was dark brown on the ends and hard as a rock. I ultimately found 11 more that way, weak contacts and cooked, and replaced them. This campground is right on the Colorado River and summer use is minimal but the A/C's are on max 24/7. I wish I would have taken a few photos. Those wire ends were crazy cooked. Would have been a good presentation of how hot the wiring and breaker can get and still function. I have a bad/good habit of telling campers about cleaning plug contacts and how to check for good connection with the receptacle. Thank goodness we have 50amp!!
TT-30 receptacles are notorious for loose contacts that overheat and burn the wires. I’m beginning to teach classes for campground pedestal maintenance this fall.
Thanks again Mike for a very informative article. I’m really surprised it takes so long for a breaker to trip at its rated current (>>10 sec according to your chart). I always knew there had to be a delay but I thought it was much shorter than it actually is. In another example, my AC compressor has a 30A double pole (240V) breaker. The LRA rating is 79 Amps. According to your chart it would trip in something like 2 seconds which is fine since the start-up transient lasts for less than 1 sec. It all work out quite well in practice. Thanks also for your very well done high speed pictures of transient start ups. I struggle making sense of inrush measurements made with clamp on Amp meters but there is nothing like a real graph of current vs time! Nice work Mike!
Thanks very much. I had been working with all sorts of electric motors for decades before I began gathering data and making high speed graphs of what the various inrush currents actually looked like. That’s when I really began to understand the relationship between inrush current and circuit breakers tripping.
Don't crush that dwarf, hand me the pliers.....
🤣🤣🤣
Another thing I often hear is when there is a fire people will say "the circuit breaker should have tripped and prevented the fire" and they blame the breaker. There are plenty of ways an electrical fire can start without tripping a circuit breaker.
So true!