tripping main

[lazorko]

Today I got a call to troubleshoot an outdoor post light that hadn't been working for several weeks. The light has its own branch circuit, wired in UF. I verified a dead short on the 15A breaker, and opened the panel cover to disconnect the branch from the breaker for further troubleshooting - and noticed that the 100A main had tripped, too.
I thought that maybe I had hit the handle when removing the cover, and reset it.
After dissassembling and reassembling the post lamp I could find no apparent problems - oddly, the short had cleared and the lamp was operating normally. I assumed that maybe the last installer had left a few strands of wire loose from a wire nut that had caused an intermittant. I left the lamp on test for an hour (doing other work) before leaving.
Tonight, the HO called to say that her lights had gone out - the main breaker had tripped. The branch circuit breaker for the post lamp had also tripped. The meter and panel are back-to-back on the garage, and I suspect that the UF shares the trench with the underground POCO laterals on its way to the post light. Is there a possibility that there is some insulation damage allowing intermittant current to flow directly between the two sets? Or should I suspect bad coordination between the main breaker and the branch breaker (both Siemens)? Your insights are appreciated.

 

[Dennis Alwon]

I don't have a direct answer to your problem but I will tell you this... Many years ago I blew a 120 volt circuit at an IBM and it also blew a 400 amp breaker. Why I don't know but I bet if you fix the short in the cable (I don't think Podco has anything to do with it) the main will be fine.

 

[lazorko]

Thanks - I was hoping that maybe the main was just a little dodgy. As for the shorted post light . . . maybe I'll try changing out the photo cell and hope that the UF isn't damaged underground.

 

[mdshunk]

Do a search on "fault coordination". It can be completely normal for a branch circuit fault to trip the main under certain circumstances.

 

[brian john]

Circuit breakers (molded case circuit breakers typically utilized in commercial and residential applications) are thermal magnetic devices with an inverse time current operation, the magnetic portion of the CB operates for faults. It is possible (VERY LIKELY IN THIS CASE) as Marc stated for the time curves to overlap. Hence the fault is of such a magnitude that the 100 amp CB operates (per design) as the 20 amp operates. Th current fell with in the curve for both devices. Or the 100 amp can be defective is also an option.

The electrical tech I worked with as I was being taught testing use to say. "What do you want they are only molded case circuit breakers, you ought to be thankful the operate at all"

 

[templdl]

You hit the nail on the head Brian. With bolted faults the fault current is on great enough magnitude that it falls within the magnetic trip pickup of both breakers. Then it is potluck which one will trip. It's not all that uncommon that the main will be taken out.

The mag trip calibration of a 100a breaker will most likely be 10x its rating or 1000a +-20, most likely on the higher side. The 15a-breaker may be calibrated anywhere from 6-7x to 10x or more it?s rating dependent upon the manufacturer. But it you had a bolted fault with 10ka of available fault current it would be not unusual to see more than 1000a of fault current.

Regarding coordination, it's impossible at best to coordinate breakers when it comes to bolted faults unless breakers are equipped with an electronic trip unit with a short time delay. This would provide a very brief delay in the upstream breaker adjustable in order of 100-500ms to allow the down stream breaker to trip and clear the fault.

 

[chateau mike]

hidden short

hidden short

first try disconnecting the circuit feeding the light ,see if the main trips again. if not, further investigate the pole and feed.

 

[ramdiesel3500]

I would bet the short circuit is very close to the panel for the main to trip out. I would check the branch circuit conductors inside the panel gutters very carefully! Look at the condition of the cable as it passes through the box connector. May have been over-clamped and shorted. Possibly only shorts out after the conductors heat up for a couple of hours!