277 Branch Circuit Tripped The Main

goose134

I don't have a lot of details about this problem as I didn't have a lot of time to spend on this but here goes. There is a 4 wire 277/480 volt panel with a 400 amp main feeding various fan power boxes, baseboard heaters, and fan coil units. Some of the branches are single pole, others 3 pole. Yesterday, the control guy says one of the fan coils tripped, so I reset the breaker single pole 20 A). I figured something he was messing with tripped it. Today he comes to me and says that NONE of the fan power boxes, or fan coils have power. So I look and see the 400 amp main breaker is tripped. Well I'm not about to turn this thing on without doing a little bit of digging, so I start ringing out the panel for shorts.
I have one of those meters that give a "clear" tone for continuity, and "buzz" tone if you're reading through some type of coil or element. Some of these circuits are giving me a buzz to neutral (normal in a resistive heating load) But on more than one I read a clean to neutral. None of these had anything to do with the breaker that tripped in the first place. I don the flash suit, close the panel and turn it on. Flip all the breakers on one by one and they hold. Before anyone asks, no I don't know what type of breakers they are. I'm sure that it is critical. But any thoughts on this?

mdshunk

It's not uncommon at all for the main to trip when a branch breaker hard faults. The main just reacted more quickly than the branch breaker, and it was probably more heavily loaded at the time, thus it took less to trip it. You might want to look into "selective coordination" to solve this issue.

goose134

You are probably right about it being heavily loaded at the time. As I said, it is a panel loaded with heating elements. I'm not sure what you mean by 'selective coordination', could you elaborate?

Sparky Joe

I'm not sure what type of meter you are using, though I'd like to know. Either way it's beside the point.

Selective coordination is an engineers resposibility, but if the panel has an electronic main breaker you could turn the little dial to it's highest setting. This should, but may not prevent the main from tripping when the branch circuit breaker should.

About your meter again, it won't give a buzz when testing the motor loads, because motors are not resistive loads and do not have any impedance until AC is applied with the motors designated frequency.

goose134

I don't know the model # but the meter is a Fluke with the forked ammeter. No, I know about not reading the motor loads, I think what it's actually picking up is the control transformer. I was simply curious about why I'd get a "clean" tone when reading on some of the elements. I'm probably describing this poorly. When I touch the two leads together it sounds like a truck backing up, but steady. The "buzz" sound sound is like that sound with a blown speaker. Could it be that some of the lower wattage elements would show a low enough resistance that it would read as a dead short? (This meter does not have a range selector for resistance.)
The Main breaker on the panel does not have an adjustment. The distribution panel in the basement does.

gravtyklz

The meter I use does the same thing, but i've found it to be kind of inconsistent, i'm not sure why either. Today for instance there was a dead short somewhere in a circuit and in one spot there was a solid tone on the meter rather than a "buzz" so I thought I found it...but it was just reading through a bulb.

goose134

See, that's just it. Any Low resistance, and it will read "clean" as a dead short. A bulb is a good example of the type of resistor it'll read right through. Good way to test bulbs I guess. When I was an apprentice, I chased a short in a parking garage for an hour before I finally asked the journeyman, "Are we just reading the lamps?" So Like I said, I'm sure it was simply a lower wattage element I was reading through. Come to think of it, maybe I'll take an actual ohmmeter to the circuit and see what I find. Have to bring it to work and see if I have time. I really like the meter but it does have its limits!

gravtyklz

Oh i see how your meter works now. Thats pretty nifty.

jrclen

It is fairly common for a good solid short to trip the main in a 480 panel. I've seen them do that without tripping the branch breaker at all.

A few years back I got an night time emergency call from the main postal complex in the city down the highway. I'd worked in there before. They told me their 1200 amp switch gear main was tripping. They had 2 banks of 1200 amp, 480 volt, with tie over, on 2 separate circuits from the power company. Their maintenance guy had already tried to tie over to the circuit that didn't trip, and that didn't work either. Now these people get really excited when they can't sort the mail. They have deadlines. I isolated myself with the maintenance supervisor and the maintenance electrician in the switch gear room. Leaving the clamoring hoards outside. I donned the suit. There was a radio call to the supervisor from one of the other buildings. They said they heard a buzz in the panel before the lights went out. Perfect time for that call. I turned off all the feeders and energized the mains after untying. I didn't energize the feeder to that other building. I had them sorting mail again in less than a half hour. I was the hero. I went over to the other building and rang out the circuits. I found a shorted 277 lighting circuit in the office. It turned out to be a pinched 18 gauge wire to a ballast. So that 18 gauge hot, shorted to ground, tripped the main in the switch gear through the ground fault protection it was provided with, and shut down the entire operation. It did trip the main in the 480 panel in that other building, but not the branch, and not the feeder breaker on the switch gear.

MechanicalDVR

It's a T-5 meter and when it makes that odd buzz in the ohms mode it's because there is some power on the circuit.

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mdshunk

That's one reason I use the T5 for my every day meter. You can't really blow it up. It'll warn you.

goose134

Yes, that is true. As MD said another good reason to use this meter. But I was getting that noise with the circuit de-energized. I must have been picking up a capacitor in the motor. Listening to you guys and typing these things down is a great sounding board. It's like I can remember these things but I'm forced to talk about them and think them through a little bit. Anyway, thanks, now I don't have to stress about a huge blowup.

Sparky Joe

I have the same T-5 as my on the job meter, I don't have it in front of me, nor have i used it so often that i can picture all the settings in my head, but that tone is not a "dead short" tone, just a continuity tone.
It's a good meter down to 5 ohms(+or- 2 ohms), but will give a continuity tone no matter the load you are reading through. And this is definetly not a troubleshooting meter.

And by the way, a lower wattage bulb is higher in resistance compared to a high wattage bulb.

60 watt bulb = 240ohms
100 watt bulb = 144ohms

gravtyklz

What do you consider a good trouble shooting meter?

mdshunk

Maybe you asked Joe, but I'll answer. The T5 is a good every day troubeshooting meter. I think that among electricians, the T5 might be the most popular meter presently in use. It really is a Fords vs. Chevy's type of thing, but most can agree on the brand name Fluke. I was a Fluke 73 man for many years, but the Fluke T5 is what I've been using for maybe the past 5 years for an every day meter. There's always a need for specialty meters for particular purposes, but you can't go wrong for the money if you get a T5. There's a newer verion out, called the T5 PRO, but I really don't see the need for it.

randomkiller

Sometimes it's not need as much as to try out something new, I use the T-5 as my regular OTJ meter and have been looking at the new pro meter (this week the closest Grainger is having Fluke counter day and the T-5 pro will be @$80, so I have been kicking it around) I do have a need to check amp often so I may just skip it for now.