lighting and receptacle breaker tripping

[Ravenvalor]

A 5 - year old home has a lighting and receptacle circuit tripping a circuit breaker. The breaker is 15amp and not an arc-fault. Since it is only doing it once daily I would like to find or develop some machine that will locate the assuming short. When I get to the home it will not be tripping therefore I need to find a short that is not occuring in my presence.
I am an electrical contractor who has been fixing these types of problems for 24 years. Usually I have to check every accessible part of the circuit including the appliances. I am wondering if there is a theorem associated with ohm's law that will enable me to calculate varying voltages and amps at each outlet to reveal the short.

Thanks,
Jim

 

[kwired]

A 5 - year old home has a lighting and receptacle circuit tripping a circuit breaker. The breaker is 15amp and not an arc-fault. Since it is only doing it once daily I would like to find or develop some machine that will locate the assuming short. When I get to the home it will not be tripping therefore I need to find a short that is not occuring in my presence.
I am an electrical contractor who has been fixing these types of problems for 24 years. Usually I have to check every accessible part of the circuit including the appliances. I am wondering if there is a theorem associated with ohm's law that will enable me to calculate varying voltages and amps at each outlet to reveal the short.

Thanks,
Jim

I already have a machine that helps find this kind of problem. I call it megohmeter.

 

[480sparky]

Divide the circuit in half, take it apart at that point, then see if the problem still occurs.

If it does, start working back towards the panel. If not, work away from it.

Take the half that's known to have the problem, and divide it in half again.

Repeat as necessary.

 

[Ravenvalor]

How does it work?

 

[kwired]

Unlike a regular ohmmeter with a test voltage of less than 10 volts in most cases, a megohmeter puts out a high test voltage usually selectable of ranges like 250, 500 or 1000 volt DC. The higher test voltage will result in return current at higher easier to measure levels even across a fairly high resistance.

Intermittent short circuits will have carbon deposits and other vaporized metal deposits in and around them but will often be a high enough resistance that there is no immediate high intensity effects when energized. Temperature changes vibration and other factors may temporarily reduce the resistance enough there is a short circuit condition and that is when your circuit breaker is tripping, but by the time you try to check it out it is back to a condition where it will not trip.

The megohmeter can measure the high resistance and help find the problem faster.

Because you are using a high test voltage some loads may need to be disconnected before testing or they may be damaged by the test voltage.

Search this forum as well as others for the term "megger" there is a lot of information out there.

 

[480sparky]

One thing to know about meggers: You need to remove ALL loads from the circuit. Umplug everything, and make sure there's no GFCIs, dimmers, devices like that. Doorbell transformers, night lights... all need to be taken out.

Meggers are made to induce higher voltages than what your service provides, up to 1000v, maybe more.

If you don't remove all the loads, you may end up frying something.

 

[kwired]

One thing to know about meggers: You need to remove ALL loads from the circuit. Umplug everything, and make sure there's no GFCIs, dimmers, devices like that. Doorbell transformers, night lights... all need to be taken out.

Meggers are made to induce higher voltages than what your service provides, up to 1000v, maybe more.

If you don't remove all the loads, you may end up frying something.

exactly what is removed depends somewhat on what the test voltage will be. If testing from line to ground at say only 250 volts I would expect the devices you mentioned to pass a short duration test and still be uncompromised. If they don't pass the test they may be the cause of the trouble you are looking for. You do need to isolate so that you do not pass test current through the load , through the neutral and equipment bonding jumper and back to the megger through the equipment ground. Not only does that keep the test current from passing through the load it keeps you from thinking you have a low resistance that you do not have.

The fault you are looking for could be in a load. Knowing what may be damaged by the test voltage is important though and those items need removed - if everything else tests good then the removed items start becoming suspects of the problem.

 

[480sparky]

That's why you get a megger that can test at lower voltages. If there's something still on the circuit, it will fail the test without letting any of the magic blue smoke out.

 

[G._S._Ohm]

When I get to the home it will not be tripping therefore I need to find a short that is not occuring in my presence.

A Time Domain Reflectometer, $300 or so or you can rent one, may show a glitch in the cable at the probable fault location.
It shows changes in the cable Characteristic Impedance vs. distance, within 1/10 of a foot. It will probably even register overly tight cable clamps.

 

[hurk27]

I get a few of these, first I look to see if any outdoor underground is ran from this circuit like post lights sheds Ect... if they still have power with the circuit off then move on, look for obvious, such as those plug in heaters/electric fireplaces Etc..., then I make sure everything on that circuit is turned off, unplugged etc, go to the panel and check Hot to ground and hot to neutral to see if there are any loads, remove the neutral off the buss, then like 480 said divide and conquer.

Intermittent shorts or overloads can be a pain to track down, burred damaged UF is the most common to find around here going to a post light, it will be ok for a while but next time it rains it will short, but this can be tested with a common ohm meter anytime by just checking between the hot and ground after you isolate both ends.

Also another little trick I use for this to check to see if a plug is hitting the side of a box, is to install a receptacle between the breaker and affected circuit using an extention cord and a plug in lamp as an indacator, turn on the circuit and using your 3-lamp plug in tester go around and wiggle each receptacle on that circuit and see if one lights the lamp, then look for a wire or screw hitting the box if metal box's are used.

Last but least is the good ole ceiling fans, if you have one that the little notch that is supposed to hold the fan rod from turning is bent or broke off, it will allow the rod to turn everytime the fan is turned on, this will wrap the wires around till they short, just somthing else to look for.

 

[Ravenvalor]

remove the neutral off the buss, then like 480 said divide and conquer.

.

What is the purpose of removing the neutral off the buss?

 

[Ravenvalor]

A Time Domain Reflectometer, $300 or so or you can rent one, may show a glitch in the cable at the probable fault location.
It shows changes in the cable Characteristic Impedance vs. distance, within 1/10 of a foot. It will probably even register overly tight cable clamps.

Here is a Wikipedia article on TDRs
http://en.wikipedia.org/wiki/Time-domain_reflectometer

It looks like a versatile toy to have. I will try to do more research on its uses on building wiring analyzing.

 

[Ravenvalor]

Thanks to all for the friendly advice.
My former boss likes to use a space heater to find loose splices in circuits. He plugs the heater into the first receptacle and then the second and works toward the end of the circuit, when he plugs the heater into the receptacle with the loose splice the heater starts cutting off and on.
I am wondering if this same method could be used to find a short but maybe start from the last receptacle and work towards the panel.

If I may quote kwire "Intermittent short circuits will have carbon deposits and other vaporized metal deposits in and around them but will often be a high enough resistance that there is no immediate high intensity effects when energized. Temperature changes vibration and other factors may temporarily reduce the resistance enough there is a short circuit condition and that is when your circuit breaker is tripping, but by the time you try to check it out it is back to a condition where it will not trip."

The space heater might just introduce a load onto the circuit which will generate heat which may temporarily reduce resistance enough to trip the breaker.

Just a thought.

 

[kwired]

Fairly recently I had a service call for an intermittent tripping ciruit breaker. There really was no significant pattern to when but seemed to be doing it more often by the time they called me there. Of course I could not find any load on that circuit that would cause it to trip, and there really was not much for load connected to it either.

This was concealed EMT with tw conductors installed 40+ years ago. I turned off lights unplugged cords etc to make sure there was no connected load even unhooked neutral for that circuit from the bus. Hooked up my megohmeter between hot conductor and ground and it indicated there was a ground fault. Went to a light fixture that I was sure was a place where the circuit split into about 3 or 4 directions and took the wires apart and tested them again. only one of them showed there was a ground fault - that narrowed it down to one section of raceway that did not have too many other switches or outlets beyond that point. Still found no fault within any boxes but narrowed it down to being between two boxes. Pulled the conductor out and sure enough there was a bad spot in the insulation. Replaced the conductor with a new one and have not been called back.

I don't think I would have found this as quickly without the use of the megohmeter.

To begin with there was no fault when I was there that was high enough magnitude to cause breaker to trip. This thing probably only tripped when temperature changed and caused just enough movement or when there was enough carbon tracking to carry enough current to start acring, or something like that.

 

[Ravenvalor]

Fairly recently I had a service call for an intermittent tripping ciruit breaker. There really was no significant pattern to when but seemed to be doing it more often by the time they called me there. Of course I could not find any load on that circuit that would cause it to trip, and there really was not much for load connected to it either.

This was concealed EMT with tw conductors installed 40+ years ago. I turned off lights unplugged cords etc to make sure there was no connected load even unhooked neutral for that circuit from the bus. Hooked up my megohmeter between hot conductor and ground and it indicated there was a ground fault. Went to a light fixture that I was sure was a place where the circuit split into about 3 or 4 directions and took the wires apart and tested them again. only one of them showed there was a ground fault - that narrowed it down to one section of raceway that did not have too many other switches or outlets beyond that point. Still found no fault within any boxes but narrowed it down to being between two boxes. Pulled the conductor out and sure enough there was a bad spot in the insulation. Replaced the conductor with a new one and have not been called back.

I don't think I would have found this as quickly without the use of the megohmeter.

To begin with there was no fault when I was there that was high enough magnitude to cause breaker to trip. This thing probably only tripped when temperature changed and caused just enough movement or when there was enough carbon tracking to carry enough current to start acring, or something like that.

Were you not worried about damaging electronic ballasts? What was the voltage setting on your megohmeter?

Thanks.

 

[hurk27]

What is the purpose of removing the neutral off the buss?

So you can see if it is a hot to ground or a hot to neutral resistance fault, this narrows down as to what you could be looking for.

I myself am not to found of loading down a circuit when a possible fault is lurking somewhere in it, can lead to a fire, there are many ways safer like the indicator lamp system I mention in my other post, but I have placed a load on a circuit to use an IR camera before, but with a fault loading down the circuit might not show up a bad wire, or it could heat it up enough to cause it to fault, I try to map out the complete circuit with my wire tracers first and look for common failure points, damp basements is a good place to look if this circuit feeds anything down there.

Murphy's law always gets me on these, but one thing you will always know, it will always be the last place you check:grin:

 

[kwired]

Were you not worried about damaging electronic ballasts? What was the voltage setting on your megohmeter?

Thanks.

There was no electronic ballasts on that particular circuit. I used the 250 volt setting on the megohmeter. By disconnecting the neutral an electronic should be safe to test from a line to ground but is probably wise to disconnect it anyway. You should test with low volt setting or regular ohmmeter first anyway. If you have low resistance with that there is a load still connected or the resistance of fault is low enough you do not need a megohmeter to measure it.

 

[Ravenvalor]

Great advice kwired.
I don't know what I enjoy most about electrical work, troubleshooting or learning from other electricians.
A megohmeter and a time domain reflectometer area both tools I will buy some day.
Thanks,

 

[LarryFine]

For an intermittent open, I like to plug a large-wattage lamp into one of the dead receptacles, and watch (or have a helper watch) it while wiggling other dead (and nearby working) receptacles.

For an intermittent short, I use a large-wattage bulb wired in series with the breaker, and have the same helper watch it while I try switches and other loads, and I wiggle stuff if necessary.

 

[Ravenvalor]

For an intermittent short, I use a large-wattage bulb wired in series with the breaker, and have the same helper watch it while I try switches and other loads, and I wiggle stuff if necessary.

Are you assuming that there is something loose associated with the circuit that will be revealed by the light bulb while wiggling devices and switching loads?