Intermittent Circuit Failure

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mphillipps

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I been called back to a house three time now because of a loss of power on one circuit.

Here is what I know:
  • Every time I leave the house the circuit is working.
  • The circuit originates at a FPE box (circa 1958).
  • The original 20 amp breaker was never tripped but the power was loss.
  • I've replaced the old breaker with a new breaker.
  • Within the panel, the breakers position was moved.
  • The circuit includes, kitchen, two bedrooms and two bathrooms.


I'm at the point to start looking for a loose neutral or a back-stabbed receptacle with a poor connection.

Are there any suggestions or thoughts on the breaker or the best way to hunt for a loose neutral?

Thanks
Mark
 
mphillipps said:
Please define MWBC.
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Multiwire branch circuit.

Branch Circuit, Multiwire
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. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.
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As far as looking for a loose stab in a receptacle, look for the one that gets the most play, where do the plug in the vacuum cleaner.
 
Put a load on the circuit plug in some heaters or a few drop lights with large wattage bulbs.

Perform FOP on the CB at the panel, across the CB, from line conductor of the panel to the load of the CB.

Based on the circuits involved I assume this is a lighting circuit?


If you can determine it is not the panel, you may have the old pull and visual inspection. If you do this LOOK FOR that hidden outlet in the closet or behind the stove.
 
When you go back is it working again? If not and you havent done this yet, do not touch anything get your meter and go to an outlet (rec, light with screw in lamp) check and see if you have voltage, if nothing to neutral see if you have something to ground. You may have done this just starting from the beginning. If you get no voltage you can check the neutral (of the circuit giving you trouble by using a close circuit that you know is working( at a rec or whatever and see if you can read that hot to ground. If so you can be pretty sure you have no neutal problem unless of course someone wired this ciruit at some point in series instead of using pig tails, then I would say you may be on the right track looking for bad connection. Before ripping out the devices maybe look for some new wiring on the circuit that may go to a jbox maybe someone broke a wire making a connection or just did not get it tight.

I once had a similar issue and I just happen to be there the next time it happen and it was when someone walke across the floor and I had a jbox open and saw the arc. Good luck that can be lots of fun at time and material.:grin:
 
100904-1334 EST

mphillipps:

You could have a high resistance or open in either or both the neutral and/or hot wiring.

To add to Brian's comment --- try to find the most extreme end of the circuit. There might be more than one depending upon the wiring layout. Pick one. Plug in a 1500 W heater. Measure the voltage at other outlets on this circuit. The measured voltages will probably isolate the region of the problem.

In my house at the most remote outlet I get a 5 V change from a current change of 12 A. There is about 50 ft of #12 and 75 ft of #6 and two breakers between the outlet and the main panel bus. A QO15 A has about 0.14 V drop. Assume 0.25 for the two breakers. Assume the voltage change for the wiring is 5-0.4-0.25, then there is about 4.35 V for the wire and joints which also includes some in the receptacle. The calculated wire resistance from the voltage drop is 4.35/12 = 0.36 ohms. The calculated wire resistance from wire size and lengths is 0.2 ohms. So is there 0.16 ohms in joints or are some of my estimates off? I do not know and I am not going to do a more accurate evaluation now, but I would expect a closer correlation.

If you do not understand my measurements and analysis, then ask questions. You clearly have large voltage differences and troubleshooting to the local area should not be too hard.

Something else you can do is use a long extension cord from the main panel to serve as long test leads for EGC, neutral, and hot. Then make difference measurements on the conductors when under load. This will help isolate hot and neutral problems or both. Do not derive the hot test lead from the circuit you are testing, but derive it from a breaker on the same phase.

.
 
Brian / Gar

I don't quite get the FOP test and what I'm looking for.

Am I to run a load on the circuit in question, then check the mV form that circuit against any other circuit? I'm not sure what I'm looking for if I have a intermittent open neutral.

My Fluke 117 only reads tenths on 600V scale. Will this work for the FOP?
 
100904-2050 EST

mphillipps:

The electrical supply to your main panel has a source impedance that is moderately small. This impedance is a combination of all the series impedances from your main panel back to the power plant generator. But from a practical point of view it is only the series impedance in the pole transformer and the wiring from the transformer to the main panel that is of importance to you.

In the measurement I made above the rough value of the impedance at my main panel looking back to the power company is about 0.4/12 = 0.033 ohms. So a 200 A load would produce a voltage drop at the main panel when connected of about 0.4*200/12 = 6.7 V.

Internal to your problem house you want to produce a change of load on the problem circuit and monitor the change in voltage at different places along that circuit. The simplest test is to measure the voltage between hot and neutral at various outlets on the problem circuit while applying and removing the test load. Your problem is so sever that maybe a 250 W bulb is an adequate test load.

If you use a 1500 W heater with an internal fan then only momentarily use it as a load to avoid burning out the motor.

Since you do not know where the open or high resistance point is in the circuit you want to draw current from the farthest point in that circuit. Therefore, the test load gets connected and disconnected at the end of the circuit.

If the problem is a high resistance at a single point, then:

To the main panel side of the problem the circuit voltage won't change much, with and without the test load connected.

On the other side of the problem point there will be no voltage at all, or a large voltage change will occur when the test load is connected and disconnected.

.
 
I got this from someone here

I got this from someone here

Fall Of Potential.

You know that when you have a difference in potential, electricity will flow. In the case of a breaker, contactor contacts, and other such connections, you can use a meter set to millivolts (sometime microvolts) and check from line to load of each pole while energized. The higher millivolts you read, the more contact resistance you have from line to load.

The exact millivolt reading isn't as important as comparing them across all three phases, or across all the similar connections for a group of breakers. If you find a bad pole of a breaker, you can check from the buss tang to the breaker tang, on each the line side and the load side. This would let you know if the resistance was at the bolted connection or inside the breaker.

usually anything over 100mv is bad and over 35mv should be watched. done this method in a house that had a breaker that kept tripping. checked connections on the circuit then measured voltage drop across the breaker. measured around 98mv, replaced the breaker and it was all fixed
 
Maybe someone replaced a switch incorrectly, and half of that circuit is now being controlled by a switch.
 
mphillipps said:
I been called back to a house three time now because of a loss of power on one circuit.

Here is what I know:
  • Every time I leave the house the circuit is working.
  • The circuit originates at a FPE box (circa 1958).
  • The original 20 amp breaker was never tripped but the power was loss.
  • I've replaced the old breaker with a new breaker.
  • Within the panel, the breakers position was moved.
  • The circuit includes, kitchen, two bedrooms and two bathrooms.


I'm at the point to start looking for a loose neutral or a back-stabbed receptacle with a poor connectio

Is the wiring old BX there could be a loose clamp somwhere.
Click to expand...
 
Sometimes I'll put a toner on the dead side of a circuit and follow it back to where it stops. (Gotta be careful so you don't smoke your toner though.)
 
mphillipps said:
I been called back to a house three time now because of a loss of power on one circuit.

Here is what I know:
  • Every time I leave the house the circuit is working.
  • The circuit originates at a FPE box (circa 1958).
  • The original 20 amp breaker was never tripped but the power was loss.

    Thanks
    Mark
Click to expand...


  • An FPE breaker that actually trips? :roll: Havn't seen one yet, it takes a lot, like a house burning down to get one to trip.:D

    I would check for a loose wire at the main (or bad main breaker)if this panel has one. Replacing the breaker may have jarred it enough to make connection again. When you leave, it heats up again and breaks contact again.
 
gar said:
100814-1618 EST

mphillipps:

Have you found the problem? If so what did you find?

.
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Follow up... After I had tighten all of the neutrals in three panel and not hearing from the customer for two weeks, I thought for sure that I'd fixed the problem... but alas, he called with the news the circuits were off, than on, then off, than on.

I tried a FOP test but found nothing conclusive.:confused:
 
mphillipps said:
Follow up... After I had tighten all of the neutrals in three panel and not hearing from the customer for two weeks, I thought for sure that I'd fixed the problem... but alas, he called with the news the circuits were off, than on, then off, than on.

I tried a FOP test but found nothing conclusive.:confused:
Click to expand...

I don't get it what have you done to get the power back on?
If you don't know then how can you feel you have fixed it.
 
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