Troubleshooting a short in a home K & T wiring


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Old 02-26-15, 11:07 AM
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Exclamation Troubleshooting a short in a home K & T wiring

Hello all. This is going to be a long post as I want you to have all of the info that I know. The house was built in 1905 and the Knob & Tube wiring is still in place. They upgraded to a 200 amp service and whoever did the upgrade left the K&T in the old service box and ran wire from the new box to the old one and made the connections there. It's spaghetti, but it worked . Nothing is tagged and nothing is written on the new service door so there is no map of the circuits. From this point, I am going to refer to the old service box as a "pass-through box". The problem affects (3) rooms on the first floor: Room A) has a ceiling mount light (light switch), and a receptacle. Room B is the same as room a. Room C) has only (1) receptacle connected to this circuit and the rest of the room is on a different one. In addition, there is a wall mounted light fixture (looks original) in the hall behind room a that has (3) sets of wires tied and in the basement there are (2) ceiling mount gangs in the circuit. To the problem:

The HO turns on the light switch in Room A. He stated the lights came on and then immediately went out. He goes to the service and see's the tripped breaker and actuates it. It arcs and immediately trips. So he calls me and I figure either a bad breaker or a short and hoping it's not in the plaster/lath walls . He identifies the circuit as told to him by the electrician who upgraded the service and the path he showed me is: Ceiling mount Gang (2) as the first drop (I call this gang 2 because I later found it was not the first drop) --> Hall fixture --> Room A --> B --> C. Continuity test proved this as I was also getting continuity from the load side (identified by the wires on the first light switch in room a) to the wire I pulled from the tripped breaker. Ohm test on the wire from the tripped breaker to neutral showed resistance. Volt test at the hall light where the run from the basement ties to rooms a,b,c showed 50 volts ?!. So I untied everything in rooms a,b,c and gang 2 no resistance was found anywhere else. I did not untie the wires in the hall fixture as the tails were short and the tips were soldered. With everything untied, I still showed continuity from the wire pulled from the tripped breaker and one wire from gang 2 again identified upstairs as the load side. So I completely pull the wire connected to the tripped breaker from the service (note that I had not untied the neutral or ground at this point) so I can inspect it and hand trace the connections in the pass-through box. Found where the load/neutral were landed and untied them from the K&T. Ohm test on the K&T still showed resistance and both load/neutral still showed continuity to neutral in the new service. Nothing was wrong with the wire pulled from the new service But I did not re-connect it. So the short is identified. The HO asked if I could just bypass the K&T identified and run a new wire to put energize the circuit. Sounds reasonable but I want to do more investigating first because the 50 volts was haunting me. So I start with gang 2 (still thinking this was the first drop) and the (2) wires that showed continuity to the shorted wire had 120 volts. So I had the HO start flipping breakers until that voltage dropped and found that the circuit for rooms a,b,c was indeed powered by a different breaker and gang 2 was not the first drop, gang 1 was. I also found that the load side from gang 1 is actually tied to the neutral in rooms a,b,c. For clarity, I identified the load and neutral at the light switch in room a. That gang only has (2) wires, primary side of the switch rings to one side of the connected wires in the hallway gang (my load side). secondary side of the switch rings to the single wire at the fixture. The connected (2) wires at the ceiling fixture rings on the other side of the connected wires in the hallway.

Now the HO wants to reconnect everything and energize the circuit seeing as though the shorted wire has been isolated but I have reservations because 1) I have not been able to trace the shorted wires to anything in the house. It's not touching the service but still rings to neutral so it's connected somewhere in the house. Even though the HO went through the house and says with the exception of rooms a,b,c everything else has power. The way I'm looking at this now is that the shorted run has a neutral connected to the room a,b,c circuit (even though I have not found that connection yet) and since the load and neutral was reversed before the hallway ties, the neutral in rooms a,b,c was carrying the load so when the short occurred it essentially put 240 volts on the circuit that tripped while at the same time cutting the voltage on the circuit for rooms a,b,c in half. Also, we opened the ceiling where the service is and around gang 1 to view the condition of the wires and see routes and the K&T wire looks good. I can't visibly trace the path of the shorted wire but I can see that it's path is not going anywhere near the run for the circuit that powers rooms a,b,c.

So my question is: with the short isolated is it safe to power the circuit for rooms a,b,c even though the shorted wire stills rings to neutral?

Any knowledge/wisdom shared is greatly appreciated! Thanks.
 
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Old 02-26-15, 01:06 PM
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"Safe" is a very sketchy term to use when you're talking about 110 year old K&T. It is probably not any worse than it currently is. You are allowed to replace short sections of K&T with Romex for repair purposes only, so it's alright from a code perspective. I would strongly recommend a GFCI breaker or receptacle as the first device on this circuit.
 
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Old 02-26-15, 04:12 PM
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You have a lot of words there that could be reduced to a schematic

But, overall, the short exists ONLY in the switched lamp run, correct? That is easy to isolate/shut down.
And then, you have a shared neutral fed from two breakers, on both L1 and L2? That sounds like a multi-wire circuit run, maybe intentional. Maybe OK.

Does at least the hot side of each branch make it to the correct side of the recepts?

GFCI are a great idea, but if the neutral is shared with another circuit, they will trip out.
 
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Old 02-26-15, 04:38 PM
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Thanks for the reply. Got sort of busy so sorry for the late reply. I will take the suggestion to use a GFCI breaker on this circuit under advisement. I am going to do some more investigating before I power the circuit. I just think there is a gang somewhere in the house with multiple ties that will tell the whole story. Again, thanks for the reply.
 
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Old 02-26-15, 05:35 PM
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You can use ground fault circuit interrupter receptacles on a shared neutral circuit if you do not hang anything off of the load terminals of the GFCI unit. Install a separate GFCI receptacle unit at each wall ocation.
 
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Old 02-26-15, 06:38 PM
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I don't think this is an intentional multi-wire circuit with a shared neutral as the circuit for rooms a,b,c has a neutral landed in the service and the run with the short (which I pulled from the service) had a neutral landed also. Again thanks to all for the ideas.
 
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Old 02-27-15, 09:46 AM
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OK, understand that. It's obviously not a good idea to wire neutrals together (or let them touch in K&T). That would mean unexpected shocks when you expect a circuit to be de-energized by turning off it's only breaker, but there is still current running in its neutral from another breaker. I've seen this in the field, and finding that connection can take time, for sure.
 
 

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