Neutral/3way Problem

[ElectricianJeff]

I went on a service call yesterday. The HO said that Saturday his wife came home and when she turned on the lights in the kitchen the ceiling fan bulb blew and all the ceiling lights in the house went dim and remained that way.

This is a 1920's 1 1/2 story. It has a 15 year old 200 amp. service. Initially, I figured this was a classic case of a lost neutral. I started by checking all the connections in the main panel, everything was tight. I also found it odd that two circuits were effected by this condition, one circuit fed the ceiling lights on the main level and the other fed the ceiling lights and a bathroom recep. on the 2nd level. Both of these circuits where on the same leg in the panel, and were the old cloth covered 2 wire romex. I checked the entrance conductors and all the circuits under load and had 125V to neutral everywhere. I pulled out the affected breaker to inspect the buss bar and discovered in the process "ouch" that I had 125 Volts to grounds on the disconnected conductor when the other breaker was on. This condition went away when the other breaker was opened.

I moved one of the breakers to the other leg in the panel and had 240V at the lights, real bright, and I read 240V at the recep in the bathroom when both circuits were closed.

I went into the attic to inspect the wiring above the ceiling fan in the kitchen and found a real rats nest. I wish I had taken a picture. There where about 6 conductors wire nuted together there, no box just flying splices everywhere. I showed this to the HO and he said he had a "electrician" friend work on this about 10 years ago.

I was beginning two run out of time so I saw a suspicious splice into the neutral of the old knob and tube wiring. I cut it and backfeed the wire from a known good neutral from an extension cord. The lights on the main level returned to normal operation. Lights where still dim on the second floor. I checked the recep in the bathroom at 125V but it dropped to around 50V under the load of a small space heater. Again pointing to a lost neutral.

I am going back Friday. I told the HO that the first thing I was going to do was to cleanup the mess above the kitchen and hopefully discover the problem in that process.

I got to thinking about this having read posts on here about "California 3ways" which I don't believe I have ever encountered in the field. I still don't understand why 2 circuits are affected by this condition unless the 3-way in the kitchen is being supplied by both these circuits.

Sorry about the long post but I'm curious if anyone has experienced a similar situation and can shed some light on the outcome.

 

[Dennis Alwon]

I would check to see if everything on that phase is affected. It does sound like a bad neutral to me. You know what happens when you get an open neutral, I assume.

You need to check the power across the phases and then from neutral to hot. The breakers must be off. Turn one breaker on and test to neutral. Turn it off then proceed to the next. My bet is that you will not read 120v to ground on one phase.

If not and only 2 circuits are affected then I would look for a loose neutral on that. Is it a MWBC from the panel?

 

[ElectricianJeff]

Is it a MWBC from the panel?

Thanks for the advice Dennis and I will do just that.

No, this is not a MWBC. Two seperate 2 wire conductors. However, I'm thinking the neutrals may be both connected to the old knob and tube neutral giving me the same characterristics of a MWBC. Am I thinking right?

 

[Dennis Alwon]

Thanks for the advice Dennis and I will do just that.

No, this is not a MWBC. Two seperate 2 wire conductors. However, I'm thinking the neutrals may be both connected to the old knob and tube neutral giving me the same characterristics of a MWBC. Am I thinking right?

well I guess it is possible that somewhere down the line the 2 loads are connected to one neutral. Anything is possible in those old places.

It does not seem to be a MWBC problem simply because they are on the same phase and getting 240v. Not likely.

 

[al hildenbrand]

No, this is not a MWBC. Two separate 2 wire conductors. However, I'm thinking the neutrals may be both connected to the old knob and tube neutral giving me the same characteristics of a MWBC. Am I thinking right?

Yes, and no.

The "no" is that the voltage swing extremes of a real MWBC are a lot different in the double 120 V shared neutral circuit that I'm hearing described in your OP.



There are two key points I take from the description of the property.

1. Built in the '20s
2. The two circuits are for the lighting.

Because it is built in the '20s, there is a real possibility that the original service was, in fact, only 120 V. I would be reluctant to install these two circuits in the 240 V configuration that you experimented with, unless I knew for sure how they are circuited.

The fact that the lights are on each of these two circuits indicates that they are highly likely to be original circuits.

I suspect there are at least two problems. First, the neutrals of the two circuits got cross circuited, that is, tied together. Second, both of the neutrals lost their connection to the service center (one neutral could get lost, when cross circuited, and not be noticed). When the last tie of the neutral to the service center failed, that's when you got the trouble report from the homeowner.

When you patched in the temporary neutral with the extension cord, it sounds like it picked up part of the old circuits, but that a part of the existing neutral is still floating, disconnected from the neutral bus, between your extension cord tie in and the actual failures in the neutral continuity.

This is a hard troubleshoot.

May I suggest that you pay attention to the line side neutral continuity of each branch circuit by lifting it at the service center neutral bus to be sure that continuity is interrupted (no cross circuit) and that nothing else is affected (taps no discovered yet).

Good hunting.

 

[ElectricianJeff]

I suspect there are at least two problems. First, the neutrals of the two circuits got cross circuited, that is, tied together. Second, both of the neutrals lost their connection to the service center (one neutral could get lost, when cross circuited, and not be noticed). When the last tie of the neutral to the service center failed, that's when you got the trouble report from the homeowner.

Thanks for your insight, Al

Based on my time there I believe you have probably hit the nail on the head. I will find out for sure on Friday.

It still puzzles me that the act of turning on the light and the bulb blowing resulted in the presence of the condition become known.

Jeff

 

[kwired]

sounds like you lost a main fuse and the voltage problems you are having are because of feed through of 240 volt appliances such as water heater.
did you check line to line as well as line to neutral voltage at main panel. in this situation you will see 120 both lines to neutral or at least what looks somewhat reasonable. but line to line will be close to zero.

Never mind main fuse issue I reread your OP. you did mention having 240 volts at one point. probably is a neutral issue on load side of main panel good luck.

 

[al hildenbrand]

It still puzzles me that the act of turning on the light and the bulb blowing resulted in the presence of the condition become known.

The report of the client is probably incomplete, which is understandable, given that the probable cause (IMO) is a break in the continuity of the neutral, which the client wouldn't be directly aware of.

I just did a somewhat similar troubleshoot on a Unit in a 1913 four plex. As originally wired, the Unit had seven overhead lights, only three of which were controlled by wall mounted switches. No wall mounted receptacles.

Over the decades, other circuits had been added and that original lighting circuit had six receptacles extended from it.

Some time ago the neutral broke near the beginning of the branch circuit. Some one "fixed it" by tying the neutral in the living room switches (living room overhead, light outside the unit door, front porch light) to the living room overhead dimmer heat sink EGC pigtail.

This connected the floating neutral of the branch circuit to the Rigid Metal Conduit wiring method through the dimmer device mounting screws ( the #6-32 screws ) and returned the circuit to "normal behavior".

I got the call when the tenant reported that turning on her living room light caused the whole apartment to flicker and go out. She said the pull chain switch in the bedroom was also bad 'cause it would flicker from time to time as well.

I found the double problem - the immediate symptoms were that the #6-32 contact to the heat sink was affected by the mechanical movement caused by doors slamming, the bus on the street outside hitting fresh glorious MN potholes, and using the dimmer.

The real problem, however, was the original neutral break way back near the beginning of the branch circuit in a long forgotten Jbox.

 

[LarryFine]

Some time ago the neutral broke near the beginning of the branch circuit. Some one "fixed it" by tying the neutral in the living room switches (living room overhead, light outside the unit door, front porch light) to the living room overhead dimmer heat sink EGC pigtail.

OMG! The worst way to do a bad thing.

 

[al hildenbrand]

OMG! The worst way to do a bad thing.

Heh! It's truly malign!

I started to remove the 3 gang cover plate and the lights went on. I thought, "Yay! I'm close to the answer now." I wiggle things . . . get more response.

I finish removing the cover plate and operate the rotary incandescent dimmer and all these pretty sparks fly out between the #6-32 and the heat sink. :roll:

I think the only thing that kept the wall from open flame was dumb luck.