went on a service call today HO lost half his lights, plugs & all his 240volt equipment, i get there & start checking voltages at the panel, phase A to G & N 95 volts (no load with load 50v), phase B to G & N 120v, (load & no load) phase A-B 215v. then i went to the meter main panel phase A to G 120v phase B to g 120v,phase A -B 240v. this a newer service, with 100' of direct burial cable between panel & meter main panel. the only thing i can think of is a bad cable. meggered the cable got low reading on all 3 wires ( i don't trust my old megger) looking for a second option before redigging the trench
weird voltage readings
- Thread starter rickl
- Start date
- Status
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Call the poco, it is a classic situation of a bad phase. Generally it is the power companies problem.
kwired
Electron manager
- Location
- NE Nebraska
What locations did you connect megger to, and did you disconnect conductor being tested from other paths so that you are only testing the underground conductors.
You have to turn off breakers or disconnect conductors from other conductors - including the neutral if you are testing it to isolate them from potential current paths that will not give you a good picture of what you are looking for.
The fact that you said you have a low reading on all 3 wires indicates to me that you likely did not do this. If you connect your megger to phase A and it has a line to neutral load connected to it you will get a low reading through this load to ground through the grounding electrode(s) or to other conductors if they are also connected via line to line or line to neutral loads.
Why not trust old megger? If you send high voltage out and get a lot of it back it is probably working fine -at least for simple good/no good tests. If you have high resistance on everything you test then you may want to know if it actually works correctly.
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Jim W in Tampa
Senior Member
- Location
- Tampa Florida
Looks like your A leg is about to open . Perhaps it is aluminum and melted some. Who ever owns that wire best fix it fast before anything worse happens.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
I agree with Jim, you have lost your A leg, and since these conductors are between the meter/main and the house panel then they would not be the POCO's
kwired
Electron manager
- Location
- NE Nebraska
I have never seen underground aluminum that is 'about to open'. Every time I have ever located a fault on aluminum it is definately open. I'm sure there is a period of time when it is about to open but there are no symptoms of there being a problem until it is actually open - at least not for the average electric power consumer.
Jim W in Tampa
Senior Member
- Location
- Tampa Florida
One morning when i arrived at the shop to pick up material for a job the boss asked me to look at an office trailer he rents out that is supplied power from our main warehouse. They had been having trouble similar to this for days and i was now the 3rd one of our men to attempt to find the intermitent problem. Took about hour to be 100% sure. Ran some temp wire on the ground (used the still good ground wire as a hot and wire on ground as temp ground) Luckily the 150 feet of digging got handed to another crew.
Point is aluminum will melt and cool for short time. It will not take any large loads. Never seen copper do it.
If your smart you will run conduit.
kwired
Electron manager
- Location
- NE Nebraska
It will seldom drop voltage enough to be noticable except for under large loads which will accelerate failure time. Maybe you were experiencing moderate load that heated the conductor pretty good but not quite enough to cause failure, although I can't imagine that time period would be very long at all. Soil conditions could be a factor in how long it will last.
I have fixed many underground aluminum failures never have received a call before it actually failed completely.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
I have one the other day at a trailer park, I would have bet my money that it was a bad underground failure, but one thing was strange, all the 120 volt loads worked just fine but would dim from time to time, checking for voltage, I had A-N 120 volts B-N 120 volts, A-B 0 volts, ok so I thought I was back feeding from a 240 volt load (only one AC) so I turned off the AC breaker, and it was still there, didn't change, I opened the main, still the same, went out side and pulled the disco still the same, went to the main distribution panel, checked the lug on the 100 amp Zinsco had 0 volts between A-B and 120 volts between A-N and B-N, ok bad breaker, but this was the first time I have ever seen a breaker fail like this, it was actually feeding the A buss to both A and B lugs out, changed it out and everything was back to normal, 37 years and that one through me.
dennis: i have normal voltage at the meter
kwire: all breaker were turned off in the metermain & lifted in the panel
jim w: they are aluminum wires, is that normal to lose voltage on a bad cable, you would think it would eventually just melt the wire in two & then you would have 0 volts to ground,
thanks for the help
kwire: all breaker were turned off in the metermain & lifted in the panel
jim w: they are aluminum wires, is that normal to lose voltage on a bad cable, you would think it would eventually just melt the wire in two & then you would have 0 volts to ground,
thanks for the help
Jim W in Tampa
Senior Member
- Location
- Tampa Florida
It would not last long. In my case the feed was #2 aluminum and actual load was likely under 10 amps normally. Only real load was coffee maker and the AC 240. We are talking a very small trailer of maybe 400 sq feet. Made me look good to the boss for finding it. Problem is even with a company of 40 to 50 employees we had very few men qualified to really do service work. Others likely were reading threw loads and with breakers off they would get the right readings. Think of it like a bad splice inside a wire nut arching.
jcole
Senior Member
- Location
- Lawrenceburg, Kentucky
Is there a main breaker at panel? If so, had same problem before. It was a bad connection where main breaker connects to bus in panel.
What voltage did you megger at? I dont see a reason why you shouldnt trust it. Get to diggin my friend.
What voltage did you megger at? I dont see a reason why you shouldnt trust it. Get to diggin my friend.
jetlag
Senior Member
- Location
- Eatonton, Georgia
about to open
about to open
the aluminum underground will about to open , always from a nick in the insulation , it will arc to the ground until a section 2 or 3 inches long has turned into a powder that is still trying to make some connection
about to open
the aluminum underground will about to open , always from a nick in the insulation , it will arc to the ground until a section 2 or 3 inches long has turned into a powder that is still trying to make some connection
kwired
Electron manager
- Location
- NE Nebraska
The powder is aluminum oxide which is not a very good conductor, soil conditions will determine how much current leaks to ground. Nicks in insulation before installation or rodent damage are two most common causes I have seen for underground failures. Once aluminum has been exposed it begins to oxidize and a little moisture around it speeds up the process. Conductor likely does not get very hot until it has deteriorated enough that it starts having a noticeable resistance and the larger the load the worse things will be.
wptski
Senior Member
- Location
- Warren, MI
240V won't jump a air gap greater than .001" and the insulation is much thicker than .001". If it's wet that's a different story!
A 1000V megger can jump a air gap of .006" but still the insulation is thicker. A arc isn't going to jump from a insulation nick under dry conditions.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I still think you have a bad phase. If the phase conductor splice is bad at the transformer there can be enough contact that you could read 120V. As soon as a load is on it the voltage will drop to 0V. I have seen this many times.
No 240V is an indicator of a bad phase. The reason you read voltage could be that the phase is being fed thru any 240v equipment such as a water heater.
Say phase A is bad. Phase B would connect to the water heater element and then fed back on the A phase which in turns energizes the entire A phase in the panel. Because it is being thru a high resistance of an element there isn't enough current available to turn motors etc but it may allow the lights on that phase to shine dim. The more 240 volt circuits that are turned on the brighter the lights will burn.
Turn off all the 240V circuits in the panel and see what happens.
kwired
Electron manager
- Location
- NE Nebraska
I had a call today with a lot of similarities to your situation - maybe about 80 volts line a to neutral normal voltage line b to neutral and maybe about 200 line to line.
These were all taken with no load connected. I was already convinced we lost one of the lines just had to narrow it down to which one so I did not even take any readings with any load connected.
Disconnected both ends and had low resistance from all three lines to ground rod at the pole with megohmeter set at only 200 volts.
Assumed that all three conductors had compromised insulation but were not all three open circuits because I had 120 volts on one line but bad insulation resistance on all lines.
Located the fault on the open line and proceeded to dig at that spot.
When we found the conductors one was completely open the other line was not open but maybe 1/3 the way disintegrated, and the neutral had a slice in the insulation and after inspecting the conductor was starting to corrode away some at the surface.
My theory was that the conductors were damaged before or during installation and have finally degraded to the point of failure.
Repaired all three lines and all was fine again.
kwired
Electron manager
- Location
- NE Nebraska
Do you have underground fault locator? Makes several hours or even days of digging turn into a 2 hour job real fast. That is how I fixed the problem I mentioned I had yesterday - I did not spend too much time checking voltage and megging before going straight to the locator. Voltage and megger readings just tell you which conductor you should try to locate first. Had similar situation to what you described, myself and my apprentice diagnosed and repaired it in about 3 hours (included digging up the fault with hand shovels @ about 24 inches deep). Without locator it would have taken more than a day digging by hand especially if we started digging on the end farthest from the fault.
kwired: no i don't have underground fault locator, but luckily i found the fault & repaired it under 2 hrs. started asking HO question about a newly poured sidewalk & he said you know they used rebar to hold the forms in place, so that where i started digging & found the fault.
the concrete people got very lucky, he must have just nicked the cable, & yes the cable was 24" deep
thanks again for the help
the concrete people got very lucky, he must have just nicked the cable, & yes the cable was 24" deep
thanks again for the help
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