I have someone reporting some abnormal readings on a residential 240/120 system. I cannot be there to verify. They are claiming, with no load, L1=120V L2=125V and N-G=1V. When loaded the measurements change to L1=120V L2=143V and N-G=20V (This person is not an electrician) Yes this points to an open or compromised neutral but why the 120/143 when loaded? All the open neutrals I have seen result in one leg dropping while the other increases the same amount, but in this case the total voltage appears to increase from 245V to 263V. For now I am figuring error on the part of the meter user, but is it possible for this to happen?
Here are the numbers. This is an RV pedestal (240/120) which is fed from a storage building sub panel (SB) which is fed from the main at the house. The SB is fed with 4 conductors as is the pedestal and the only bond exists at the house main.
What's confusing is when the load is placed on the RV pedestal the problem shows up at the sub panel, yet when he loaded the sub panel the problem did not appear. Also the L-L supply voltage again increased to almost 255V when the problem appears.
The actual L/L seems pretty constant. The imbalance still looks like a bad neutral but I wonder what the load is and what kind of protector he has. It is possible with a reactive load and a big inductor in the protector that you might be seeing a harmonic thing. I would address the neutral problem and see what you have then.
He has tried 2 different loads, one was the RV and the other was the surge protector alone with nothing else plugged in. Not sure if it's just a MOV surge protector or an actual EMS. Do you agree that, with the readings at the sub panel when the RV pedestal is loaded indicate the issue is upstream of the sub panel?
The numbers all make sense, the L-N numbers are all off by the approximate potential of 18 volts on the neutral. The L-G measurements are accurate since the EGC is at or near zero.
What doesn't make sense is the measurements taken at the sub panel. When the remote pedestal that is supplied by the sub panel is loaded, measurements at the sub panel indicate the open neutral is further upstream, yet when the sub panel is loaded directly in the storage building with a 120V load everything looks normal.
The only thing I can come up with is the problem is in the sub panel somehow got fixed when the 120 load was applied, otherwise the 120V load would not function. The sub panel contains only a single pole circuit for lights and plugs and the 2 pole 50 for the RV pedestal. The 2 pole was switched off when the sub panel was load tested on the single pole circuit.
If you look at the blue column below, the open neutral appears to be upstream of the sub panel, but looking at the green column now it appears to be between the pedestal and the sub panel. It's not making sense to me. Where is the break??
My wild second hand guess is you will find something wrong on the sub panel neutral bus. It is probably worth inspecting and retorquing the whole neutral run from end to end. I doubt it is a bad conductor, just a bad termination somewhere. (maybe more than one)
