Here is the deal, for long time we have had a customer that has some breakers that have been tripping.
Here is the panel I am dealing with, it has a 200 amp main breaker at the top and down at the bottom you can see two - 3 pole breakers, a 70 and a 100.
The other night I went there and the 200 amp main, 100 amp feeder and 70 amp feeder were all in the tripped position.
Below is the main breaker.
Here are the two 3-pole breakers
The 200 amp breaker supplies the panel from an ATS right beside it. In turn the ATS is supplied from switch-gear very close by. I would say the supply is very stiff, over done in house engineering. (Not a complaint, just saying they use a lot more copper than is likely required)
The 100 amp breaker supplies a panel 400'-500' away in a separate structure. This is an underground run, around 500 Kcmil copper in PVC. The load in less than 20 amps on all three legs. The panel supplies LED egress and exit lighting, a fire alarm booster panel and other small loads.
The 70 amp breaker is the primary supplying a transformer directly above the panel, lets say 15' to 20' of conductor from breaker to transformer. I did not check but suspect the transformer is a 30-45 kVA unit. 480-208Y/120.
We all suspect the underground run is failing, each time there has been a trip over the course of maybe two years and five or six trips the ground has been wet or moist.
I did use a basic mega meter on it and it went to the end of the meters scale.
Monday night I am meeting a testing company on site to do some type of advanced testing on the conductors.
Now here is the real question I have.
Regardless of the fault being on the 100 amp breaker or the 70 amp breaker why are they both tripping?
I understand the main and one feeder going to the tripped position at the same time but why are two unrelated feeder breakers tripping at the same time?
This has happened each time, always both breakers tripped.
Grasping at straws while there the other night I decided to move one breaker away from the other on the thought one was somehow tripping the other?? Magnetic field?? Just the sharp rap of one breaker tripping causing the other to go??? I have no clue but I did move the breaker.
Anyone have some insight? A technical reason for this? Poltergeists?