So here’s one that I have been working on for the past few weeks. I was called out to a booster station where the main breaker in the outdoor switchboard keeps tripping on Ground Fault when the customer tries to start one of the pumps. When this happens, the station back-up generator comes on and the pump then tries again to start. The pumps successfully start every time on generator power.
I connected my Dranetz to the MCC and monitored the power hoping to catch one of the instances in which this is happening and we did. Funny thing, in going through the report I did not see anything during the start
cycle (on utility power) that would lead me to believe there was a ground fault issue. I did however see a huge ground current event when the power is transferred to the generator, more on that later.
Due to the fact that I am not seeing anything unusual on the input of the Motor Control Center (which also has a Solid State Breaker with GF), I can only assume that the issue is happening somewhere between the utility transformer to the outdoor switchboard, or to the transfer switch that is also connected to the MCC and the generator. We decided to go to the site and pull some covers to try to get some answers. Here is what we found.
The MCC main breaker is equipped with GF in the trip unit. The ground is picked up via a current transformer located on the neutral. This is not connected as the contractor did not pull a neutral into the building. There is
a ground but no neutral. The ground is connect back through the transfer switch and heads out of the building to the main switchboard. We go out and pull these cover and there we find that the Main Breaker here (which
happens to be the one tripping on GF) is also a breaker that picks up GF via a current transformer located on the neutral bus which is connect to the ground bus by a neutral-to-ground jumper which is installed. The problem
here is that the grounds are all tied to the ground side but yet again, there is nothing connected on the neutral side thus making it impossible for current to flow across the bus portion that contains the CT.
The only thing we can point to at this time is that the trip unit in the main breaker located in the Switchboard must have a problem.
More on this to follow…..