Hi guys

Just looking for a bit of advice. I had a breakdown today on a 3 phase system. The problem started with a burnt out contactor which is only running a DOL motor. I replaced the contactor and when i powered back up i noticed that the supply to the contactor from the isolator wasnt giving 400 volts across the 3 phases.i was between the first and second but not the first and third or second and third. I was able to trace it back to the main panel where i was getting 400 volts on all 3 phases across the bottom of the MCB but again i wasn't getting the three phases across the top of it. i was getting 400v between the first and second but not the third. I swapped over to a spare mcb in the panel and everything worked from there out and i was getting three phases all along the line and back at the motor and new contactor and overload i installed. I went back and with the load removed off the mcb i was getting 400 volts across the three terminals of the mcb which i thought was strange? If the mcb is faulty should it still have been showing less voltage across the terminals the same way it was when the load was connected? I have never encountered this before where an mcb has behaved like this. The failed terminal or phase shall we say was giving me 155volts instead of 400
Everything is running fine since i hopped the supply over to the spare mcb in the panel but the spare was a bigger mcb but all i had and i had to get the plant back up and running but when i can i will replace it back on to the correct amperage mcb.

 

Is the motor running at line capacity?
If so, maybe some kind of Star-Delta starter may be needed?
Max.

 

No Max it is just a DOL starter and it is a small motor too only 1.3kw. Have you ever seen such problems with the 3 phase mcbs? Oh and the motor doesnt run constant it is only turned on manually for pumping waste oil out to drain

 

Not heard of an MCB, but assume it’s your branch breaker. What’s your resistance on each phase, line to load, through the breaker? Contact resistance can cascade quickly.

 

By 'Burnt out contactor' is this the contacts rather than coil?
If so, what was the rating for this relatively low size of motor?
Max.

 

the motor overload was set for 3.5 amps this circuit breaker was powering more than i think it was rated for (20amps) i have a feeling it was tripping a lot before i had to repair it but i cannot prove it

 

I have had to replace circuit breakers that have had one phase fail. Single pole failures do happen, and sometimes just one phase is lost and the breaker does not trip, especially if the motor overload trips first. Why does one phase contact fail? Possibly because that contact opens first when the breaker trips.

 

No Max it is just a DOL starter and it is a small motor too only 1.3kw. Have you ever seen such problems with the 3 phase mcbs? Oh and the motor doesnt run constant it is only turned on manually for pumping waste oil out to drain

Have you sized the contactor correctly and is it fitted with correct O/L's ?
Never seen a MCB go open with lower than rated load?
Max.

 

Hi Max

Yes all the switchgear is sized properly according to manuals and meter readings. I think the problem was before i got there the circuit breaker was tripping a lot and just being reset over and over and not being reported which i think eventually lead to it being burned out i also found that this breaker was powering other lines as well and wasn't tripping because it was powering 2 phase inverter modules and therefore not needing the three phases/ Everything appears to be running well now and i have been checking it regularly with the meter to make sure. No trips or excessive amperage so it all looks good