Mcb keeps tripping, Why? Help!

[lees8110]

Hi All,

I have been working on a heat pump package unit that keeps tripping out the main c5o amp 3 phase circuit breaker, and I am struggling to find out why.

It only happens usually when on full demand for a couple of hours, its not over load its going out on short circuit because I monitored my clamp meter while it did it.

I first suspected the bearings in one of the motors were getting hot and ceasing up but I have observed most of the fans and they all seem to be spinning freely just before it trips.

I think the most logical explanation would be maybe a cable is chaffing somewere? and the fact when it is on full load vibration speeds it all up.

I will megger the cable from the unit back to the board as a next precaution but any help shared would be very gratfefully recieved!:)

[frank]

As it's a packaged unit, does it have an electric back up heater?

If so, have you tested this?

As you say, it only happens when on full load, and this is usually when the electric heat is brought into use.

[VRVIII]

Hi All,

I have been working on a heat pump package unit that keeps tripping out the main c5o amp 3 phase circuit breaker, and I am struggling to find out why.

It only happens usually when on full demand for a couple of hours, its not over load its going out on short circuit because I monitored my clamp meter while it did it.

I first suspected the bearings in one of the motors were getting hot and ceasing up but I have observed most of the fans and they all seem to be spinning freely just before it trips.

I think the most logical explanation would be maybe a cable is chaffing somewere? and the fact when it is on full load vibration speeds it all up.

I will megger the cable from the unit back to the board as a next precaution but any help shared would be very gratfefully recieved!:)
It could be the circuit breaker that’s faulty and tripping prematurely. I’ve had faulty MCB’s in the past.

[wigs]

I think that your problem could be a need to replace the C50 3 phase breaker. I have had this many times.

Has it got an inverter compressor? Inverter technology can and dose trip RCDs in my experience they sometimes trip, sometimes don't with no real pattern. has the power supply been change to an RCD recently. Had a site with a similar sounding problem. they changed the fuse board to one with a 100amp RCD trip, and every now and then the main power tripped, the problem was traced to the air conditioning it turned out to be that some RCDs are just too sensitive for inverter technology. The client reverted back to an MCB and the problem was no more. However I have installed many connected to RCDs without any issues.

I think that your problem could be replace the C50 3 phase breaker.

[Quality]

Could possibly be a phase imbalance on one phase I know you said you checked the current but what about the voltage, it could be dropping only 20V and the current could appear very similar across each phase - just a thought

[lees8110]

Its not an inverter driven compressor, So I will replace the mcb I think as a next precaution.

I have checked the electric heater but this seems ok also although I will disconnect this out of the loop too to eliminate.

I will let you all know how I get on.

Cheers
Lee

[marc5180]

Could possibly be a phase imbalance on one phase I know you said you checked the current but what about the voltage, it could be dropping only 20V and the current could appear very similar across each phase - just a thought

I had this the other day on a motor wired in star delta, it kept tripping on the overload which was set to 15a but when ever it switched to delta it was pulling 60a and triping the breaker.

Finally traced the problem to the isolator that had let water in and one of the phases on the isolator was losing 22v so 240 going in but 218 coming out which was enough to cause problems........

[frank]

I will let you all know how I get on.

Cheers
Lee

Any news yet Lee?