Welcome to the unknown...

[mattjcatchpole]

Okay so I'm on a site with a colleague of mine and we came across an Airedale DF60X2-EZ that has been intermittently tripping mcb 10.

We have replaced/swapped the following components:
Hp and lp switches
Condenser fan motor
Pco controller
Transformer
air flow switch
On/off module
both compressor contactors, overloads and aux contactors
Indoor fan motor
Both indoor fan motor contactors
Fan speed controller
The mcb itself
we have also megga'ed the clixons for both the compressors and the discharge sensors...

As you can tell, we are clutching at straws here!

The cable going into the mcb is from the transformer and feeds the block which supplies the 24v circuit. Everything we have checked is fine...

Can anyone shed some light?

Thanks peeps

[frank]

What is the type and rating of the MCB?

What is the maximum operating current of the unit (from the nameplate)?

Why would you megger the Klixon's on the compressors?

How did you megger the Discharge Sensors?

Have you megger'd the compressors?

[Brian_UK]

So you are saying that mcb10 on the 24volt control circuit, is that correct?

[The Viking]

Follow the wire my friend, always follow the wire...
The truth will be out there.

[The MG Pony]

Ask your self how does a circuit breaker work, and what is its function, go from there.

I'll throw a bone: they work on 3 principles, Current, time and temperature they all inter act with each other.

High current means less time for it to trip, raise the ambient this takes less current or a shorter time for it to trip.

Under sized wires have voltage drop means for more current as an example

I all so didn't read of you replacing the circuit breaker! as they can become weak over time.

[install monkey]

10413373_252880418236070_1484131287118667676_n.jpg:d...........

[gsracer]

Follow the wire my friend, always follow the wire...
The truth will be out there.

But which wire Viking ;-)

[Hamza Alg]

Disconnect components (loads) each a time and see if the Mcb will trip until you find it. It works every time,
No need to check switches like klixon, switches are not loads and they will never trip any mcb. If a switch fails the unit will not run. If anything trips you need to check the loads eg solenoid, compressor, fans, crank case heater, evap heater ..ect
Also have you megged the compressor?

[install monkey]

from original post
"The cable going into the mcb is from the transformer and feeds the block which supplies the 24v circuit."
thus its on the control circuit, it could be "a switch like a klixxon" as they are on the control circuit, they will not pull a load but they are capable of failing

[Hamza Alg]

So if a switch like klixon fails, the circuit will break and stops the compressor. But why would that cause mcb to trip?

[install monkey]

grounding if its faulty

[Hamza Alg]

thats true, i remember i had an isolator grounded and tripping the breaker.

[nike123]

Grounding 24V control circuit cannot trip fuse because transformer is isolated from grounding, therefore, circuit cannot be closed through grounding. Same thing as isolation transformer at your bathroom for shaving machine. Also, that is the reason why it is transformer installed to control circuit.

It is overloaded by some of the components or wiring is faulty or fuse is of wrong type or wrong size or wrongly dimensioned.

To OP:

First check that all wiring is as on wiring diagram.
Remove ALL components and transformer from control circuit and leave only wiring and terminals, than masure resistance with multimeter, and if it is infinite, meger (100V) wiring between two poles starting from transformer then each branch of control circuit wiring.

[Hamza Alg]

Matt, any update on this fault?