Air con blowing 20a MCB a few weeks into box change in shop/commercial

[The Ghost]

Strange day, changed DB in shop couple of weeks ago from old wylex with cooked neutrals and burnt out neutral bus. Wiring was the usual sargasso sea of cables above suspended ceiling, old circuits, redundant circuits and really bad light switch arrangement. Pulled out the switch wires (six) and re-located into grids at reachable height and so on. Fully expected to be called back for RCD problems.
Got a call regards the air con MCB popping a couple of weeks in. Thought mhmm just megger cables (SWA) and flexible control cable from outside fan to unit. Not possible as could not disconnect or access cables on outside unit. I suspect the outside fan is jamming but can't even look at it as it is inaccessible. Do not really know about air con apart from the obvious general principles. Anyone up on air con? Its a Fujitsu in suspended ceiling quite old by the looks of unit.

 

[westward10]

When was it last serviced and what Type of protective device have you put on it.

 

[LankyWill]

If you can’t access it how can you be expected to diagnose the fault. Either sort out access or get the client to get a specialist in so he can service/fault find. Sounds like you’ve gone above and beyond with fuse board change anyway so the client can’t be too arsey.

 

[The Ghost]

60898 MCB 20a type B, it was on an HRC 20a. It is serviced every two years and is due in August to be serviced.

 

[The Ghost]

I did say he should get a service engineer in for air con as it was not my area. I only went to see if I could identify any obvious reason for the tripping in the supply cable. So he is going to get in the guy he uses.

 

[westward10]

Has it been okay on the Type B or have they just started using it.

 

[The Ghost]

It has been ok for a couple of weeks and then started popping off.

 

[AJshep]

I dont know much about Aircon but usually they need Type C MCBs due to high startup currents.
Shame you cant access it, they also usually have a max fuse size stamped on the rating plate too.

 

[The Ghost]

Checked all around the ceiling cassette in the shop no rating plate or any model info either. The MCB does not go off straight away it may do, but then it can stay on for several minutes, then only a couple of minutes very random in timing terms.

 

[westward10]

No guarantee to resolve it but I would stick a Type C on it and see what happens, aside from that they need an A/C engineer to look at it.

 

[The Ghost]

Ok sounds like a plan. They will have the engineer come in, will try to be there as well if I can. Appreciate the input all.

 

[westward10]

A Type C may well resolve it as you have twice × In tripping characteristics compared to the Type B.

 

[Marvo]

Unlikely to be a fan problem because they're usually supplied via a speed control board and fused internally.

Did you check the running current?

As per Westward, if you're not familiar with HVACR kit I'd suggest trying a C or even D curve MCB, assuming the cct can make disconnect times. If problem persists hand it over to an A/C techie.

 

[littlespark]

Can you see a model number on the outside unit, even if you cant access the connections? Google it and see what the ratings should be.
It maybe runs at just over 20A and the HRC was holding fine.

 

[plugsandsparks]

Older a/cs can be quite brutal on start up and of course the compressor drops in and out frequently. Best bet is to c or d curve and to max of cable capacity.