Discuss Air con blowing 20a MCB a few weeks into box change in shop/commercial in the Electrical Forum area at ElectriciansForums.net

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T

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.
 
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.
 
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.
 
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.
 
A Type C may well resolve it as you have twice × In tripping characteristics compared to the Type B.
 
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.
 
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.
Any of the filters blocked? this can cause the compressor to work overtime, what temp does the unit produce at the evaporator outlet?
 
Is there not a DP isolation switch?
Not that I can see but can't get to outside where the outside unit is which is where all the business is like compressor and heavy duty fan.
Thanks again for the input it appears you are all spot on! I spoke to tech. dept. today for Fujitsu. They told me it is an obselete fifteen year old model and as stated by @plugsandsparks are brutal on the inrush side. I was told the initial draw is at least 61a so undersized on the 20a they recommended type C at least 25a or 30a. They also said it has probably been regassed with a substitute gas reducing the efficiency by up 30% considering age of the components and the gas is banned (R2 gas) since 2014 and re-filling it with that would attract a hefty fine/imprisonment! There was no info on the exact model but the guy seemed to know it well and was great, really chatty. So I have learnt a few things which is great. And thanks all again.
 

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