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.

 

[Pete999]

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?

 

[Toneyz]

It needs a C curve MCB due to an inductive load.

 

[littlespark]

Here cometh the "But it worked before you touched it" bolux from the customer

 

[Octopus]

Is there not a DP isolation switch?

 

[The Ghost]

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.

 

[The Ghost]

Any of the filters blocked

Pete I don't know to be honest. It is due for service so no doubt that will be checked but that is outside my skillset and competence so I left that side of it all well alone.

 

[The Ghost]

Here cometh the "But it worked before you touched it" bolux from the customer

Yep had that already from the customer. Basically due to lack of info on rating. I have underrated it. But all's good as now I know I can fix that and no cost to customer just a small amount for the type C MCB.

 

[Pete999]

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.

The only reason any refrigerant is required, is if there has been a leek in the pipework, as for the excuse replacement gas that's horse poo, the /C equipment will have been designed and set up for a particular Refrigerant, I think they are talking about R12, which has now been outlawed, almost like the garage telling you that a top up of Gas is required, more BS if the system has been installed properly the the refrigerant, should last for the life of the A/C unit, far to much BS spoken about refrigerants.

 

[plugsandsparks]

I do a/c , if its been re-gassed, will be probably from squirrel stock. Pressures are very low on the old stuff so a small leak can exists but take years to have an effect and they are hard to find hence many a/c engineers top up the gas. Easiest way to test it in your situation is with a thermograph temp device like the Flur, Set a/c to cold, max fan and measure temp of air coming off, anywhere between 5-10 degrees is what you want to see.

 

[Pete999]

I do a/c , if its been re-gassed, will be probably from squirrel stock. Pressures are very low on the old stuff so a small leak can exists but take years to have an effect and they are hard to find hence many a/c engineers top up the gas. Easiest way to test it in your situation is with a thermograph temp device like the Flur, Set a/c to cold, max fan and measure temp of air coming off, anywhere between 5-10 degrees is what you want to see.

What you need is a refrigeration High and Low Pressure gauge set thermometers etc to set up an A/C system relying on temp off is a quick fix method, what people don't seem to understand is that, is that HVAC is a Trade on it's own, don't muck with it if you don't understand the dangers, and believe me it can be as dangerous as a Chippy , Plumber messing with electrical work, I was lucky I took many A/C courses, but I would never in the world consider myself a competent Fridge Mech.