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

The Ghost · Jun 24, 2018

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.

GBDamo · Jun 25, 2018

The fact that it is not tripping on start up rules out the MCB. Fitting a C or D type would be masking a problem that will sooner or later start tripping a C or D

Get a clamp meter on the live at the CU and look how it’s drawing current. If you see the start up spike followed by a drop off then a slow creep back up until trip then it’s probably one of three things.

1, Bad run cap, or relay stuck in the soft start.

2, build up of detritus on the outside coil,

3, low gas.

It’s highly unlikely to be the fan or the compressor.

But to discharge you’re responsibility just clamp the feed and demonstrate it is drawing more amps that design.

ipf · Jun 25, 2018

Pete999 said:
Think maybe you are giving Fridge Mech the wrong signals IPF, it's what they have bee taught to do, up to the likes of us to put them right, OK they may be using inappropriate cabling, but then again so do many new Sparkies don't they, I have had Fridge Mech come away with me on trips overseas solely because they know far in excess of my understanding of A/C systems.

Only a point, Pete. I just don't like completing an industrial/commercial kitchen, all to spec, only to return and find SY strung around, including inside DB's, after they've been in to do their little bit. Not always, but sometimes, they seem exempt because they're coming in last. Please note, most of these are not engineers, just installers.

kevincoolnomix · Jun 30, 2018

Vortigern said:
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.

kevincoolnomix · Jun 30, 2018

Pete999 said:
Any of the filters blocked? this can cause the compressor to work overtime, what temp does the unit produce at the evaporator outlet?

The problem is likely doe to compressor short cycling. This is due to either a lack of refrigerant or (more likely) the condenser fins outside are blocked and the unit therefore cannot get rid of enough heat (a bit like a car radiator overheating). As a result the compressor will shut down on high temperature/high pressure and then attempt a restart while there is still a significant differential pressure across the compressor. The resulting load is just too much for the MCB and it will trip. The solution is first to clean the outside condenser fins and then top up the refrigerant (as a lot of this will have vented to atmosphere through a high pressure relief valve).

Pete999 · Jul 1, 2018

kevincoolnomix said:
The problem is likely doe to compressor short cycling. This is due to either a lack of refrigerant or (more likely) the condenser fins outside are blocked and the unit therefore cannot get rid of enough heat (a bit like a car radiator overheating). As a result the compressor will shut down on high temperature/high pressure and then attempt a restart while there is still a significant differential pressure across the compressor. The resulting load is just too much for the MCB and it will trip. The solution is first to clean the outside condenser fins and then top up the refrigerant (as a lot of this will have vented to atmosphere through a high pressure relief valve).

Never seen an A/C system with a built in refrigerant relief valve that vents dangerous refrigerants cfcs to atmosphere, as it's a sealed system, more likely to be a leak via a loose or bad joint. A properly installed A/C system will have been filled with refrigerant, using a gauges and scales. Most A/C systems have HP and LP controls which will shut the unit down due to either condition, and allow the refrigerant pressure to equalize, NOT vent it to Atmosphere.

Pete999 · Jul 1, 2018

ipf said:
Only a point, Pete. I just don't like completing an industrial/commercial kitchen, all to spec, only to return and find SY strung around, including inside DB's, after they've been in to do their little bit. Not always, but sometimes, they seem exempt because they're coming in last. Please note, most of these are not engineers, just installers.

Sorry for the late reply ipf, I know how you feel, looks bluudy awful doesn't it, you could say, well I didn't do it, but being the Electrician on site, it will almost always be attributed to you, awful stuff, cable tied every where. As you say Installers not engineers.

The Ghost · Jul 1, 2018

kevincoolnomix said:
The problem is likely doe to compressor short cycling

Thanks for the input @kevincoolnomix. Air Con is not within my skillset and it is a discipline outside of my ken. I was only there to rule out the supply circuit contributing to the problem. As already stated, the manufacturers told me that the unit should be fused to allow for a draw including inrush currents of 61-80a. So at 20a that was not going to cut it. I have in fact put in a 32a MCB type B. Although I had a type C just in case I was loathe to put it in as the EFLI may not have complied with a type C. At this time I cannot check the Zs as I can not get to the outside unit (furthest point) to check such things. I have an arrangement to get there but it is when the people upstairs grant access to measure Zs to ensure it complies. So far the 32a MCB has not switched off and the unit is working fine.

Toneyz · Jul 1, 2018

What size cable is it? we always fit C type MCB's based on the label plate on the unit and have no problems, is this one with a feed to an isolator at the outside unit. As you know what Ze is you could take a good guess of the cable run and work out R1 + R2 to give an approx EL I for the MCB. The shop should have got access for you originally so that you could have tested prior to energising.

Pete999 · Jul 1, 2018

Vortigern said:
Thanks for the input @kevincoolnomix. Air Con is not within my skillset and it is a discipline outside of my ken. I was only there to rule out the supply circuit contributing to the problem. As already stated, the manufacturers told me that the unit should be fused to allow for a draw including inrush currents of 61-80a. So at 20a that was not going to cut it. I have in fact put in a 32a MCB type B. Although I had a type C just in case I was loathe to put it in as the EFLI may not have complied with a type C. At this time I cannot check the Zs as I can not get to the outside unit (furthest point) to check such things. I have an arrangement to get there but it is when the people upstairs grant access to measure Zs to ensure it complies. So far the 32a MCB has not switched off and the unit is working fine.

What is the distance fro the inner unit to the outside unit?

Toneyz · Jul 1, 2018

Pete999 said:
What is the distance fro the inner unit to the outside unit?

Not knowing much about A/C when we installed supplies to these outside units the A/C guys lashed in (sorry installed)small flexes to the indoor unit is there not internal fuses protecting these?

Pete999 · Jul 1, 2018

anthonybragg said:
Not knowing much about A/C when we installed supplies to these outside units the A/C guys lashed in (sorry installed)small flexes to the indoor unit is there not internal fuses protecting these?

On SP units the compressor (outside) unit provide a SP supply plus control wiring, usually protected at the outdoor unit,

The Ghost · Jul 1, 2018

Pete999 said:
What is the distance fro the inner unit to the outside unit?

20m I guess, and yes I was going to calculate R1/R2 but thought no there is nothing like measuring it physically. The SWA is 2.5m² 3 core. The control cable coming back is possibly 1.5/2.5 at a guess.

The Ghost · Jul 1, 2018

anthonybragg said:
The shop should have got access for you originally so that you could have tested prior to energising.

Beleive me when I say access was/is impossible until the guy upstairs lets me in.

Toneyz · Jul 1, 2018

How do you know by putting a 32A MCB complies if it is only PVC SWA it's borderline.

plugsandsparks · Jul 2, 2018

Pete999 said:
On SP units the compressor (outside) unit provide a SP supply plus control wiring, usually protected at the outdoor unit,

Pete999, as mentioned previously, i am a/c refcom certified installer / maintainer. I mentioned the air off simply for the OP to stick his hand near it perhaps with a thermometer, not start to be a refrigeration engineer, politely you have some knowledge of a/c but its either out of date or you have not experienced the full spectrum. Modern systems manipulate pressures and have quite robust protections in place, air off is very reliable. Many systems now are VRF the indoor unit is not powered from the outdoor unit, just saying