Three Phase Air Con Unit On Single Phase Mcbs-Help

Hi ,

New to forum and could do with some advice.

I recently was asked to temporarily isolate some external air con condensers to allow some roofing repairs to take place.

On opening the board I found it had no ability to accept three phase MCBs and had been connected to three individual single phase MCBS (correctly configured) across the three phases.

 Recognising the danger behind this I have refused to reconnect the units and this was understood and agreed with the maintenance company in the building concerned.

Temporary air con units have now been hired in to cool the areas concerned and a price submitted to change the pan assembly in the switch panel to accept triple pole breakers.

My problem is that the FM company overseeing the building have got involved asking how the configuration breeches regulation and asking me to quote specifics.

Though I think this is obvious I am not sure which regulation(s) it breeches.

Any advice appreciated as this guy is a bit of a know all and is trying to blind me with science

so what would you have done if it was 3x BS88's?

+1

Although if used as an isolator isn't it meant to be able to isolate all the live conductors at the same time....

Aren't BS88 fuses normally in a 3ph enclosure with a 3ph isolator?

I would love to see a picture of a 3 phase board that is incapable of accepting a 3 pole MCB, and requires a replacement "tray" to enable them to be fitted. As a 3 pole MCB is usually just 3 single pole ones riveted together, with the actuator commoned together, I can't actually imagine of one that can't accept a 3 pole mcb. I have certainly never seen one so please enlighten us.

I have as a temporary measure (to keep a machine running) connected a 3 phase load to three single pole mcb's, but only while I go off and order the correct 3 pole mcb then go back to swap it.

NozSpark said:

+1

Although if used as an isolator isn't it meant to be able to isolate all the live conductors at the same time....

Aren't BS88 fuses normally in a 3ph enclosure with a 3ph isolator?

Click to expand...

Plenty of old BS88 and indeed rewireable boards, fed each phase from one fuse, with only one isolator for the whole board. So it was possible, indeed common to drop one phase.  

And phase sequence / failure relays were not common then either.

Rough as a Badgers, yes, illegal, no, against reg's, not one I can think of off the top of my head, apart from 132-134 groups, and that is not specific.

As has been suggested only perhaps Manuf' Inst. thus reg 510.3.

There may be a reg that says that you can't do it, but I can't think of one.

Again as has been suggested 3 separate BS88 fuses would offer the same protection.

Labelling would however, need to be exemplary to ensure that the work was safe.

The only situation I can find that it would DEFINITELY not be allowed would be under external influence BE2.

three lock off devices needed and exemplary procedure.

just wondering if you can get a bar to link the MCBs from somewhere? Failing that, think I would look at a 3 phase isolator rather than change the board components, (though I also don't understand why that is necessary), exemplary labelling, and for arguments with others point out that it isn't designed to run on single or 2 phases, so risks blowing the machinery.

Or add a phase failure relay and contactor into the machine?

how much are those then?

At our old HQ site  the TP Aircon units were on BS88's 

The hotel in Manchester has several TP loads, that are coming from Ottermill DBs, with individual OCPDs - not healthy - but that is how it used to be.

( look at 4th or 5th pic here

http://talk.electricianforum.co.uk/topic/9428-some-pics-from-that-hotel-pir/

Hi all,

In a big rush to go out, so have not got time to go looking things up, BUT......

What about EAWR? Thought this included words to the effect that you had to take all reasonable precautions to prevent people from danger. Yes i agree about three BS88's. but these would be in a situation where you would be expecting to find them. I know labelling has been suggested, but say you affixed a notice to the dashboard of your car saying, "the brakes on this vehicle do not work as expected" that would hardly do would it.

I would be sorting the situation out if for no better reason than if one trips, the rest of the contraption will go up in smoke....

About 40 years ago i saw a compressor where one phase had dissapeared. Oh yes, there was lots of smoke, well, steam actually [it was kept and used outside] coming out of that one!!!!!

Anyway, well done to the original poster for standing up and making his concerns known.

Still, would like to know how a three phase board cannot take a TP MCB

Pictures please!!!!!!

john..

NozSpark said:

+1

Although if used as an isolator isn't it meant to be able to isolate all the live conductors at the same time....

Aren't BS88 fuses normally in a 3ph enclosure with a 3ph isolator?

Click to expand...

What about a 3ph DB?

Hiya,

Reg 431.1.1 (page 79 in your green book) might help in any FM discussions as this specifically mentions the case of three phase motor protection. It's not an absolute, but gives more strength to your argument.

New Venture! said:

Hiya,

Reg 431.1.1 (page 79 in your green book) might help in any FM discussions as this specifically mentions the case of three phase motor protection. It's not an absolute, but gives more strength to your argument.

Click to expand...

GOOD FIND!

I could not have recalled that, so, in fact that could be used to prevent the use of separate 1ph breakers or individual BS88 fuses for a balanced 3ph load, if the use of these could cause damage or danger.

Considering the undetected loss of one phase on a running 3ph motor might not cause an issue, then this could be extrapolated to require linked breakers on ALL 3ph motors that come under BS7671...