A phase is only giving 41v im told

[Rather be Fishing]

Hi guys

I know I don't exactly contribute much but can anyone maybe stop me killing myself tomorrow.

I know to do the 10 point 3 phase dead test

And I know how to test each phase.

I've had the job below come in and I wondered if anyone can give me some advise.

I've never had a job like this before!

 

[Lucien Nunes]

The most likely scenario is that the 41V is just induced voltage from the phases that are still OK, and in fact the bad phase has no supply at all. A multimeter is sensitive enough that it will give some kind of a voltage reading with induced voltage, but there's nothing very significant about the actual reading, whether it says 20V, 41V or 100V. It's just detecting that a tiny amount of current is being coupled into the cable from others nearby. A low-impedance tester or test lamp probably wouldn't show anything at all.

So you are probably looking for a simple loose connection, isolator not making contact in one pole, MCB with one pole not screwed to the busbar etc. Or, if the supply comes via fuses, one fuse blown.

 

[Rather be Fishing]

Thank you @Lucien Nunes that is really much appreciated mate. I genuinely thank you for passing that information/knowledge onto me.

I'm there 100 percent tom so at least I can have a look with a vague idea of what could be he issue.

Thanks again @Lucien Nunes

 

[Rather be Fishing]

Morning guys

Well I'm out at the location of the ac unit that isn't working today

Everyone's probably at work so unable to reply but you never no there might be someone about

So below I've got some pics that I've taken

The first is of the board feeding it that I have tracked down

The second is the legion telling me that 7 and 8 l1,l2,l3 are the breakers feeding it.


The third shows the insolators for each of the ac units. I'm not quite sure what is beneath that white panel. Is seem to have been riverted on though!!

And the forth is of the acunit itself. Stood right beneath it looking up.
This is a whole new experience to me but im guessing underneath that silver plate held on by 2 screws will be my incoming supply to the unit.

 

[Rather be Fishing]

Thanks to @Lucien Nunes my plan is as follows.

Go to the main panel in pic 1 and carefully take of the cover. I would flick the main switch to kill the power to it but there are other circuits on there like meat chiller that would probably sound an alarm if power is lost.

I'll use my insulated screwdrivers to tighten up the mcb to the buzz bar. If any are loose going on the advise that might have been my problem. If they are all right I'll put the cover back on and move over to the isolater ruling out loose buzzer connection.

One at the isolater I'll open it up and have a look.

I'm not sure if I'll be able to visually see if the isolater is not making contact in one pole but if I can find out where the supply cable comes into the ac unit I'll be able to see if it's the isolater as on one of the poles I'll be getting a low reading.

If there are no loose connections

Could the mcb feeding it be dodgy.

Going on the above

If the terminations are all tight, I should probably look to change the isolater.

I haven't got an isolater on me but seeing as there is one next door maybe I could temp disconnect that and use it to confirm if it's a brocken isolater on as I know that one works as the other ac unit is on

Does that sound reasonable/logical

 

[Lucien Nunes]

No, don't start at the busbar screws! Don't work live unless absolutely necessary, and you don't yet know whether it's necessary.

I'd start at the isolator because it's easiest and safest. If all three phases are OK on the supply side then there's probably no need to touch the board at all, therefore no need to work live. If all three phases are OK at the load side of the isolator too, then the problem is likely at the A/C unit itself.

If however there is a phase missing at the supply to the isolator, you need to do a risk assessment about working live in that board. If you deem it safe to proceed, check the output of the MCB first. Touching the busbar side of things is an absolute last resort.

Note - busbar (bus as in 'omnibus') not 'buzz bar'