detecting 230v circuit supplied from two seperate phases

Talk Electrician Forum

Help Support Talk Electrician Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

DaveS79

Well-known member
Joined
Mar 20, 2017
Messages
238
Reaction score
18
Location
London
Hi everyone

Ive been helping out with some testing on a job on at moment. Today noticed a plug from a fan was overheating and melting socket. This appliance is plugged into rcd protected sockets which are fed from electrak run underneath floor. At moment everything points to faulty appliance although was informed this had been recently pat tested although there was no sticker on it. I tested sockets and confirmed only 230v present. MCCB rating is 32A and wiring is 6mm both phase CPC. This did get me thinking though that in circirmstances where electrak or for that matter any other circuit is incorrectly wired and a phase (say L1) from one circuit is incorrectly cross connected into another circuit fed from (say L2) thus creating  400v on circuit 2 at what point in testing process should this show up. If R1 + R2 results seem quite satisfactory with the length of cable that you have been advised is wired in. I cant see how an IR detects this and to be honest I'm not sure if ZS test does either (would appreciate if anyone can clarify this) . Thanks



 
erm... what makes you think you have 400v L-N because a plug has over heated / socket melted? its a very common issue caused by dodgy contacts / loose connection

 
Confirmed 230v with both MFT with plug adapter to L - E and L-N. I'm aware its very common for plugs to overheat because of bad connections but it did cause a bit of suspicion that this has happened soon after circuit gone live. Like I say have proved 230v but more out of curiosity than anything I still cant see how a circuit wired across phases is obviously detected in testing process. Probably missing something silly but better to look dumb than be dumber not to ask

 
if it was 400v then the heater would have lasted seconds and the plug wouldnt have burnt much.

your R1R2 would confirm that live & earth are connected. R1Rn (or R2Rn) would then confirm live & neutral are connected and polarity is correct. this would highlight any problems before IR

and if you do somehow manay to wire a socket between 2 phases then something is seriously wrong in the first place

 
Not really answered the question Andy R1 + R2 and R1 + RN Proves polarity but nothing else. Other than saying it will go bang or melt when powered up which I'm obviously trying to see how testing avoids this. It isn't a socket I'm asking about Its large electrak circuits that multiple people could be involved in wiring and could easily have incorrectly wired and terminated into wrong circuit. Once most of floor is down it can become almost impossible to estimate circuit lengths and so calculate expected R1 + R2. I suspect only dead test that can really prove 100% is IR between phases but then if circuits supplied from an already live board (as in this case) this test cannot be done anyway

 
if you have correct polarity, in that you have verified that live neutral & earth are all there and in the correct places, then you cannot have it wired between phases or your testing would have shown a problem

if you connect your live into the 'wrong circuit' then its still only 230v L-N, exactly the same as if you wired it to the 'correct' circuit

IR cannot be used to check if the wiring is in the right terminal.also, you dont suddenly loose all ability to do any dead tests once the board is switched on. you know that switch to turn it on? well it can also switch it off again. you have a serious lack of knowledge of I&T. go have a read of GN3 for a start

 
This did get me thinking though that in circirmstances where electrak or for that matter any other circuit is incorrectly wired and a phase (say L1) from one circuit is incorrectly cross connected into another circuit fed from (say L2) thus creating  400v on circuit 2 at what point in testing process should this show up.


So basically 2 live (or line, or phase, or whatever they are called nowadays) cables are connected into one MCB, or you mean a live from a circuit on L1 and a live from a circuit on L2 are connected together somewhere down the line?

Neither situation would end up with 400V being on a single circuit, the former would still work fine, the latter would just make a lot of things go bang and pop.

 
So basically 2 live (or line, or phase, or whatever they are called nowadays) cables are connected into one MCB, or you mean a live from a circuit on L1 and a live from a circuit on L2 are connected together somewhere down the line?

Neither situation would end up with 400V being on a single circuit, the former would still work fine, the latter would just make a lot of things go bang and pop.
Oh yes, I remember it well, I used to do a lot of shopfitting, big supermarkets and stores, one day me and the lad got sent to this store just outside Nottingham, they'd got behind with the work and decided to run 24hrs to catch up. We arrived and it had all been 1st fixed and was ready for 2nd fix, now because we were only helping out I told the lad to just fit accessories, leave the 3 phase board to Dave, it was his job after all.

We went round and fitted a load of switches and sockets and all he had to do was a few lights and the mains. We were driving back home and the lad's phone rang, it was Dave and he was ranting and raving, calling me all the names under the sun. Apparently he'd connected his ends into the board and was powering up when things started to go bang. He'd a ring connected to red 1 and it powered up fine, when he closed blue 2 there was a bang and it and red 1 tripped! , this had happened with several of the circuits and it was my fault!

I got on the phone and had a right go at him, how was it my fault when we'd only fitted accessories? I asked how he'd marked the cables, I knew they were marked up but wondered how he'd done it. Apparently he'd been on the roof of this unit, it was a building within a building, and after pulling a load of cables in, he'd cut them off the coils, "remembered" which was which, and pushed them down the trunking once inside he'd wrote on them with a pen, obviously anyone who can take about 10 or 12 identical cables, unmarked and remember which is which is a better man than me!

I pointed out that if he'd done the dead tests, end to end etc, that he'd have found the problem before connecting up, he replied that he didn't need to as "I've wired it so I know it's correct". I couldn't decide which was worse, his arrogance or his stupidity!

 
Top