Strange Supply Fault

[Iggifer]

So we got called out to a cake shop/bakery today with a new 3P+N oven that wasn't working.

Tested all 3 phases to E/N - 230V

Testing phase to phase
L1-L2 - 0V
L1-L3 - 415V
L2-L3 - 415V

Clearly not a phase down, but seemingly L1&L2 connected to the same phase somewhere.

Definitely a 3P+N supply - Head, meter, isolator all standard 3P+N so not a 2P+N that somebody has tried to convert to 3P+N.

Can only assume the DNO have done this deliberately somewhere to get around a problem and forgotten about it. Can't imagine they've accidentally connected two cables to the same phase and if it was further back then presumably somebody would have noticed this before now (her new oven hasn't worked for about a fortnight since she acquired it)

Anyone ever come across this before?

 

[JohnW2]

Do I take it that there is no other 3-phase equipment which is (or has been) working normally?

Where did you measure those voltages - at the feed to the oven or somewhere more 'centrally'?

Kind Regards, John

 

[Iggifer]

Do I take it that there is no other 3-phase equipment which is (or has been) working normally?

Where did you measure those voltages - at the feed to the oven or somewhere more 'centrally'?

Yea just one 3P MCB feeding the oven, the rest are all SP.

Tested it at the isolator to begin with and worked our way all the way back to the suppliers isolator. Same fault all the way back.

Just spoken to my boss and WPD have identified the fault and are digging up the road as we speak. They’ve promised her phase restoration by 9am or they’ll get her a generator in so definitely they’re fault.

I just can’t see how you could accidentally connect two phases to the same core in a piece of wavecon but that must be what’s happened

 

[JohnW2]

Just spoken to my boss and WPD have identified the fault and are digging up the road as we speak. They’ve promised her phase restoration by 9am or they’ll get her a generator in so definitely they’re fault.

Fair enough.

I just can’t see how you could accidentally connect two phases to the same core in a piece of wavecon but that must be what’s happened

It does seem very hard to understand. I wonder if there is something else they could have done, which we haven't thought of, that could produce the same result?

Kind Regards, John

 

[Guitarguy]

What could of happened is that a fault on the cable has caused two of the phases to weld together so that at the substation -
R- fuse healthy, red core healthy
Y- fuse blown, yellow core at red potential(because of fault)
B- fuse healthy, blue core at blue potential.

So all three cores are live, just R+Y are at the same potential. This sort of fault is only noticeable to customers with three phase equipment.

It can also be done deliberately to restore supply’s to single phase customers by temporarily strapping a dead core in the faulted main to one of the live cores to restore supplies.

 

[JohnW2]

What could of happened is that a fault on the cable has caused two of the phases to weld together so that at the substation -
R- fuse healthy, red core healthy
Y- fuse blown, yellow core at red potential(because of fault)
B- fuse healthy, blue core at blue potential.

Wouldn't two fuses be likely to blow if two phases got welded together?

Kind Regards, John

 

[Iggifer]

That sounds like a reasonable explanation.

Also seems like a risky ‘fix’ when there are 3P+N supplies on that supply.

John, no because once one fuse has blown, L1 and L2 are simply now L1. The connection to L2 has now gone.

 

[JohnW2]

John, no because once one fuse has blown, L1 and L2 are simply now L1. The connection to L2 has now gone.

Yes, but if enough current flowed between L1 and L2 to blow the L2 fuse, the same current would presumably have had to flow through the L1 fuse, wouldn't it? Admittedly, one fuse might blow before the other 'had a chance' to, but I wouldn't think that would be a foregone conclusion - particularly if (as I imagine would be the case) the current were very high.

Kind Regards, John

 

[Iggifer]

Yes, but if enough current flowed between L1 and L2 to blow the L2 fuse, the same current would presumably have had to flow through the L1 fuse, wouldn't it? Admittedly, one fuse might blow before the other 'had a chance' to, but I wouldn't think that would be a foregone conclusion - particularly if (as I imagine would be the case) the current were very high.

Kind Regards, John

Oh I see what you mean. Yea it could be one of those either/or situations.

Or as Guitarguy has suggested, they’ve had to replace at least one of the fuses to get those two phases back on line.

I don’t suspect this is the case in this situation though because it would appear they were unaware of the fault until I reported it this afternoon. If there had been intervention on their part they would have been investigating the fault.

And from the limited information I’ve been given this evening, they do seem to be digging directly outside the shop, which would lead me to believe it’s a connection error vs a wavecon dead short

 

[JohnW2]

Oh I see what you mean. Yea it could be one of those either/or situations.

Yes, that's what I meant.

I would suspect, although don't know, that when the current involved is much higher than the current needed to 'operate' the device, with two (identically rated) fuses in series it is probably more likely that both will blow than is the case when one has two (identically rated) 'breakers' in series - but, as I say, I don't know if that 'intuition' is true.

And from the limited information I’ve been given this evening, they do seem to be digging directly outside the shop, which would lead me to believe it’s a connection error vs a wavecon dead short

If you ever find out, we'd obviously be interested to hear!

Kind Regards, John

 

[Guitarguy]

The scenario I suspect is that the main has been faulty a while and wasn’t known about until the OP needed the three phase supply.
As for two fuses blowing, yes that does happen, you will have red and yellow blown for example and on replacement of the red, the yellow comes up live due to the fault.

 

[winston1]

Something wrong with your meter as well. If phase to neutral is 230v, phase to phase should be 400v. On the other hand if phase to phase is 415v, phase to neutral should be 240v.

 

[DetlefSchmitz]

Yes, well... It's close enough for government work.

 

[SUNRAY]

Something wrong with your meter as well. If phase to neutral is 230v, phase to phase should be 400v. On the other hand if phase to phase is 415v, phase to neutral should be 240v.

Unless there is a fault and neutral is in the wrong place.

We had one where motor overloads were tripping and one phase was in the wrong place, approx 90°/150°

 

[JohnW2]

Unless there is a fault and neutral is in the wrong place.

Indeed. I would think that there are some situations in which all bets about the 'expected' relationship between phase-neutral and phase-phase pds are 'off'!

Kind Regards, John