Phase to Phase fault


Postby wws4029 » Sun Apr 12, 2009 1:03 pm

I have been told that a friends electrical system has a phase to phase fault on a circuit on one of his boards. What does it mean and how is it caused?
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Postby kbrownie » Mon Apr 13, 2009 10:55 am

Depends what the fault is, is it a loss of power or an insulation resistance fault.
What is the system used for?
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Postby BLAKEY1963 » Mon Apr 13, 2009 11:07 am

WWS4029
PHASE TO PHASE as you say , could be on a 3 phase system
where the voltage is usally 400 volts. To have a fault between
them is a high dissapation of fault current , and shock risk.
An electrcian is needed to , isolate immeadiately , and investigate cause . not a diy job

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Postby wws4029 » Mon Apr 13, 2009 3:03 pm

Right, I have spoken to him again. The fault has come up on an inspection report.The system is a 3 phase to main D/B then from there goes out to 7 other D/B's ( in single phase, small hotel ) It says on report phase to phase fault on circuit 10 D/B 1 ( furthest away from main D/B.) On closer look the fuse is a 32mcb feeding an immersion heater ( or at least thats what label says)

All it says on report is phase to phase fault on D/B 1 circuit 10.

All 7 boards are 240V

Immersion heaters are very large commercial ones which are pressurised and have 2 heaters each.

Not going to attempt anything, just asked what it meant and I had no idea. Any explanations would be gratefully received.
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Postby sparx » Mon Apr 13, 2009 9:13 pm

wws4029,
What you have is a fault between line & neutral on 230V circuit 10 on dist.bd. 1.
If as you say it is supplying an immersion heater from a 32A mcb then it should be either a 4mm2 cable or the mcb should be 16A.
Unfortunately both LINE & NEUTRAL conductors are now refered to as 'lives', however the inspector is quite wrong to call it a phase to phase fault.
hope this helps,
one other thing, if there is a fault the circuit will trip, it may just have a low insulation resistance reading, which is not unusual on water heaters, look for the readings on the test sheet, an IR reading of over 0.5M.ohms will normally allow the heater to still work!
regards SPARX
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Postby wws4029 » Mon Apr 13, 2009 10:38 pm

Thanks for the info.

I must actually stand corrected as the fuse is a 20 mcb NOT 32mcb as previously stated. He looked at the wrong circuit. Most of his electrics are in need of an update. Nothing been done for 10 years +.

According to his report, this circuit is on 20mcb on 2.5mm cable. Is this correct?

Correct me if I am wrong, but as the D/B is just the other side of the wall to the immersion, would it not solve the problem if that circuit was just rewired?

He has also asked me if there is anywhere ( other than a local spark ) he could get a copy of/ be able to read a copy of the new 17th edition regulations? Is such a thing available to all or is it only to the trade?

I must admit, I am getting the feeling he does not trust his spark!
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Postby sparx » Wed Apr 15, 2009 8:44 pm

Hi again .

2.5mm2 on 20A mcb ok except in exception installation method!

the IEE regs 17th edition is freely available to anyone with £47 to spare!
eg Amazon, IET publications etc
ISBN#978-0-86341-844-0 ,
however they are written in very bad 'LEGALISE' and there are several guides which try to make 'English' of them!
There are several trade forums where many very experienced electricians spend an awfull lot of time argueing the details of the 'red book' so don't think a 'layman ' has much chance, but good luck to him.
As a full scope 'NAPIT' member I spend most of my time doing Periodic Inspections & am amazed at differences between mine & previous reports from other people!
As to the circuit in question, if local immersion isolator switch turned of what is the reading? if high then no need to rewire as heater element needs replacing, not the wiring.
regards SPARX
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Postby ban_all_sheds » Fri Apr 17, 2009 11:02 pm

line is now the proper way to refer to phase.

line to neutral fault is what you have
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