Burnt Main Downstream Neutral

[ennaress]

Did an EICR opened the CU and found this burnt charred Neutral running downstream of the (main switch). What should I be looking for (other than a replacement tail)?
There were two or three C2 observations that may well have caused an overload - eg 5kW of water immersion wired into the ring final and the oven run off a 13a plug, but the live conductors looked OK.
Installation over 35 years old.
OC or very low IRs on all circuits.
R1+R2 ok as was Zs.
I know one answer would be a whole re-wire, but I want to know what caused this.
Thanks for any helpful replies

 

[Ian1981]

Only the lower heater should be connected in a two immersion heater cylinder if no off-peak/E7 supply is available.
The upper heater is only there to be a boost element in an off-peak/E7 setup. It will do little to no work if it is switched on at the same time as the lower heater, the heat from the lower heater will quite quickly operate the thermostat of the upper heater and switch it off. Also with modern immersion heaters the lower element can operate the overheat trip of the upper element and switch it off permanently (until reset)

Why are you quoting the 230V power ratings? The public electricity supply in the U.K. is 240V, the mythical 230V only exists on paper and not in the real world. These immersion are will be operating at 3kW and using the lower power ratings will lead to you designing and installing a circuit with a small overload which is a dangerous situation.

What is a rubbish/null IR reading? All of the IR testers I have used have only ever had a numerical scale so I haven't a clue what those readings equate to.

He's probably using 230 v because that is what we are taught to use from college and bs7671.
We are required to record the nominal voltage on our test certificates as 230 volts and not the measured value.
Also voltage drop calculations are based on 230 volts it's 3% of lighting etc based on 230 volts yes it takes the worst case scenario.
Maximum zs values calculations require it to be 230 x 0.95.
Where I do agree that most of the time the voltage is around 240-245 volts it's not the voltage we are told to use maybe common sense tells us different.
Maybe it's not 'the real world' but it is whAt it is.

 

[static zap]

....Maybe it's not 'the real world' but it is whAt it is.

Maybe a harmonized EUROPE ... or What it WAS ...
(did we ever intend to comply -or just upped the + to 10%)

 

[Ian1981]

Maybe a harmonized EUROPE ... or What it WAS ...
(did we ever intend to comply -or just upped the + to 10%)

Yes it was to harmonise if you read page 316 of bs 7671 it was agreed by CENELEC within Europe from as early as 1988.

 

[static zap]

CENELEC within Europe from as early as 1988.

Politicians promises , (-sounds good-) !!!

 

[telectrix]

latest one being corbyn's u turn on student fees. promise.. we'll scrap them.. now you students have voted labour, we wouldn't.

 

[static zap]

Was it about all debt , vs future debt ?
(Just heard U turn ,and sighed/switched off)
Vagueness is normal in their arean.

 

[ennaress]

Typical....someone mentions a relatively minor point* about Voltage (each immersion carried a plate stating 230v...not because that's what I was taught at college - thanks all) and the thread goes way off into Labour party election promises...

The IR readings for the Ring Final returned 0 and 5.5 and 5 MegOhms (must I state that these were for readings through L-E, L-N and N-E...?) and c.350 MOhms or thereabouts for the other circuits - 6A lighting and 2 No. 32A Radial (yes, I know a 32A on a radial is unusual). Either way, they're rubbish so that's why I assumed (wrongly) that you'd all be able to assume I knew what was a good, reasonable, unacceptable and utterly rubbish reading.

*Please - do not start arguing the ---- - of course Voltage is a critical element of everything we do - I mean minor as in the relevance to the problem - low/no IR readings.

 

[DPG]

Are you saying the IR reading between Live and Earth on the RFC was zero ohms ??

 

[westward10]

On the ring final did you unplug / remove all loads to get the L/N reading.

 

[davesparks]

He's probably using 230 v because that is what we are taught to use from college and bs7671.
We are required to record the nominal voltage on our test certificates as 230 volts and not the measured value.
Also voltage drop calculations are based on 230 volts it's 3% of lighting etc based on 230 volts yes it takes the worst case scenario.
Maximum zs values calculations require it to be 230 x 0.95.
Where I do agree that most of the time the voltage is around 240-245 volts it's not the voltage we are told to use maybe common sense tells us different.
Maybe it's not 'the real world' but it is whAt it is.

We are given 230V as the nominal voltage yes, but that is nominal.
Nothing in the regulations tells us that it must be used to calculate current ratings, as far as I know the regulations don't tell us anything about how to calculate current.
The regulations do state at various points that nominal values should be used when no better information is available, the actual voltage is always readily available to us.

The regulations do however require that we do not allow small overloads to be designed in to a circuit.
Designing a circuit based on 230V power ratings when the actual voltage is higher will lead to such small overloads being designed in to the circuit, which is dangerous as the ocpd will not operate on such overloads.

 

[static zap]

Still confused with order
Ring Final returned

0 and 5.5 and-- 5 MegOhms ...
L-E, ..L-N and N-E...?
If N-E have not been isolated L&N swapped ..
(or is it just LE,LN&NE sequence off)

 

[westward10]

I don't find it confusing.

 

[ennaress]

Zero was the reading for Ring Final between Line/Earth. 5.5MOhms was the reading for Line/Neutral. 5 MOhms N-E. c.350 MOhms or thereabouts for the other circuits. This was a soft test (250V) - and yes I know what 7671 says...but disconnecting all loads was not practical at the time. I knew the CU would have to be replaced and that the circuitry was shot due to the burnt N bar so I hoped a soft test would suffice knowing a new CU would then have to be fully tested with 500V DC.

 

[westward10]

If you haven't disconnected all loads how have you got a L/N reading and differing L/E and N/E readings. You should have used 500v it would have done no harm.

 

[DPG]

But the zero ohm reading shows that there is either a fault or something is still plugged in. Without unplugging everything how will you know which it is? The test result is meaningless.

 

[HandySparks]

But the zero ohm reading shows that there is either a fault or something is still plugged in. Without unplugging everything how will you know which it is? The test result is meaningless.

Not entirely meaningless. Just not the value you would have hoped for. If you can get an acceptable IR value with appliances connected, you know it's going to be OK without them. Obviously, if the IR's too low then further investigation required.

 

[DPG]

Not entirely meaningless. Just not the value you would have hoped for. If you can get an acceptable IR value with appliances connected, you know it's going to be OK without them. Obviously, if the IR's too low then further investigation required.

But he hasn't got acceptable values.

 

[westward10]

It is odd he hasn't got zero both sides to earth but he also has a reading for L/N.

 

[HandySparks]

But he hasn't got acceptable values.

Quite agree. Hence the need for more testing.

 

[DPG]

Quite agree. Hence the need for more testing.

Indeed. As I said, stuff needs unplugging and then some proper testing doing.

 

[mrloy99]

The burnt neutral looks to me to be on the lighting circuit..must have been loose.
I would agree that all would need to be unplugged for IR test and the imm htrs would need to be switched off if double pole switch or disconnected if only single pole kit eg some spurs maybe for washing machine?

Wonder what live to earth on ohm scale showed?