Burnt neutrals

[Jonspark]

Why is it always the neutral that overheats on shower pull switches? I've never seen a burnt live conductor.

 

[HappyHippyDad]

I love shower threads so !!

Once current has flowed through a fixed resistance the returning voltage drops.
That drop in voltage creates a back emf that is harmonically amplified by an increase in current.

Now the harmonics become resonant, and over time create mechanical vibration that work the terminal screws loose.
This in turn creates a bad connection producing heat.

So effectively if line and neutral are torqued to equal values, over time it is the neutral that becomes susceptible to a loose connection. :icon12:

You're going to have to tell me if you're joking? I don't mind admitting I just don't know if you are or not!!

I don't think I'm going to say anything else :smile5:

 

[willy fixit]

You're going to have to tell me if you're joking? I don't mind admitting I just don't know if you are or not!!

I don't think I'm going to say anything else :smile5:

Why is the word "gullible" not in the dictionary?

 

[HappyHippyDad]

Had a proper read now! pity.. I really liked that explanation..:dunce2:

 

[baldsparkies]

You're going to have to tell me if you're joking? I don't mind admitting I just don't know if you are or not!!

I don't think I'm going to say anything else :smile5:

Have you ever watched Pearl Harbour ??

The bit where he says I did it to inspire the men sir, because I wanted to be like you.
And the reply is "That's BULL **** !! But its very good bull **** :icon12:

And sometimes it baffles :conehead:

On the other hand when performing mechanical maintenance checks on composite panels and switch gear.
Have you ever noticed its always the neutrals that need tightening up, more so than anything else ??

 

[NickD]

Personally I think the Klingons are to blame here, they're always causing trouble in the neutral zone....

 

[GB.Kayak]

Standard R1+R2 testing on a radial carries out no test on the neutral therefore no poor connection in the neutral will ever be picked up. I'm sceptical about this having any kind of statistically significant part to play though.

You have to bear in mind that on AC, if you ignore potential difference to earth, the neutral spends exactly half of its time being the 'live', really

N, thanks for this ... a really useful and inspirational thought! There is so much focus and emphasis on proving the 'safety' function of an electrical installation that proving the functional aspects of the system has been lost which prevents basic errors being identified on the functional side. 'Useful and inspirational ... ' nay brilliant! I am going to pore over BS 7671 and GN 3 with a new found emphasis!

 

[Resu]

What about burnt neutral and live?

 

[telectrix]

you need to replace the accessory and back box with steel ones, sealing any holes to prevent the spread of fire... oops. it's only CUs that have poor terminations acc. to amdnt.3.

 

[Jonspark]

So if it's down to not giving neutral terminals the same attention/tightening up as lives, why doesn't it show in cu as well as shower switches?

 

[telectrix]

as Trigger (RIP) would have said...... "look after your neutral".

 

[NickD]

This must be why people use bare terminal strip on the output side of SELV lighting circuits. There's no neutral, therefore it can never ever burn out....

 

[GB.Kayak]

So if it's down to not giving neutral terminals the same attention/tightening up as lives, why doesn't it show in cu as well as shower switches?

Correct me if I am wrong but the only test in GN3 that has any link to the neutral conductor in the functional Line-Neutral circuit, other than the check for RFC polarity, is that for the Prospective Short Circuit Current, Ipscc. This is covered together with the Prospective Earth Fault Current, Ipefc, under the heading Prospective fault current, Ipf in section 2.7.16 on page 54 of the GN.

The purpose of this test is to ensure that the breaking capacity of the OCPDs is adequate. This test is carried out at the outlet of the OCPD in the CU or DB in order to identify the minimum resistance of the Line-Neutral circuit which will be the point at which a short-circuit fault will generate the maximum current.

There is therefore no test in the standard Code of Practice for the industry that requires the entire Line-Neutral circuit to be proven! Is it therefore any wonder that high resistance joints in the Neutral conductor are: a) not found because the system is only proven by the most diligent sparks; b) a potentially serious incident waiting to happen; c) highly likely to create ignition temperatures for plastic materials and d) unlikely to trip an OCPD? The last 2 points may be slightly over dramatised it is an area that I have not experimented with, other than using NiChrome wire to cut polystyrene, I do not know whether sufficient heat would be generated without tripping an OCPD.

Does the theory ring true? Might it bear further investigation?

Further investigation ... see research at link ... is there an incipient failure mode here that is not widely recognised? http://www.interfire.org/features/electric_wiring_faults.asp

Worse still, are most sparks blind to the possibility that their work may be affected and they have no test in their Codes of Practice that picks up the problem?

 

[NickD]

There is therefore no test in the standard Code of Practice for the industry that requires the entire Line-Neutral circuit to be proven!

Ring final circuit continuity/polarity test sequence steps 1 and 2 check the neutral continuity, but obviously this only applies to a ring final circuit.

 

[GB.Kayak]

Ring final circuit continuity/polarity test sequence steps 1 and 2 check the neutral continuity, but obviously this only applies to a ring final circuit.

As I commented in my first paragraph. Most importantly on the subject of the polarity test is that it takes place at and requires the removal of the wiring from the board. Therefore, other than the skill and experience of the tradesman or use of a torque screwdriver ... helmet on and running for cover ... on reconnecting the cables, there is no 'proof test' of the entire circuit post reconnection. In my mind a test to prove the entire circuit must take place after reconnection at the board and be carried out at the furthest point in the circuit; even then, it would need to check every spur as well ... hmm ... I don't think that would go down well with anybody!

 

[HappyHippyDad]

as Trigger (RIP) would have said...... "look after your neutral".

Rodney 'and the neutral will look after itself?'

Trigger 'No, just.. look after your neutral'.

Absolutely love that clip!

 

[DPG]

My theory is this: The loose neutrals are the ones that stick in the memory as perhaps more unexpected, and so the brain remembers this. I've just re-read what I've typed and thought 'what a load of cack', but I'll post it anyway. Somebody might think it makes sense! Where's my whisky gone? Daz

 

[shanky887614]

never had a burnt neutral but have had burnt brown lives.

the previous testing firm forgot to tighten the terminal before they turned the mcb on and jumped in there van

 

[Plonker 3]

Well just been to my sisters has her shower pull cord has stopped working. I only swapped it a few months ago as the old one also fitted by myself failed too both times a burnt neutral cable, I even double checked the terminations last time I fitted it and also before disconnecting it today and they were still tight.

 

[NickD]

Both scorched equally so maybe a faulty switch. Not sure that counts.

 

[NickD]

List of memory biases - Wikipedia, the free encyclopedia

He's right you know.

My theory is this: The loose neutrals are the ones that stick in the memory as perhaps more unexpected, and so the brain remembers this. I've just re-read what I've typed and thought 'what a load of cack', but I'll post it anyway. Somebody might think it makes sense! Where's my whisky gone? Daz

 

[Jonspark]

Bull****

 

[NickD]

Absolutely. Human beings never choose (either consciously or unconsciously) to note things that concur with their preferred view of the world and quietly overlook things that don't. That never happens ever.