Spot The Fault

[apprentice87]

See above!!

Anyway, referring to the photograph.

Unearthed armour........ All exposed conductive parts must be earthed in installation that contains the protective device. Armour is an exposed conductive part.

There are more circuits than there are MCB's One appears to be feeding TWO circuits.. How you supposed to isolate the one circuit without inadvertently shutting off the other, that is a non compliance in itself, never mind other issues that may be involved.

john..

 

[Sidewinder]

There were 5 outgoing and 5 breakers.

If it shares the same breaker then it's the same circuit.

It might be inconvenient to shut one breaker off and loose the two things being fed, but hey ho.

My lathe & miller share an MCB, there was not enough spares in the board, and I did not want to change it, I can only use one at a time really, mind I can run both the circuit is large enough.

There are rotary iso's for each machine so that covers machine isolation.

Anyway, going to try and dig out some other regs, if I've got time.

 

[Andy™]

There are more circuits than there are MCB's One appears to be feeding TWO circuits..

not quite, a circuit is anything connected to OCPD, so even if there are 2 cables going in opposite directions, its still one circuit

 

[apprentice87]

Hmmm, Yes... See what you both mean!!!!

john..

 

[Lurch]

if it is ONLY there to provide mechanical protection, HAS to be earthed at the supply end to the main earth terminal, [by whatever means] for various reasons...

Not necessarily.

 

[Nicky Tesla]

App 87.............nobody said the SWA would be connected to a DIFFERENT earthing system at the other end.  The gland and earth would be connected at the far end meaning that the armour would be earthed all the way back to the gland at the supply end.  

Sides........i was right then,  needs to be earthed at both glands under these conditions

But as another scenario.....all conductors and armour servered,  but live conductor still managing to contact the armour on the side furthest from the supply,  means the armour remains live.

Evans.......seen loads banjoed on the inside + there is no bolt poking through if it was done on the other side.

 

[Sidewinder]

Not necessarily.

I disagree

If the armour requires earthing, which it invariably does, as it is required to provide the mechanical protection, which therefore must be earthed.

Thus it must be connected to the means of earthing from which the supply is derived.

Whether that is TT or not, it still must be connected to the source end as this is the means of earthing for the circuit.

I don't see how, one can say that the load end can be connected to anything as long as it's connected to earth somehow.

The whole definition of a circuit goes with the supply, neutral and means of earthing being at the supply end.

 

[Nicky Tesla]

armour is connected to the cpc.....cpc is connected to the earthing conductor at the supply.   Therefore no matter what end the armour is connected the armour is still connected to the supply earth.

 

[Sidewinder]

Nicky,

You've confused me now!

Not difficult this time of night!

What and why does it need earthing at both glands, I'm missing something.

How if all conductors are severed and the disconnected live now contacts the armour can the armour become live?

Where have you got a cpc from, the crmour is the primary cpc?

 

[Nicky Tesla]

Extra protection for different scenarios in your FMEA survey.

The severed live may have a bare conductor still in contact with the armour on the load side.  Just similar to your  scene apart from  contact to a different side of the armour.  (far fetched i know)

CPC...if you check the photo,  you see the SWA has L N and CPC.    If the swa armour is connected to the cpc at the far gland then the armour is indeed connected to the supply earth.    I dont know why people keep thinking it is joined to some other earthing system. 

 

[Adrian]

Do you ONLY do something because the BYB tells you to, or do you sometimes do something because it's good engineering practice?

See post 24

I said it was not good practice to earth load side of SWA but it is not a non compliance as there is no reg against it ,that is all that I was saying .on a eicr what code would you give it? At best a code c3

 

[Sidewinder]

The issue is I think the medium of communication.

 

[apprentice87]

You design a circuit yes??? You make sure that at the extremity Zs is within limits yes??

If you have not earthed the armour at the supply end, then you are obviously not using it as the CPC and are using an internal core of the cable instead, Yes????

You now have an earth fault near the supply end. What happens?? The R2 part of your circuit is now twice as long as you thought it was, [all the way up the armour to the appliance, and all the way back down the CPC] even worse, the resistivity of the steel armour is 8 times as high for the same area of copper. Result is, at the POINT OF THE FAULT, Zs is now VERY much higher than you thought it was from your design. You OCPD will now not operate as intended.....

And back to the original problem...

The RCD will not operate on the test button???

Is the supply to the CU TNCS ?? Is there a fault with the neutral??? Do things connected to the CU work??

Faulty RCD or break in the neutral i reckon.... ooerrrr...

john..

 

[Lurch]

You now have an earth fault near the supply end. What happens?? The R2 part of your circuit is now twice as long as you thought it was, [all the way up the armour to the appliance, and all the way back down the CPC] even worse, the resistivity of the steel armour is 8 times as high for the same area of copper. Result is, at the POINT OF THE FAULT, Zs is now VERY much higher than you thought it was from your design. You OCPD will now not operate as intended.....

That could happen, it is not a fact that this is the case in 100% of the installations where armouring is connected to the CPC at the far end.

Don't know why you are getting so worked up over something you admit you know nothing about.

 

[Andy™]

Don't know why you are getting so worked up over something you admit you know nothing about.

at least he is trying, unlike some 'electricians' on this forum

 

[Lurch]

at least he is trying, unlike some 'electricians' on this forum

Quite. He's going to do himself an injury though, all this shouting and raving.

 

[apprentice87]

I am not getting all worked up, just cannot understand some of the total nonsense some supposed "electricians" come out with....... makes no difference to me...

john...

 

[ProDave]

That could happen, it is not a fact that this is the case in 100% of the installations where armouring is connected to the CPC at the far end.

Don't know why you are getting so worked up over something you admit you know nothing about.

Why are you trying to defend a situation where the SWA is earthed at the wrong end and arguing that's "correct"?

Anyone with an ounce of sense can see it's FAR better (and not exactly difficult) to earth it at the source end.

But you seem to be saying "show me the reg" and if there isn't one then it must be fine and dandy then.

No wonder there are so many regs in the book these days, they have to spell out EVERY detail for those with no ability to apply good engineering practice.

What's the saying, make something idiot proof, and someone will find a better idiot?

 

[Rob_the_rich]

You design a circuit yes??? You make sure that at the extremity Zs is within limits yes??

There are two Zs limits, one for overload current protection and one for fault current protection (1667ohms)

 

If you have not earthed the armour at the supply end, then you are obviously not using it as the CPC and are using an internal core of the cable instead, Yes????

You now have an earth fault near the supply end. What happens?? The R2 part of your circuit is now twice as long as you thought it was, [all the way up the armour to the appliance, and all the way back down the CPC] even worse, the resistivity of the steel armour is 8 times as high for the same area of copper. Result is, at the POINT OF THE FAULT, Zs is now VERY much higher than you thought it was from your design. You OCPD will now not operate as intended.....

It depends whether the fault current is travelling up the armour and down the cpc, or just down the cpc, either way it is unlikely to meet resistance greater than 1667ohms 

If the cable is severed and, as seems certain in this case, re-energised, there are 4 possible scenarios

1. live cable is touching cpc-> the RCD trips

2. live cable is touching neutral-> the mcb trips

3. live cable is touching un-earthed armour-> armour becomes live (assume mostly still enclosed by outer sheath)

4. live cable is not touching anything

With 3 & 4 the rcd should still provide protection if there is a fault current (puddle, floor, finger).

Hopefully the operative will realise that the machine(s) doesn't work and will disconnect the supply and investigate. 

If damage to the cables is likely, perhaps burying or re-routing might be considered. 

I did find a sort-of-relevant reg 523-100, which mentions solid bonding, but it is single core, therefore there is no internal cpc.

It does make sense to earth the armour at the supply just to provide an easier path for fault currents, but whether it is necessary is debatable, some might say that having a larger surface area of live conductors at the cut end allows for more chance of the fault currents being large enough to trip the rcd.

Just my thoughts on this.

 

[Lurch]

Anyone with an ounce of sense can see it's FAR better (and not exactly difficult) to earth it at the source end.

I am not disagreeing with this.

But you seem to be saying "show me the reg" and if there isn't one then it must be fine and dandy then.

I have said no such thing and did not intend to imply it either. I was merely saying that shouting things loudly does not make them fact.