Pinched wire

[BeeLZeebub]

A quick question following on from BAS's comments in Glyn 1959's thread to further my educashion ...

Why does a pinched wire knock out an MCB? ** What's happening to produce the over current ..... or have I leapt to conclusions about cause?

Thanks

** (Had this on putting in a new light switch a while back. Switch fitted, power up, MCB off. Wires checked - all OK, re-fit, power on, all well)

 

[wingcoax]

A pinched wire causes a short to earth.

 

[BeeLZeebub]

A pinched wire causes a short to earth.

... which to my simple DIY-mind would knock out an RCD; but MCBs are for over-current protection no? What am I missing ....?

 

[ban-all-sheds]

It is an RCD which is tripping.

An MCB would trip¹ if there was no RCD, and it was a L-E fault.








¹ Subject to the normal considerations of fault loop impedance

 

[BeeLZeebub]

It is an RCD which is tripping.

An MCB would trip¹ if there was no RCD, and it was a L-E fault.

As I have no RCDs, it must then be a L-E fault ... at the risk of being left in the corner with the pointy hat, I'm just trying to get an idea of how it it works: you pinch the insulation, so what? Where's the leakage if there's no split?

Edit - and am I to infer therefore that the bit I'm missing is that MCBs are not solely for over current protection? Missed that one .....

 

[ricicle]

There must be a direct contact between the L and E (or enough breakdown in insulation) to cause fault current to flow.
An MCB will provide overload protection and L to E fault protection (subject to enough current flowing - usually short circuit)
An RCD will provide L or N to E fault protection. They are more sensitive so will operate in the region of milli-amperes depending on their rating. They will usually operate first although an MCB could still operate as well under the short circuit scenario.

 

[BeeLZeebub]

Thanks very much for that Ricicle: the fact that MCBs provide L to E protection had escaped me

Must confess though I'm still a little perplexed as to why my MCB tripped per my first post. Given that ...

There must be a direct contact between the L and E (or enough breakdown in insulation) to cause fault current to flow.

... what L&E contact there could have been by my screwing back the plate?. There were no nicks in the wires; I'm always careful to terminate them neatly and securely (and I checked this was the case) ... ??

 

[sparkwright]

Often in light fittings, if space is limited, a wire can inadvertently be squashed up against some (earthed) metal work and cause a short circuit.

This demonstrates why earthing is so important.

Sometimes (though not often) the damage to a 'pinched' wire can be so small you can hardly see it.

Always keep the wire away from any stubby or sharp bits of metal.

 

[Taylortwocities]

Thanks very much for that Ricicle: the fact that MCBs provide L to E protection had escaped me

Many DIYers are unaware of what the earth does and its relation to the other 'live' wires. The earth connection is more or less directly connected to the neutral. This is doe either at the substation down the road, or in the service head in your house - depending on the service delivery type.

So an MCB will trip if there is a short circuit between earth and line, or between line and neutral providing the fault current exceeds the trip characteristics of the MCB.

Have a look at the different earthing systems here
http://www.tlc-direct.co.uk/Book/5.2.2.htm

 

[securespark]

Thought this was gonna be about copper theft.

 

[JohnW2]

Many DIYers are unaware of what the earth does and its relation to the other 'live' wires. The earth connection is more or less directly connected to the neutral. This is doe either at the substation down the road, or in the service head in your house - depending on the service delivery type.

Quite often, when I've explained that, in roughly those words, to people who didn't previously realise, they have responded by asking me why, then. we bother to have separate neutral and 'earth' wires in electrical installations. I don't know about you, but it's a question I don't find too easy to answer satisfactorily for them, particularly in relation to TN-C-S !

Kind Regards, John

 

[plugwash]

Combining neutral and earth has a few issues.

1: If the neutral breaks then your combined neutral and earth core becomes live.
2: Currents in the neutral can create voltage gradiants in the earth which can push current down paths that were not originally intended to be current carrying
3: A cable with a combined neutral and earth cannot be RCD protected

Electricity supplier networks are designed to keep the risk of a lost neutral very low and bonding and multiple earthing requirements somewhat mitigate the risk if it does happen. For household wiring it would be very difficult to mitigate the risks from a broken CNE core and the way typical house wiring is done makes a broken conductor far more likely than in an electricity supplier network.

 

[BeeLZeebub]

Thanks for all the responses

Many DIYers are unaware of what the earth does and its relation to the other 'live' wires ... Have a look at the different earthing systems here

Thanks Taylor: I'm aware of all the differing earthing systems, what earth does and so on - in this instance, I was just unaware of the simple point that MCBs provide some earth leakage protection. Typical patchy learning of a DIYer I suppose ....

Sometimes (though not often) the damage to a 'pinched' wire can be so small you can hardly see it.

- then this would certainly explain the earth leakage in my example ..... thanks

Thought this was gonna be about copper theft.

I dont think I could lift one .... it's all those donuts

Combining neutral and earth has a few issues ...

Thanks Plugwash - interesting stuff

 

[JohnW2]

Combining neutral and earth has a few issues.

It does - and, as I presume you realise, I was not suggesting that we should have CNEs within installations. However, as I said, explaining the issues to someone who needed to be told that neutral and earth usually be joined 'somewhere' is not necessarily at all easy!

1: If the neutral breaks then your combined neutral and earth core becomes live.
2: Currents in the neutral can create voltage gradiants in the earth which can push current down paths that were not originally intended to be current carrying
3: A cable with a combined neutral and earth cannot be RCD protected

Of those, (3) is probably the most straightforward, the easiest to explain to 'laymen' and probably the most compelling reason for not having a CNE conductor within installations. (1) is also pretty compelling, but probably not quite so easy to explain to 'laymen'. I'm not quite sure what you have in mind in your (2).

Kind Regards, John

 

[ban-all-sheds]

I was just unaware of the simple point that MCBs provide some earth leakage protection.

They don't, not in the accepted sense of "leakage".

An MCB limits overcurrent - it doesn't matter what the current path is, if it's too high, the MCB will trip. But a B50 used for a shower, for example, will let 56.5A flow to earth indefinitely, and 72.5A for up to an hour. If the current path is through you you'll be dead long before the MCB trips. When people talk about leakage they mean currents in the mA range.