Archaeological matters....??

[Careful_Bodger]

Hmmm ..... I have uncovered some 3 stranded twin & earth in a lighting circuit, with red and black PVC insulators, bare solid CPC, and white PVC outer sheathing. From the outside it looks very much like 2.5mm T&E.

Is this a Bad Thing? (sorry to the TM holder....) What might be the options? If the load is say within that acceptable for the next lowest equivalent solid T&E, is it okay to leave it alone?

Also, I have found a wooden back box behind a modern plastic switch plate. Assuming this must be replaced, are the modern replacements drop-in, or will I need my bolster.

Many Thanks

 

[plugwash]

nothing wrong with stranded T&E provided it hasn't been abused

as for the wooden box yes it should be replaced no i don't know whether the hole will be big enough for a modern box.

 

[ban-all-sheds]

nothing wrong with stranded T&E provided it hasn't been abused

Isn't the cpc undersized?

as for the wooden box yes it should be replaced no i don't know whether the hole will be big enough for a modern box.

The ones I've removed have left a hole big enough (but not deep enough for my taste).

 

[plugwash]

Isn't the cpc undersized?

maybe its a bit smaller. But its rare to be runnign lighting cuircuit cables anywhere near there limit anyway.

 

[Careful_Bodger]

Thanks guys....

nothing wrong with stranded T&E provided it hasn't been abused

How do you 'abuse' stranded T&E ??? Must be a punch-line there somewhere. Mind you, I have noticed its tendency to disintegrate under tightening, compared with a solid core.

But its rare to be runnign lighting circuit cables anywhere near there limit anyway.

Agreed - but isn't the CPC size requirement based on the voltage likely during fault conditions, and not based on a nominal load. Sorry - not meaning to be critical of your answer - just trying to get my head around things.

 

[plugwash]

yeah true but if you have mcbs you really don't need to worry too much because mcbs cut so fast so the CPC simply won't have time to heat up

its not really something to worry about.

the main form of abuse cable can have is long term small overloads. over time the softer plastic can cause the cable to migrate through it at the ends.

 

[andy]

But its rare to be runnign lighting circuit cables anywhere near there limit anyway.

Agreed - but isn't the CPC size requirement based on the voltage likely during fault conditions, and not based on a nominal load. Sorry - not meaning to be critical of your answer - just trying to get my head around things.

dont u mean current, not voltage

 

[Careful_Bodger]

Andrew2022,

dont u mean current, not voltage

No - I did actually mean shock voltage in the event of a fault current to earth.

Since the max fault current and timings can be determined by an MCB, and since the CPC resistance is a characteristic of the installation, whether the resulting shock voltage is <50V can then be calculated.

 

[Damocles]

Off the top of my head I would say the voltage on the cpc at the point of the fault will be mains/2 or higher because the cpc resistance is always going to be greater than the supply cabling resistance.To change this you would need thicker cpc than live.

cpc and live resistance must be sufficiently low for sufficiently high current to flow during fault to trip. Heating of cables is pretty irrelevant for one-off design trip as it is so fast. Though fault current needs to be low enough not to destroy mcb from the arc explosion as it opens.

 

[plugwash]

in the extremely rare case where a mcb fails short the fuse behind it should deal with the problem.

service fuses can interrupt insanely high currents.

 

[Damocles]

Well I'm not insane enough to try interrupting it personally

 

[kendor]

Andrew2022,

dont u mean current, not voltage

No - I did actually mean shock voltage in the event of a fault current to earth.

Since the max fault current and timings can be determined by an MCB, and since the CPC resistance is a characteristic of the installation, whether the resulting shock voltage is <50V can then be calculated.

Slight confusion over timings (disconnection times), shock voltage,
An MCB's selection using the disconnection time tables in the regs are to stop the prospective short circuit current value from getting too high to cause damage to the cables it is protecting as for the voltage this could be at a peak at the time of the fault anyway! but the danger is thousands of amps flowing through relatively small conductors for any length of time that would cause an unwanted thermal effect of an excessive proportion.

 

[plugwash]

of course there is a world of difference between a fuse which may take of the order of seconds to disconnect a short and the mag trip and arc suppression on modern mcbs.

the regs require dissconnect times of 0.4 sec or 5 sec depending on cuircuit type.

with an older mcb that interupts on zero crossing you are talking about an intterupt in under 0.02 sec and with a mcb with arc supression interruption faster still.

if the current is correct then cutting through a flex has been known to take out a B32 before a plug fuse!