Twin & Earth

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the_mouse

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Hi guys, why is the c.p.c. in T&E not insulated also why is it smaller than the live and neutral conductors? I've put this question to my college tutor but got no explanation.

TIA

Mouse

 

Badger

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Hi guys, why is the c.p.c. in T&E not insulated also why is it smaller than the live and neutral conductors? I've put this question to my college tutor but got no explanation.TIA

Mouse
thats a very good question i think its down to cost of not insulating the extra conductor.

I think the reason it smaller is because as it is only a protective conductor NOT a current carrying conductor in norma circumstances this cpc cable is not live through normal use, so if it does go live it carries the excess current to the earthing arrangment thus causing the protective device to trip out.

 

seajayess

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Been thinking on this and yes I think it is a cost issue, but do you think it is bare so that if you cut through the cable while it is live, you stand a good chance of coming into contact with the CPC as there is no insulation round it.

Where if it were insulated you would have to cut through the insulation to get the Earth return path if you see what I mean ; \ CJS

 

lee mcgregor

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Twin & earth has no insulation on the CPC to save cost and to ensure its use as a CPC

The requirement to have no insulation is stated in the relevant standard, in this case BS 6004.

My understanding of the main reasons are that in a fixed installation it does not need insulation (so why add additional weight, size and cost), and to ensure it is only used as a CPC. If it had insulation, it might be improperly used as a conducting core (for example in 2-way lighting) without a circuit CPC being fitted.

This does mean that there is a bit of extra work to do when terminating the cable, adding sleeving.

Some cable manufacturers have produced flat twin and earth with an insulated earth for specialist applications, but these cables do not comply with the British Standard.

 
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Also since the conductors have to carry current for years and the earth only needs to handle it for a very short period of time? This makes common sense but I may be wrong.

 
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no1electrician

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also regards the smaller size the manufacturer's work out the size using the adiabatic equations

 

springcrocus

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If the cpc was sleeved, it would make it a bugger to strip ]:) ] :)

 

steptoe

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also regards the smaller size the manufacturer's work out the size using the adiabatic equations
unless its 1.5mm X 1.5mm cpc. LOL,

its all down to the expected current ,

ie, the cpc only has to carry the fault(ie , >6amps) for a very short period of time ( <0.4secs), the L&N have to carry 6a indefinitely.

 
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unless its 1.5mm X 1.5mm cpc. LOL,its all down to the expected current ,

ie, the cpc only has to carry the fault(ie , >6amps) for a very short period of time ( <0.4secs), the L&N have to carry 6a indefinitely.
s'what I said but you phrased it better ;)

 
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I always assumed as above , keeps size down, cost, not nessessary to insulate.

It comes in useful for quite a few central heating installers I ,ve come across of late, they all seem to use the earth wire as a neutral to their room stats, and sometimes even the switch wire, very naughty !!

 

Admin1

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We discussed this the very Wednesday before Witson Hols.

The answer was cost and what steps said too. ;)

 
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Just to add the the points so far,

One of the functions of the CPC is to connect all exposed conductive parts,

e.g. those metal light fittings, the case of the washing machine or microwave or fridge and those metal-clad sockets & switches in the garage etc..

If all the exposed conductive parts, (by their description are not insulated), there is little point in insulating the conductor that is joining them all together with the exposed metal MET, ROD or sheathing of the incoming supply.

It just needs identification as to its purpose at each termination, so a small length of sleeve does the job quite nicely.

None T&E

When wiring is done in singles, (not T&E), the CPC still requires a level of mechanical protection and correct colour identification and so to meet both requirements, comes manufactured with the appropriate coloured insulation. ;)

 

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