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Topic Title: Protective Conductor Size
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Created On: 19 March 2019 04:59 AM
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 19 March 2019 04:59 AM
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Sorusorma

Posts: 8
Joined: 09 December 2018

Hi,

According to the good book on table "TABLE 54.7 -
Minimum cross-sectional area of protective conductor
in relation to the cross-sectional area of associated line conductor" page 198.

The size of the protective conductor shall be half the size of the associated line conductor if the c.s.a of the line conductor is more than 35mm2 & is same material.

Now, why are we following that? Is there a technical reason behind it? Appreciate if someone can shed some light on the reason.

Thank you
 19 March 2019 06:16 AM
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mapj1

Posts: 12039
Joined: 22 July 2004

yes, this is about estimating the worst case diverted neutral current that may flow in that bonding conductor - the assumption is that the line and neutral conductors are similar in cross section - there is little point in having bonding so good that the neutral conductor in the supply cable fails before it, equally we do not want a credible magnitude of network fault to be likely to cause the bonding conductors to start a fire.

the estimate of how much current is possible is however an approximation - it will depend totally on the path any diverted neutral currents actually take via metallic services and the bonding at other properties.

An alternative approach if fire reduction was the only aim, might have been to have a fuse in the bonding conductors, but this is not compatible with the main aim of bonding when there is not a supply side fault, which is shock voltage reduction when there is a load side fault, and for that reason earth fuses are not permitted by UK regs.

Note that the modern approach usingplastic service pipes for gas and electricity, or at least insulating sections in metal mains near the entry points, greatly reduces the scope for large diverted currents, but greatly increases that likelihood of the CPC wiring in the building not being at quite the same voltage as the true terra-firma, so 'tickles' from outside taps, outside lights and similar fittings are more likely.

-------------------------
regards Mike
 19 March 2019 11:52 AM
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AJJewsbury

Posts: 17795
Joined: 13 August 2003

I think Mike's mis-read the question - it's about 54.7 (c.p.c. sizes when not calculated) not 54.8 (PME bonding sizes).

I'm not sure of the exact thinking behind the 'half size' rule. I suspect it's just based on a lot of years of practical experience. Certainly in much older editions of the regulations, before we had the adiabatic approach for calculating conductor sizes, there was a general rule that as long as a conductor was at least half the rating of the protective device then it was deemed to be protected from faults (but obviously not from overcurrent) - so that was the rule for sizing c.p.c.s (as well as live conductors for circuits that required only fault protection - e.g. industrial motor supplies with overload protection at the motor). Typically the adiabatic gives results in the same general region as the old half-sized rule - sometime a little bit higher, sometimes a fair bit lower - so the half-size rule could be considered an early approximation to the adiabatic, usually erring on the safe side. But even the adiabatic isn't perfect - it's based on the assumption that no heat is lost from the cable during the fault, but as disconnection times get longer that becomes less and less true and so the adiabatic will tend to over-estimate the size that physics actually requires - so even if you calculated (for a large cable with a longish disconnection time) that a half-sized c.p.c. that was a bit too small according to the adiabatic, I wouldn't necessarily be too worried.

As for why they kept the half size rule for above 35mm2 when requiring full size below 16mm2 I'm not sure, but I suspect it was down to practical considerations - e.g. cost and being able to continue to use exiting cable types.

- Andy.
 19 March 2019 12:04 PM
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mapj1

Posts: 12039
Joined: 22 July 2004

Oh dear, that'll be my error, please accept my apologies !
I was indeed answering the wrong question. I'll leave the wrong answer up there just in case that was going to be the next question, but ignore me if it was not.
For small cable sizes there is a consideration of mechanical damage - is the earth likely to get snapped off and not notice, the size at which you start to worry about this is arbitrary, but 1mmsq final circuits (as opposed to 1.5mm) and reduced earths in T and E are unique to the UK regs. You see a similar considerations in the minimum CPC size for circuits with high leakage circuits, where you can have two thin CPCs combined to one fat one, though of course, the choice of threshold sizes is not the same

-------------------------
regards Mike
 19 March 2019 12:48 PM
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broadgage

Posts: 3170
Joined: 07 August 2007

With a classic, non PME, system another way of justifying the reduced size of the earth was the fact that it should only carry current short term, unlike the live conductors.

Example, is 16mm suitable for a 100 amp load ? Answer, in general no it is not. There is no short term or immediate danger in loading 16mm to 100 amps, but eventually it will suffer heat damage. It will be damaged much sooner if loaded to 125 amps for hours whilst waiting for a 100 amp fuse to blow.

Example, is 16mm suitable for the earth of a 100amp circuit ? Yes, in most circumstances it is. 16mm will easily carry enough current for long enough to operate the fuse.
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