Main earth size- why?????

[alexg]

Must admit someone really confused me at work today...

why 16mm csa for a main earth? Is it for disconnection times (assuming the adiabatic applies) for the main incomer DP to disconnect in time?

Yeh, I know...


Thanks guys as ever

 

[Sparky_Ben]

I thought it was a general rule of thumb to be half the size of the mains tails. e.g. 25mm/2 would be 12.5 mm so next size up with that not being a normal cable size would be 16mm?

 

[alexg]

Yeh
But why is my problem (amongst others!)

 

[Sparky_Ben]

hmm now that i cant help with i am still learning myself.

 

[alexg]

I thought it was the current capacity for 25mm tails, should a PSC or PEF occur at the unit/service head... without the cable failing.

But today someone confused me

 

[Sparky_Ben]

i would imagine it must be to do with disconnection times like you say. will be interesting to get other peoples opinions on this

 

[DurhamSparky]

BECAUSE THE IEE SAYS IT HAS TO BE 16MM!!!!! END OF DEBATE lol..................

 

[JUD]

Not sure if this is anything to do with it but adiabatic for 100A BS 1361 and 100A BS 88 at minimum fault currents for disconnection in 5 seconds are

BS 1361....√(630² x 5) ÷ 115 = 12.5mm²

BS 88.......√(550² x 5) ÷ 115 = 10.7mm²


Both above 10mm² so would need 16mm²

 

[mechelec]

The earthing conductor of an installation which uses the earthing facility of a Protective Multiple Earthing (PME) supply as its means of earthing also performs the function of a main protective bonding conductor. the csa of the conductor must be not less than that determined in accordance with Table 54.8 of BS 7671 for a main protective bonding conductor, or Section 543 for an earthing conductor, whichever is the greater.

 

[alexg]

Nice one!!!

Smart

 

[SirKit Breaker]

The earthing conductor of an installation which uses the earthing facility of a Protective Multiple Earthing (PME) supply as its means of earthing also performs the function of a main protective bonding conductor. the csa of the conductor must be not less than that determined in accordance with Table 54.8 of BS 7671 for a main protective bonding conductor, or Section 543 for an earthing conductor, whichever is the greater.

So Durhams correct then.

 

[dichroic]

Hi Guys, I will try to answer this one without confusing myself,

Where:
S is the nominal csa of the protective conductor in mm2

I is the value in amperes (rms. for a.c.) of the fault current for a fault of negligible impedance, which can flow through the associated protective device, due account being taken of the current limiting effect of the circuit impedances and the limiting capability (I2t) of that protective device

t is the operating time of the disconnecting device in seconds corresponding to the fault current I in amperes

k is a factor taking account of the resistivity, temperature coefficient and heat capacity of the conductor material, and the appropriate initial and final temperatures of the conductors.

The adiabatic equation is based on the assumption that the duration of the earth fault current is so short that none of the heat energy produced in the protective conductor escapes before the protective device operates. The assumption, though never true, is reasonably accurate providing the operating time of the disconnecting device does not exceed 5 s, as is normally required by BS 7671.

In order to limit the temperature rise of a protective conductor under earth fault conditions, so as to avoid damage to its insulation and/or sheath (if any) or to adjacent material, Regulation 543.1.1 requires the csa of the conductor to be not less than that:

(i)calculated in accordance with Regulation 543.1.3, or(ii) selected in accordance with Regulation 543.1.4.


The csa required for a protective conductor may have to be greater than that determined in accordance with the requirements of Section 543.1 of BS 7671, in order to limit its end-to-end impedance. The end-to-end impedance contributes to the earth fault loop impedance (Zs) of each circuit for which the conductor acts as a cpc.


Regards

Dichroic

 

[ian.settle1]

Not sure if this is anything to do with it but adiabatic for 100A BS 1361 and 100A BS 88 at minimum fault currents for disconnection in 5 seconds are

BS 1361....√(630² x 5) ÷ 115 = 12.5mm²

BS 88.......√(550² x 5) ÷ 115 = 10.7mm²


Both above 10mm² so would need 16mm²

BS1361 standard was withdrawn in 2007 before the 17th came out

BS1361 is now a BS88-3 read the appendix in the back of the regs