Earthing equation 3P 100A

[SouthernSpark]

Hi guys

I carried out an EICR a while back on a property with 3P 100A incomer and it only had a 16mm earth. I thought this was undersized, I made contact with the local DNO and had 2 replies, one said it was fine, the other said it should be 25mm.

Im trying to do the adiabatic equation for it, but its so long since I did one of these I want to ensure im getting it right.

S=(⎷(I2t))/k

so I being the possible fault current which was 2.4kA, 2400 squared is 5760000

t being the disconnection time, which for a BS88-2 at 2.4kA is 0.1

K being 143

im getting ⎷(5760000 x 0.1) = 758.95

758.95/143 = 5.3mm

5.3mm seems far too low, what have I missed?

 

[The Ghost]

The square root in the equation and have you doubled the PFC for 3 ph.?

 

[Pete999]

The square root in the equation and have you doubled the PFC for 3 ph.?

Been puzzling that Vorti, not saying you are wrong can you expand a little please

 

[SouthernSpark]

Yes its already doubled.

 

[westward10]

Fault current down the earthing conductor is Uo at single phase.

 

[Strima]

Doubling up the PFC is for phase to phase, not phase to earth.

 

[telectrix]

what's the PEFC? ( not PSCC). that's the value you need. and what csa are the L1, L2,and L3 conductors?

 

[Wilko]

Also type of earthing and incoming DNO N size .

 

[Ian1981]

16mm is more than adequate for CSA of the earthing conductor and the Adiabatic equation will confirm this.
Why do you think it’s undersized?
Also it maybe sized in accordance with table 54.7 are the tails 25mm?

 

[The Ghost]

Been puzzling that Vorti, not saying you are wrong can you expand a little please

I stand corrected in fact the square root has been applied, bit trigger happy there. Have been a bit quiet lately due to just moved into our new home in a new town. Lovely five bed semi Victorian villa with an old victorian built double garage in a private lane at the back. Just about got unpacked a week later. Anyway yes definitely wrong on that one.

 

[davesparks]

I carried out an EICR a while back on a property with 3P 100A incomer and it only had a 16mm earth. I thought this was undersized, I made contact with the local DNO and had 2 replies, one said it was fine, the other said it should be 25mm.

5.3mm seems far too low, what have I missed?

You don't even need to do the calculation for this, just look at table 54.7 in bs7671.

Based on experience 5.3mm is the kind of size that can normally be expected from the adiabatic equation on small supplies like this.