21 ohm excluding consumer's earth electrode.

[electrocuted]

Good afternoon,

Looking at page 11 of OSG ,it says maximum value of external loop impedance outside consumer's installation for TT system (earth return via consumer earth only): 21 ohms excluding consumer's earth electrode.

Can anyone please explain how you can an earth path return without an earth electrode,unless you rely on parallel paths. Apologies if I sound thick.

 

[oneofthedamned]

As I understand it if you read the note at the bottom of the page it states "21ohms is the stated maximum resistance of the distributors earth electrode at the transformer"

"The resistance of the consumers instalation earth electrode" i.e. the earth rod plus the rest of the planet " should be as low as practicable.

The figures given in the guide on page 11 are suppliers values which they have to maintain "outside the consumers instalation".

Hope this helps explain it.

If not I'm sure somebody will put it more clearly.

 

[Andy™]

this is just the resistance the DNO need to get between their transformer and earth

 

[electrocuted]

Thanks,

oneofthedamned and andyc i do understand it now .But i could'nt grasp it earlier,thanks again guys.

GREAT FORUM

 

[Robojin]

Thanks,oneofthedamned and andyc i do understand it now .But i could'nt grasp it earlier,thanks again guys.

GREAT FORUM

How it was explained to me, 21Ω is used in the calculation on the rating selection of an RCD used to meet you disconnection times

As the others have stated 21Ω is what the DNO supplies at the substation to earth for TT systems

So for you, your Ze = 21Ω, using the regs you are permitted a max 200Ω Zs, if for argument you had an R1+R2 on a circuit of 10Ω (just a number) you add 21+200+10=231 divide this by 50 touch voltage, so 231/50=0.22A convert to 220mA, your RCD must not exceed 220mA or you will not meet disconnection times, so for practical reasons that would be 100mA protecting the instalation

Of course the above assumes you have and earth electrode or equivalent in place

Hope that helps, if I missed something Andy let me down gently

 

[Andy™]

i think you have missed something. your right in the touch voltage being below 50V. this works out at a max of EFLI if 1667 ohms. but you still need to get efli as low as possible, and any rod above 200 should be considered unstable.

the 21 ohms is purely for the DNO - if they dont earth their transformer (or if they earth it badly) then you got problems. i.e if they put a 1mtr into stony ground, with a 200 ohm resistance, then there is no way you can get less than 200 ohms

 

[SPECIAL LOCATION]

in addition to what the others have said it may be worth pointing out..

(some may not realise the big obvious point)...

The figures on page 11 of the on site guide are just specifying the types and and scope of installation that where the figures, calculations and guidance from the on-site-guide would be applicable for.

If your installation does NOT fall within that scope, you should then refer back to the full BS7671.

e.g.

domestic

industrial & commercial where distribution board is adjacent to main cut out.

50hz

nominal 230v

cut out either BS1361 typeII BS88-2 or BS88-6

Max earth loop TNCS 0.35max

TNS 0.8max

TT suppliers earth 21ohm max

(a chunk of this 21ohm Earth loop value will be the line (phase) wire coming into your installation, e.g. how far your property is from the supply, not just the DNO's earth rods,tapes,plates, or whatever?)

Which covers 99.9% of domestic UK installations AFAIK..

But it is not saying there couldn't be exceptions..

just you would have to go back to BS7671 and more in-depth calcs to ensure electrical safety for the users.

e.g.

BS7671 includes exhibitions, shows, fairgrounds, other temp installations, professional stage & broadcast etc..

It is feasible that on a temporary installation with its own power generator,

some earthing arrangement e.g. rod may not meet 21ohms,

local ground condition may mean only higher values achievable

So just don't go referring to OSG even though it may be TT with B type MCB on a final circuit...

check ALL your 'i's are dotted and 't's crossed from the BIG RED book!

 

[Mr Sworld]

It is feasible that on a temporary installation with its own power generator,some earthing arrangement e.g. rod may not meet 21ohms,

local ground condition may mean only higher values achievable

A generator with an earth stake is not a TT system. It's a TN-S system!

Remember that the big three phase diesel generator is taking the place of the transformer in a standard distributors layout.