size of main bond

M cebee

customer wants a cattle shed fed from house 100m away

how do i calculate size of main bond-tails are 16sq

i've seen a formula -main bond resistance <0,05 ohm

that would mean 25sq earth?

ShamFeen

I presume you mean coming from the new local d.b you are installing? Or from the house with the mains? How long of a run is it?

M cebee

100m:)

the main bond is from the house

ShamFeen

I think (as opposed to know!!) that to satisfy current regs the impedance reading has to be less than 1.5 ohms

Bruthal

Once your loop impedence is ok i would of thought thats it, if you made the 16 square mains smaller then the earth would have to be bigger to bring the loop z down. At 100 meters you would think 16 square on all 3 cores would work, but might be tight readings from any sockets wired out from the sub board. Its an interesting one. I think 3 x 16 will work though.

M cebee

not the loop-impedance

the 'main bond' for girders from MET at house

Bruthal

O im goin blind again:D

Still would`t think it would have to be any bigger than the mains supply out to the cattle shed though, its an interesting one though, 0.05 ohms you reckon? Seems very low.

Bruthal

Makes you wonder would a TT setup be better out at that distance.

Bruthal

Not that the TT setup would be allowed, but a broken neutral before the farm house would be nasty there, thats the problem with neutralising method.

M cebee

yes-convert to TT with 30ma final circuits would be safer

but for tn-c-s i think you have to oversize the 'main bond' over that distance

that would bring it to 25

the 'live' conductors can be smaller as long as you comply with Zl at the sockets in shed

Bruthal

Sounds reasonable alright. Cant see anything wrong with the 25 earth for that. Mechanicaly strong connection as well.

M cebee

a 10 paralleled with the the 10 swa cpc will prob do-basic supply

not allowed afaik-but does a rod at the cattle shed make it safer or less safe
i wonder

Bruthal

A second earth rod at shed would bring the ground around it up toward broken neutral voltage if the neutral breaks before the residence neutralising point. Some would say the earth rod will keep touch voltages down, and it will if its only one or 2 CFL lights on, but once there is any bit of a load on in house the broken neutral voltage will go up, bringing with it all the earthing once the neutral failure is before the neutralising point.

I think milking parlours have a metal floor also bonded to everything in the parlour to reduce and points of contact having different potentials, cattle make good contact with the ground so they are very sensitive to electric shocks. Thats the idea of everything having equipotential bonding in neutralised installations, so in the failed neutral scenario mentioned above, if all earthing heads up toward mains voltage, at least everthing that can be contacted is at the same potential and so minimising fatal shocks.

In a failed neutral scenario, the neutral voltage can get closer to the Live voltage in an installation, or can actually get further away in the case of a mini-pillar neutral failure because this has several houses now effectively connected in star with no neutral, and not balanced, so in any one house the L-N voltage can reduce, or increase depending on the loads in each house. If its the neutral just to one house that fails then the L-N voltage will reduce as the load in the house increases, as in the neutral voltage moves above ground potential, only the earth rod now keeps it at ground potential but it wont be effective even with relatively small loads.

So maybe sometime a device could monitor L-N voltage and cut off supply if the L-N voltage goes outside a certain limit (+ or - 40 either way), maybe its impractical though.

M cebee

think i'll just replace rod at house

afaik additional rods not allowed?


the tt conversion would be a lot safer you would think-in fact it is

Bruthal

M cebee wrote: »

think i'll just replace rod at house

afaik additional rods not allowed?


the tt conversion would be a lot safer you would think-in fact it is

Im not sure what the situation is on multiple rods, they are used in TT alright, and along the neutral conductor in TN-C-S between the esb traffo and the meter, unless the residence is a single traffo one and the traffo is close to it where they may have just 1 or 2 rods.

I dont think multiples are allowed at the meter in TN-C-S like you say.

And yea you would think TT conversion would be allowed in situations like you mention here. A main RCD would probably have to be incorporated at the supply (house) end of the cable to be fully effective here. Then the metal of shed be connected to earth rods.

M cebee

no-do you remember discussing it ?


-2-core swa

-armour glanded supply side only to MET


-rod and 30mA rcd/rcbo's at sub-board

Bruthal

I do remember that, but the difference is in the cow shed the entire structure is steel, imagine the mains cable coming into contact with the steel structure, the RCD in the sub board in the cow shed would now do nothing. If the supply is out to a concrete shed its a different scenario i would think.

Im talkin about a 300ma RCD on the supply cable

M cebee

i would think you must avoid contact

will the rcd prevent a high touch voltage if neutral breaks on supply and armour is making contact

haven't thought about it

Bruthal

M cebee wrote: »

i would think you must avoid contact

will the rcd prevent a high touch voltage if neutral breaks on supply and armour is making contact

haven't thought about it

No i would`t think it will, the cable armour would be part of the house bonding and only turning off all loads in the house would bring this touch voltage down completely.