Rcd head scratcher

Welchyboy1 · Feb 18, 2014

Farm job im at has a problem

three phase tt system comes into main barn area, three phase main distribution board (with 100mA s type 4 pole rcd as main switch) supplying barns and general farm buildings

Swa single phase supply to farm house(around 40meters away), supplied directly from meter side of distribution board on henly blocks to switchfuse/rcd

so house supply is totally independent from the barn supply

however everytime there is a fault on the barn distribution board and its main rcd operates it takes out the rcd at the farm house too

i havent been over to the farmhouse yet but i cant understand why it would be knocking out the rcd in the house?

any ideas?

Darkwood · Feb 18, 2014

Welchyboy1 said:
It is a pain

im thinking of isolating the two earthing systems from each other, having a rod for each and insulating the SWA sheath at the house end from the barn see what happens

just wanted a few ideas for when i get back there

Ramp test the house RCD

Oppps my bad never saw post 18... lol

Ok find the location of the TX relative to the house and barn i suspect if all other scenario's are exhausted i may have an idea.

PS if it is what I suspect you get an arkward situe to address and sometimes its not easy to sort without contravening the Regs.

R G · Feb 18, 2014

I don't think he has enough info yet , he needs access to that house and some test results as well,,,,

Welchyboy1 · Feb 18, 2014

Ray Galea said:
I don't think he has enough info yet , he needs access to that house and some test results as well,,,,

I have cleared a N-E fault in the barn, all other circuits on that side are ok

will get more info on next visit

Engineer54 · Feb 19, 2014

So where is the single point means of isolation to this farm installation??

Are you absolutely SURE that an earth fault in the barn is tripping the house's downstream 30mA RCD and not the other way round?

Isolating TT earthing systems, what do you think you'll achieve by doing that?? I'd be looking at more towards as to what and how these rod(s) are connected into the system. In fact each building should be roded and then physically connected together. Done properly you could well end up with a neigh on TN Ra value.

Welchyboy1 · Feb 19, 2014

Engineer 54 wrote;

So where is the single point means of isolation to this farm installation??

there is none!

Are you absolutely SURE that an earth fault in the barn is tripping the house's downstream 30mA RCD and not the other way round?

When the flood lights to the exterior of the barn was switched on they tripped the main RCD in the barn and the isolated 30mA rcd in the house, i have removed this N-E fault but they want to sort this problem as it has happened in the past and a few electricians before me have failed to diagnose the problem, they are also two independant supplies so either way it shouldnt happen

Isolating TT earthing systems, what do you think you'll achieve by doing that??

Although i have not yet been to the house to investigate further, there was a post that was referring to fault current using the house main bonding path rather than the earth rod at the barn as there is no main bonding at the barn end as its all plastic piping , and i know that there is PME connections to local houses so that is probably the case, i was doubtful of whether this is likely to affect the house RCD, but then im a bit short on solutions

I'd be looking at more towards as to what and how these rod(s) are connected into the system. In fact each building should be roded and then physically connected together. Done properly you could well end up with a neigh on TN Ra value.

on this install i would think it fortunate if there is more than one rod lol

Engineer54 · Feb 20, 2014

Welchyboy1 said:
Engineer 54 wrote;

So where is the single point means of isolation to this farm installation??

there is none! Then one needs to be provided then doesn't it!!

Are you absolutely SURE that an earth fault in the barn is tripping the house's downstream 30mA RCD and not the other way round?

When the flood lights to the exterior of the barn was switched on they tripped the main RCD in the barn and the isolated 30mA rcd in the house, i have removed this N-E fault but they want to sort this problem as it has happened in the past and a few electricians before me have failed to diagnose the problem, they are also two independant supplies so either way it shouldnt happen

Take some photo's of this incoming arrangement and lets have a look...
Should be independent, but depends on how they have been connected, or even possibly interconnected . Farm installations are notorious for out of sight cheap that'll do DIY add-ons and obscure ancient lash-ups. So you might want to have a good walk round to see what you can find... lol!!

Isolating TT earthing systems, what do you think you'll achieve by doing that??

Isolating TT earthing systems from each other isn't going to serve any purpose whatsoever.

Although i have not yet been to the house to investigate further, there was a post that was referring to fault current using the house main bonding path rather than the earth rod at the barn as there is no main bonding at the barn end as its all plastic piping , and i know that there is PME connections to local houses so that is probably the case, i was doubtful of whether this is likely to affect the house RCD, but then im a bit short on solutions

How far away are these other houses in relation to the farm? Unless they are on the same distribution line as the farm, it's doubtful.

I'd be looking at more towards as to what and how these rod(s) are connected into the system. In fact each building should be roded and then physically connected together. Done properly you could well end up with a neigh on TN Ra value.

on this install i would think it fortunate if there is more than one rod lol

Is the farmhouse connected for PME?? On a farm installation, i would personally connect a local earth rod to any and all buildings MET/EMT. Any stray currents will then have a local N-E rod path to run too. But to be effective they need to have a single to very low double figure Ra value...