What BS number is this?

[RF Lighting]

Because the resistance of the earth stake is too high to allow enough current to flow to operate the OCPD in the event of an earth fault, so an RCD is used to automatically disconnect the supply in the event of a fault to earth. The RCD is usually 100mA which does NOT afford sufficient protection against direct contact. hence using an additional 30mA RCD for this

 

[Softus]

So, if I've got this right, when the lighting circuit EFLI is, say 500 Ohms, and the RCD trip current is 100mA, the product of those is 50V, which is too high (according to the regulations).

In my ignorance, 500 Ohms seems quite high for an stake, but is it in fact normal for TT?

 

[Spark123]

The external loop impedance / rod resistance shouldn't really be above 200 ohms.
You are correct 500ohms is too high. You'd need to lower the loop impedance by driving in more rods or a longer rod.

 

[Softus]

As much fun as driving in a longer rod must be, is the solution to use a 30mA RCD?

 

[Spark123]

Not really,
In theory only, a 30mA RCD can have a maximum electrode resistance to earth of 1666ohms.
However, in reality figures exceeding 200ohms can be considered unreliable over time and hence 200 ohms should not be exceeded. I believe the NICEIC require this to be below 100ohms. The 17th edition DPC also looks as though it will be going for 100ohms max.

 

[Softus]

OK. The unreliability of a high EFLI path - I surmise that this arises because such a path possesses elements that are likely to degrade radically, thus raising the resistance more than negligably, compared to low a EFLI path that is likely to stay low. For example, I can see that the stake-to-earth interface could be rather nebulous, and the resistance could vary wildly according to prevailing season and soil conditions.

Is this right?

 

[Spark123]

Yeah, dry conditions will give a higher resistance to wet conditions. You should really measure the resistance when the conditions are less favourable which limits it to about half a day in the british isles

 

[tim west]

OK. The unreliability of a high EFLI path - I surmise that this arises because such a path possesses elements that are likely to degrade radically, thus raising the resistance more than negligably, compared to low a EFLI path that is likely to stay low. For example, I can see that the stake-to-earth interface could be rather nebulous, and the resistance could vary wildly according to prevailing season and soil conditions.

Is this right?

Yes, recommendations are to bury rods deep enough to penetrate into wet clay if possible.

 

[Softus]

Thanks guys. Especially tw, who I'm prepared to say that I've misjudged.

tw - I've come to realise recently that there's more to you than the **** you sometimes portray. I suspect you have it within you to forgive me, so here's an olive branch - I apologise.

 

[Podooser]

.. cooker might be better off on the non-RCD side without a socket outlet on the CCU.

Socket outlet is already wired in.

.. I take it you meant 30mA?

I do, sorry.

.. Just wondering if a normal board and RCBOs wouldn't be much more expensive?

I have already purchased the MCBs

Can you convert a 6 way CU into a 6 way split?