im installing a socket on my gate post as part of my gate instillation, for very occasional use of garden machinery rather than having miles of extensions trailing. The socket and gate motor will be at the end of a 50m SWA cable running from my garage. It will be on its own MCB in the garage and the garage CU will have its own RCD, and the house has just had a new CU with twin RCDs. I was wondering if I need an RCD socket, or if a plain one will be okay? Obviously I want to to trip ASAP if I do something stupid like cut through a cable.
Should I bother with RCD sockets
- Thread starter DIYTom
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Is it safe to assume that you've taken account of the EFLI at the end of that cable when sizing it?
Assuming you have then you don't need an RCD socket.
Assuming you have then you don't need an RCD socket.
One would obviously like to be able to hope so, but ....
Can you explain that? Given that there is going to be an RCD at the other end of the SWA, I don't see that there would be a need for an RCD socket whatever the EFLI (within reason!)
Kind Regards, John
I'm uSing three core, 2.5mm SWA with one of the cores as the Earth. So from my calcs I have 100m of 2.5mm cable, with a resistance of 0.0074 ohms/m, so the EFLI is 0.74ohms. From what I've read that seems to be low enough to meet the requirements. Does that seem okay to you?
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I presume that those figures assume that the armour, as well as the third core, is used as the earth - and, not being next to my books, I'll assume for the time being that your figures are correct (if you were only using the third core as earth {not armour}, 50m of 2.5mm² would be about 0.9Ω
.Whatever, you have calculated just the resistance of the SWA. The EFLI will be appreciably higher than that, since it also includes the resistance of the line and earth paths all the way back to your local substation/ transformer.
Kind REgards, John
I believe there are, and that such was the basis of BAS's sarcastic and/or rhetorical question!
However, for those without such a meter, who are therefore reliant on calculation/guesstimation, they can at least add on (to calculated resistance of their cable) an estimate of the impedance between the cable in question and the origin of their installation, and then add on an estimate/guess of Ze - if, that is, they understand what EFLI is.
Whatever, as I responded to BAS, I don't really understand what EFLI has got to do with the need for a second RCD at the end of a cable which is already RCD-protected!
Kind Regards, John
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I would assume an incoming of 0.35 so the measured ELI would be around 1.2 ohm and volt drop on 13A of around 10.5 volt. Again assuming armour not used. But really it does not matter the ELI would need to be extremely high to stop the existing RCD from tripping in the event of a fault.I presume that those figures assume that the armour, as well as the third core, is used as the earth - and, not being next to my books, I'll assume for the time being that your figures are correct (if you were only using the third core as earth {not armour}, 50m of 2.5mm² would be about 0.9Ω
Whatever, you have calculated just the resistance of the SWA. The EFLI will be appreciably higher than that, since it also includes the resistance of the line and earth paths all the way back to your local substation/ transformer.
Kind REgards, John
I think we have to give a well done that he has even tried to work it out rather than the normal everything forward and trust to the Lord.
Even with a TT supply it would really make no difference to the answer to question. Unless a RCD is 1/3 of the size of the one proceeding it there is no point. And cutting a cable is more down to time than current even a 100 mA would trip if the end actually touched and earthed point for over 40 ms but often they don't trip simply as cut with an insulated blade with clean cut and so no current flows. Only when some one picks up the cable is contact made.
As the above posts, you obviously do not understand what EFLI is.
Even your calculation for your cable is incorrect as you have only assumed the resistance of one core. The only calculation that you could make for the cable would be R1+R2. For that 100m length, the R1+R2 figure would be 1.48Ω, using one core as live and one as earth.
As the above post you evidently can't be bothered to read an entire thread.
In my OP I said I had the socket at the end of 50m of SWA. So my calculation is correct thank you, as I will be using one conductor as live and one as Earth. I was trying to show BAS that I had thought of it, although I've learnt my lesson now. Ignore all of BAS's posts and wait for someone helpful to come along. Thank you ericmark for being that person and giving me the information I had ACTUALLY ASKED FOR.
I think the OP was a bit more clever than you suggest. His cable run is 50m (not 100m), so, presumably for two cores, he has calculated on the basis of a length of 100m. As I suggested, what I don't know is where the 0.0074 Ω/m resistivity came from (leading to his 0.74Ω R1+R2 figure). As I said, if just two cores were used (armour not being used as earth) then R1+R2 for 50m would be about 0.9Ω - so I don't know if he found some clever figure to use which took into account the use of armour as well as a core as the CPC, or what. I actually suspect that the answer he has ended up with (for R1+R2) is not far off!
Whatever, as has been said, he has calculated just R1+R2 of the cable, not EFLI. However, as I've also said, I cannot see for the life of me why the EFLI has any bearing on whether the OP needs a second RCD (which surely he doesn't, whatever the EFLI.
Kind Regards, John
There is already upstream RCD protection at the 'garage CU'. As I've said, I see absolutely no reason, protection-wise, for a second RCD (an RCD socket) at the end of SWA. The issue of EFLI introduced by BAS seems to be a complete red herring in relation to that question.Hi, I would have RCD protection on the circuit in some form. You may live not to regret it one day
Kind Regards, John
I think BAS's mention of EFLI was a complete red herring. The EFLI would have to be ludicrously high to prevent the existing (upstream) RCD operating and, even if that were the case, a second RCD (RCD socket) added at the end of the SWA would not operate either. EFLI is therefore irrelevant to the OP's question.
Kind FRegards, John
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