One of the jobs in need of doing shortly is the replacement of my Wylex rewireable fuse consumer unit. Nothing actually wrong with it, but it only has six ways and I am going to be needing eight in the near future, so it seems like a good time to bring it up to date.This brought about an investigation of the earthing setup. Never having had need to look that closely at it in the past, I had not done a detailed study of what was there, and had probably made a few assumptions! A quick check over tonight raises some interesting points...First assumption - Being a TT System (overhead power delivery) it will have RCD protection. Having looked close up at said "RCD" I am now not so sure. Have a look at the following (out of focus) picture:- that likely to be an RCD? Or is it more likely, as I now suspect, an old style ELCB?The words at the top read "Test this protective switch often by pressing the yellow button", the words above the switch simply show "500V 50~", & "60 AMP". It's made by Crabtree. Nothing else that I can see written on it - no mention of a trip current. That feed through earth wire from the CU looks a bit suspect as well!The Henley block you see in the photo splits the tails from the meter, one set go to the item in the photo - the second set go to a much newer MK 30mA RCD that ultimately feeds the out buildings.The second point of interest is the actual earth itself - So far the only earth connection I can find is to the incoming (steel/iron) gas main. Nothing that I have yet found seems to connect to the water main, and as for an earth spike - if there is one, it is well hidden. (don't even bother asking about suplimentary or equipotential bonding!) Not sure how common this was/is on 50's properties? While the gas main might be a resonable earth (not tested it yet I do know that there is no plastic in it for a good 20m minimum) not sure I like the idea of relying on only that.(the outbuildings do have their own earth spike adjacent to them - I have not...

This is without doubt an old voltage trip and needs scrapping.Many properties of this age were earthed to a rod using steel wire and overthe years lost the connection to rust so it may well have disappeared bynow.Asking how many earth rods is a bit like asking how long is a piece ofstring. You need a meter to check (not a multimeter) the final earthresistance. Don't make the common mistake of banging in rods close to eachother thinking more is better; getting them too close actually increases theresistance. In clay you probably only need one but you buy a 'proper' earthrod they will screw together to achieve a greater depth should you need to.

Down to earth - can you identify tonight's mystery switch gear?

Apr 18, 2004 21 Replies

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N. Thornton 22 years ago

Ah! I had misunderstood the earthing arrangement. Couple of points though...

You say their main problem is nuisance tripping, yet I have observed that happen very rarely. Compared to RCDs they look like angels in this respect. The RCD is vastly inferior when it comes to nuisance trips.

As far as safety goes, v-ELCBs may not add a danger but they are so inferior to RCDs that they do represent a risk relative to an RCD protected TT system. TT systems are somewhat different to well earthed ones, and do need some extra protection because of this. And a v-elcb only goes so far in this respect.

Regards, NT

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Christian McArdle 22 years ago

Yes, I thought it said something like that. It doesn't actually explicitly ban the use of a single device, just asks you to consider the consequences. However, I got the impression that this had been recently reinterpreted to effectively ban an overall non-time delayed RCD.

Christian.

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