Consumer units -how do they work?

RCD's do not provide protection against electrocution Nick. They are there to prevent fires. The claim is that the bulk of fires in the UK are caused by faults in the electrical circuitry in houses.

I have had a kick from the mains at work a couple of times. They feel the same even when there is an RCD there. Peoples immunity varies too. PAT probably has more gain in this area but as far as I know not needed in domestic environments.

Wouldn't surprise me if the bulk of house fire of this nature were down to rubber insulated house wiring that has perished but statistics are statistics and don't always account for cause on things like this - just like the effect of so called safety cameras hidden around corners to catch people for minor speeding offences.

Quick edit. I thought that a range of rcd trip currents were still available

http://www.screwfix.com/p/bg-80a-100ma-double-pole-type-ac-rcd/5948P?kpid=5948P&cm_mmc=Google-_-Product%20Listing%20Ads-_-Sales%20Tracking-_-sales%20tracking%20url&gclid=CPXUvPWRsdECFY8Q0wodbIsE_g

The low limit was either 5 or 10ma - can't remember which. Next step as I understand it is none at all and clear marking but it's hard to see why at this level.

John

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Edited By Ajohnw on 07/01/2017 23:05:09

John,

30ma tripping current of the RCCB is so designed to prevent fibrillation and so prevent death by electric shock and is well documented . Prevention of fires by electrical faults can normally be achieved by the use of a 100ma RCCB.

Emgee

Edited By Emgee on 07/01/2017 23:36:19

Yes, agree with Emgee. The downside is they breed over-confidence when working on live circuits.

.

There must be a 'Darwin' joke in there ^^^

MichaelG.

Edited By Rick Kirkland 1 on 08/01/2017 09:51:16

John, you cannot use an earth spike for bonding you will get nowhere close to the resistance required. As to the size of the bonding, don't forget you need to consider faults external to the building as well as internal. And you cannot assume that just because today your water pipe is low resistance to the supply earth it will be so next year.

I agree with the comments about RCDs breeding over-confidence, too many people think they prevent an electric, as in some how magically prevent more than 30mA going through. Whereas in practice all they do is limit the duration of the shock.

Come on Rick, don't hold back, tell us what you really think

Seriously though, electrics is no different to anything else, for example hobbyists messing with dangerous metalworking machines that they have little experience of

I think the great thing is people can post what they like on forums like this and there will hopefully be someone around to correct them if they mis-understand something. Like you say, there are plenty of "electricians" out there that don't really have a clue what the regs mean or why so it is hard to criticise a non electrician for occasionally getting it wrong.

Ok, I cannot resist this- that is not quite accurate

the external potential can be something other than earth

Edit: btw, for those that want to read up on earthing and bonding I can recommend the IET guidance note 8 (earthing and bonding). A riveting read (ok, maybe not...) which gives a lot more info although I think doesn't go far enough in explaining why as apposed to just what.......

Edited By Toby on 08/01/2017 10:24:34

btw. While I am correcting things, I need to give myself a good slap and correct myself!

Yesterday I said......

That should of course have read " In fact the 10mm2 min is actually for PME/TNCS, for TNS or TT it can sometimes be 6mm2".

Ok by me but afraid I don't entirely agree. One of the odd jobs I did was design and help install a large ev battery pack cycling installation. 200v+ packs, lots of them. We had some experts in on this particular subject. Why I don't really know as precautions obviously needed to be taken. There were concerns about the fact that it was DC too and much higher voltages when they were on charge. It turns out that much of the information that is available comes from experimentation with pigs but doesn't really match the reports from actual accidents. It can go either way and is somewhat dependent on the person and also just where the current flows. In one extreme is seems that people have caught fire in electric chairs before they die and on the the other hand much lower currents can kill.

I'd strongly advise people not to stick one hand on a well bonded sink and then poke a finger on the other hand into something live. Sums it up pretty well really. That particular path is worst case. It also looks like there is no specific call for sink bonding in BS7671 or by the IET. This may be why.

One thing for sure when I had to work on a live panel they didn't make me feel over confident. The word panel in my sense though is some box maybe a large one with all sorts of bits and pieces in it.

John

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Aw Rick. Is that too harsh? I think I've learned all I need to know about consumer units and electrical equipment from this thread. I am now even confident enough to go change my consumer unit, in case it goes up in flames due to the rubber wiring. I plan to go down the local pub and pick up some more information and 'hearsay' from the drinkers before I start. I'll probably thumb through the Screwfix catalog as well, as it contains a wealth of information for potential installers! And of course I will wear thick heavy gloves as I dismantle the consumer unit, in case the rats get me. LOL. (Ps, I'll probably do it live, as I am one of those with a different immunity to electric shocks).

Steve

Edited By Steven Vine on 08/01/2017 11:18:37

Edited By Steven Vine on 08/01/2017 11:31:02

Hi Rick

If the Zs table (A41.1to A41.6?) in the on-site guide is copied from and the same as that included in BS7671 why is it wrong to enter the Max Zs permitted for the cct by reference to a table of values ? provided of course the correct part of the table is selected for the type and rating of the protective device and maximum disconnection time for the cct.

The cct measured Zs value will be written in the relevant column in the test readings to check compliance.

Emgee

Just a point regarding the Darwin risk. If you do some stupid thing in your workshop and rip your hand off, you probably will learn a valuable lesson but it will most likely be limited to personal education. If you arse with electrics the collateral damage almost certainly will include OTHERS. This is the distinction between competent persons, they are the ones who know that they don't have the correct information and go find it rather than a quick look on wikipedia and some assumptions about what "they" think should be OK.

Mark

John, you are going to have to explain what exactly you disagree with. Safe currents and voltages are always going to depend on circumstances and the subject. Just because RCDs don't work sometimes (no surprised to anyone I hope....) doesn't mean they don't serve a purpose. Ditto earthing and bonding.

And why should the regs have something specific to kitchen sinks? The general rules on bonding cover them just fine. We could start a whole new discussion on when to bond and when not to but I don't think that would be appreciated

As to live working, it is clear Rick was talking about domestic electrical work and he is right, there is no excuse for live working there except just lazyness. I know we are all tempted to bend the rules sometimes but that isn't really an excuse. I can tell you from experience that when the DNO fuse goes with a big bang and a flash you feel pretty embarrassed and stupid!

On the other hand, if your job happens to be connecting up a new supply from the road to a house you would be well used to working live (in a hole in the road, half full of water.......)

I'd second that comment Toby. If they want to know why not just buy the IET guide.

Maybe I am a control freak but maybe not. I like to know what should be done who ever does it. It can be useful. For instance the gas pipe bonding. The guide states something entirely different in my situation where the meter isn't in the house so if some one at some point wants to wreck the appearance of the place which seems likely I can point that out.

I understand that the guide may mention things that the building regs don't and the advice is to follow them 'cause if something goes wrong an electrician might find themselves being sued.

Toby has raised an interesting point as to why. For instance if some one has a metal waste pipe that contacts the earth it must be bonded.

The gas pipe needs seems really weird to me though. I'd love to know the reasoning.

As to 100ma RCD's. I have no need. If I did I would take the trouble to find out the why's a wherefores of their use. The only reason I posted the link was to show that they were available.

John

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This thread is getting silly!

Russell.

I have to admit I am not clear what Rick means, the on site guide tables are not the same as BS7671 as the on-site has a 0.8 correction factor to take into account conductor temperature. I wonder if he is referring to Cmin, I don't have the latest copy of the guide to check if that is now included in the tables.

Or maybe Rick is refering to circuits with RCDs where Zs can exceed this figures (but Zln should not....). Actually that is something that has always annoyed me, the certs (and hence what most people test) concentrates on Zs, completely ignoring that it still needs to work with a Line-Neutral fault.

EDIT: but yes, Rick's post above mine and Russell before that are right, I am also going to shut up now

Edited By Toby on 08/01/2017 12:00:05