Bonding Clamps etc.

[andrewpreece]

Hello Folks, Looking at my earthing (TN-S) I see a 6mm2 earth wire come from the service head thingy to my CU, then another green 6mm2 earth wire exits the CU and dives below the floorboards to clamp onto my lead water pipe. Since I now have gas and central heating pipes which appear unbonded ( except by secondary means ) I decided to bond them to the main earth, but I have some questions that Googling didn't clear up.

(1) Can I use 6mm2 earth cable for the short runs from my gas and CH pipes to the main earthing point?

(2) I have some BS951 clamps which are a puzzle to me: I thought that one threaded the strap through the clamp and tightened the screw, which would tighten up the clamp on the pipe, but what I took to be the clamp section ( it has the earth terminal attached to it ) does not move at all when the screw is tightened ( yes I backed the locknut off ). Two metal dimples in the main clamp section prevent it from tightening on the pipe. I figured I was wrong about how they worked so just pulled the strap tight and screwed the screw down to lock it, but it is well nigh impossible to get the strap tight in this way - the clamp rotates and so can't be giving a good connection. What am I doing wrong?

(3) I believe that it is unnecessary to run an earth cable to my bathroom ( which has no bonding at all ), and that merely bonding all the exposed metal bits to each other with 4mm or protected 2.5mm cable is OK??????

(4) I understand that suplementary bonding is not required in anyother situation, i.e. kitchens and toilets????

(5) I cannot fit all the new earth cables into the earthing block in my CU - only one terminal is still available, so I bought a metal busblock (?) which I intend to mount exposed on the board, alongside the CU, and I will bring the main service head earth into that and then take all other earths out of that block, i.e. to the CU, to the CH pipes, to the gas pipe etc.

thanks for any answers or observations,

Andy.

[Andrew Gabriel]

That sounds more like TN-C-S. For TN-S, the earth conductor would come from the cable sheath under the service head.

Your installation isn't up to current regs. If you are going to do anything to the installation, you need to bring the earthing up to current regs, which includes replacing the existing earth bonding, based on how you have described it.

Service bonding should normally be 10mm2. Main earth conductor to supplier's earth connection should be 16mm2.

The strap goes round the pipe and then through the slots which raise it off the pipework a second time. Pull the strap tight with pliers and bend it sharply where the tail exits the bracket to stop it slipping back in. Then the screw clamps onto the strap, deforming in the bracket and pulling it tight on the pipe. The screw should not touch the pipe itself, nor deform the strap enough that it is pushed back onto the pipe under the bracket -- if it does, you didn't pull it tight enough to start with or it slipped back through the bracket before you got it tight. Once you've used a clamp, the strap is deformed and cannot be reused.

Yes. It should be connected to an earthing terminal in the bathroom (ideally the earths of all circuits in the bathroom), but does not need a dedicated run back to your main earthing terminal.

Only in rooms containing a bath or shower. Personally, I also do it within 2-3m of the kitchen sink too, but this is not done by everyone, not a requirement, and not always held to be a good idea either.

That's fine. You need 16mm2 from the supplier's earth to this terminal, and into the CU. The other bonding can normally be 10mm2.

[andrewpreece]

Thanks Andy. I understood that TN-S is most common for urban houses, and that there would most probably be a sign saying so if it was PME (TN-C-S ). I can't immediately see why if the sheathed cable enters the service head/cutout that means it's TN-C-S. Is there a foolproof way to tell?

I had a feeling someone would say that :-(

I've read conflicting stuff about this - some say the size of service bonding depends on the earthing type in question. 16mm2 is out of the question - the CU won't take 16mm2 nor will the brass earth terminal on the service head: you're going to say now I should get the 'lectric company in to replace my service head........ :-(

I figured pulling the strap tight with pliers might work. However, the screw does not deform the strap pulling it tight. The strap sits in the U portion of a metal stamping shaped thus:- U¬ , which is held in position by the rest of the clamp. All that happens when you tighten the screw is that the strap is clamped to the 'U' and prevented from moving - it does not tighten at all, hence my headscratching. All the tightening has to be provided by the initial pulling the strap tight, and neither with fingers nor pliers does this result in a good connection.

Am I to understand that the clamp as purchased has to be disassembled ( with a bit of grunting I can pull out the U¬ shaped bit and reassemble the clamp so the U¬ shaped bit is free to move down under the action of the screw ( by reinserting it in the slotted bit below the metal dimples ). I can see in that way that the strap will be dragged down under the action of the screw to eventually clamp the strapagainst the U¬ shaped bit and the pipe. Why no clues if that is the way it is supposed to work? It's like one of those Xmas cracker puzzles. Will try it anyway.

What passes for an earthing terminal in a bathroom? I have no electrics in my bathroom.

OK

Hmm, as befroe, not physically possible with my main earth terminal and CU accepting 6mm2 maximum AFAICS.

cheers,

Andy.

[andrewpreece]

Correction, I have determined my main earthing cable is 10mm2, not 6mm2 as I said, and the main earth terminal could take a 16mm2, but the CU cannot. I have plans to upgrade the CU so there is hope I could have 16mm2 main earthing cables eventually, only I have to stick with 10mm2 for now, unless I parallel two up.

I have just unscrewed the Bakelite cover where the main cable terminates, and emerging from the main cable there is a sheathed red core, probably

35mm2 or some such, a skein of smaller twisted black insulated conductors, which I take to be the neutrals, and 4 bare copper conductors, adding up to 16mm2 probably, going tothe main earth terminal.

I suspect this proves it to be a TN-S system then, as I appear to have a discrete earth return in the cable.

Andy.

[Stefek Zaba]

6mmsq is undersized by current Regs thinking: the main earthing conductor (from service-head-thingy to earthing block, and on to CU earthing terminal) is s'posed to be 16mmsq, and the main bonding conductors (to water, gas, CH and so on) to be 10mmsq. I'm not sure of the contractual/legal position on fiddling with the earth connection provided by your supply company, but ideally you'd want them to do the 6mmsq -> 16mmsq upgrade rather than d-i-y, even for The Best Of Reasons and with the Best Of Intentions. They may take the opportunity to turn you into a TN-C-S (PME) installation while they're about it, as this is the Modern Fashion (and does reduce the earth loop impedance, i.e. make it yet more likely that fuses will blow nice and quickly in the event of L-to-earth-conductor faults).

On the what-I-take-to-be-totally-standard-pattern clamps I have... the 'clamp section' indeed doesn't move. You wrap the strap round the pipe and under the end of the screw. As you tighten the screw down (yes, with the locknut backed off ;-) the two sections of the strap - the captive start and the part you just threaded through - both deform under pressure from the screw, pulling the strap tight against the pipe. Since there's not a great deal of movement, I find it useful to keep the loose end of the strap pulled tight with pliers, by rolling the outside of their jaws against the side of the clamp, until the screw's far enough in for the strap to grip tight against the pipe. And you can't really reuse the clamps - once the strap's bent in, especially the captive end, it ain't going to spring back when you slacken off the screw. Other than that - I can't offer any pearls, or even bits of frosted glass, of wisdom.

That's about it. The bathroom stuff is all about *supplementary* bonding, so that all the bits of metalwork which are capable of introducing a potential are at the *same* potential. That it ends up being earth potential is incidental (and in some circumstances undesirable, even).

You understand rightly. It's the wetness and nakedness (most importantly, bare-footedness, though an electrocution path might pass through any other parts of the skin which rest firmly against the metallic items at differing potentials) of the (immediately-post-)bathing body which are the main causes of the lowered resistance which increases the risk of shock: so it's baths and showers we care about. If you take your imitation of 'The Naked Chef' overliterally, you may want to indulge in supplementary bondage around your kitchen sink, but that's a matter of personal taste, rather than strict regulation. (Quiet at the back there, Simpkins!)

You done good: a separate earthing block is preferred practice, since it makes testing of earth impedance easier. (Our reading today is from section 4.10 of the On-Site Guide: the lord Cook spake thusly unto us, "[whilst] the earthing bar is sometimes used as the main earthing terminal; however, means must be provided in an accessible position for disconnecting the earthing conductor to facilitate testing of the earthing". For so requireth fivehundredandfortytwo-dash-ohfour-dash-ohtwo.

Y're welcome - Stefek

[andrewpreece]

Hmmm. I'm thinking about leaving it all now, it's too much trouble for no benefit. Read my supplementary message where I correct myself, I think I have 10mm2, not 6mm2 as I said. It's close enough for country work, ain't it!

Everyone says the same thing - that the straps deform under the screw and tightens on the pipe, but my straps don't deform at all, you can undo them and just see a faint circular scrape mark where the screw contacted the strap, and that's even if you apply an unfeasible amount of torque to the screw. Nor does the strap tighten even slighly, as there is no force acting on the strap which could tighten it. I am coming to believe that these clamps must be disassembled then reassembled in a different arrangement to that they were purchased in before they will work. I have pulled one apart and rearraged it and will test my theory tomorrow.

thanks,

Andy.

[Steven Briggs]

While we're on earth clamps, whats the preferred method of dealing with the embossed aluminium safety labels with a slot at both ends? a) Leave both ends captive on the strap round the pipe (but label is too long for 15mm pipe) b) Leave one end captive & the other flapping in the breeze. c) Trap it onto the free end of the strap (by folding the spare strap)? d) Toss it in the bin (Noooo!)

[Andrew Gabriel]

Look at the FAQ:

> Service bonding should normally be 10mm2.

It does. I gave you the figures which cover most residential installations. In theory, your supplier can ask for larger conductors in some cases too.

I don't think I've seen such a service head, except possibly a 5A one in the base of a streetlamp column. Does this or the CU have multiple terminals -- if so you can split the conductor strands into two terminals. Otherwise, I guess you're stuck with using

10mm2, which is probably OK if you only have a 60A cutout.

Sorry, but I can't picture what you're doing wrong. Can't find any web pictures to point you at either.

In that case, just bond.

[Lobster]

Back in the good old (pre-Part-P) days I deliberately omitted bonding from the kitchen last time I did a rewire, having read up on this issue here; and for my trouble I failed the Periodic Inspection carried out by a sparks after I'd finished!

David

[Stefek Zaba]

Yeah - there's a significantly misleading bit in Whitfield's usually reliable guide-to-the-Regs, which more or less claims that Kitchenz Need Bondage... and since Whitfield's a lot more readable and practically-oriented than the Regs, it takes precedence in many a real-life sparky's mind.

[GymRatZ]

Not having a clue (about this) myself, I found it could be _almost_ neatly attached to the strap and through the loose end of the strap whilst passing over the top of the clamp/screw. i.e. capping the screw meaning you could not undo the screw without first removing the "warning" label.

Just seemed like the logical way of doing it in today's "safety paranoia" society.

[andrewpreece]

I am informed that the label should come off the strap and be attached to the screw before the clamp is assembled. This is another thing they didn't tell me when I bought the clamps - that's now two bits that have to be disassembled and reassembled in a different position. The rationale is that the warning label should be easily readable. Of course, I now have 5 clamps all clamped badly onto pipes because I didn't reposition the sliding clamp bit and with the labels still attached to the strap and the labels obscured 'cause that's where they were when I bought them!

Andy.

[andrewpreece]

> >> Service bonding should normally be 10mm2.

[Andrew Gabriel]

I do c) when I have this type, but I also have a box of them where the label is trapped under the locking screw rather than being on the strap.

[andrewpreece]

having looked inside the service head, I can identify the live conductor, red insulated and central to the cable. Surrounding the red core are then some

16(?) smaller black insulated wires that are stuffed into a terminal to make neutral. There are then four bare copper conductors of the same size that go to the earth block. Thinking about it, I have no idea whether these are dedicated earths or simply four neutral wires with their black insulation stripped back out of sight in the cable, and co-opted into being earths, in which case I have TN-C-S. Why is nothing ever easy?

I think you're right about the 16mm2 - I 've just found a website that gives

16mm2 as the size of the main earth conductor for TN-S. And TN-C-S.

I was wrong, the service head will take 16mm2, it was difficult to see, all covered in dust at at a bad angle.

See my earlier post - think they need disassembly then reassembling slightly differently. I think they may be assembled that way to keep them all in one piece during transportation.

OK -thanks.

[Dave Stanton]

Are you sure ?, try it because I have just replaced our old 10mm2 earth cable with 16mm2 from service head to cu. The service cable was installed in 1972 and the provided terminal took 16mm ok.

Dave

[Lobster]

Talking of Whitfield's and bondage...

I'm quite confused by the explanation of bonding the incoming gas and water supplies to te main earthing terminal.

Figure 5.13 (on p96 in the 6th edn) it shows the connection being made to the gas supply on the house side of the gas meter - fine. But the connection to the rising main is shown on the street side of the stopcock, albeit with the proviso that it should go to the house side of the stopcock if the pipe is insulated on the street side.

Now why is that? Why doesn't Whitfield show the default connection to the rising main being on the house side of the stopcock? (This in fact was my other 'failure' by the same electrician - I had my bonding cable on the street side (all metal pipework)!

David

[Dave Plowman (News)]

I just bonded both sides of both. :-)

[Andy Wade]

Careful! - you're only supposed to bond the consumer's installation pipework. On metal pipe gas supplies Transco often install an insulating insert on their side of the meter, to prevent (or at least reduce the risk of) current flowing in their pipes. It won't help if this is shorted out :-(

[Dave Plowman (News)]

Right.:-)

My incoming gas main is iron barrel which is imbedded directly in the earth. A few years ago, they inserted a plastic pipe in it - very clever - no disruption to the garden path.

I'm totally confused about all the various options, so simply bonded absolutely everything together - including a vast earth rod. But not the incoming leccy SWA main sheath as it looked not that sanitary.

It measures ok. :-)