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Topic Title: Supplementary Bonding and Voltages from SELV lights
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Created On: 21 June 2012 07:19 PM
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 21 June 2012 07:19 PM
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Avatar for MrOther.
MrOther

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1)

http://www.electrics-home.co.u...cs/diagrams/supp2.jpg

Familar picture always given for a supplemenary bonding installation of a bathroom.

Between the the illuminares in this picture down to the shaver sockets (much like my own situation) could this be a common CPC or does it HAVE to be a 4mm2 bond (unprotected bond from a 1.5mm2 CPC) and then a 4mm2 from the shaver to the pipework?

I could understand a 4mm2 bond linking seperate circuits to the extraneous conductive parts, but when these circuits are in common with a CPC it seems a little excessive to run out an extra green/yellow.

2)

Plumber is in tomorrow to put our new bath in (how I smell!!!) the shower is going to be chased into the wall. If he follows previous form he's going to come off the metal existing pipe work (bonded) and then into plastic, the shower head would be metal. Would any of the older heads please advise me whether I should bond this or not?

I wasn't going to because of the run of plastic, but now my reasoning is flundering and I think some advice from more knowledgable persons would be greatly beneficial.

3)

Just volt tested (using my trust Fluke T-1000) to see if I got any volt drop. Testing between the two is no go as they are both at the same potential. I tested between the earth/screws of three different local switches. Using the earth merely as a reference.

Between two switches and the conductors I got 40v and from one I got 18v. Is Al's famous screwdriver in the ground the best method here, get a true reference?

Thanks all so so so much as always. Got in from work and brains back on thinking up anal silly things again. I'm easily worried.
 21 June 2012 07:39 PM
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MrOther

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I found this but can't find a reg to support it.

http://www.electrics-home.co.u...ics/diagrams/supp1.jpg
 21 June 2012 10:21 PM
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Fm

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I assume your lighting modifications are protected by a rcd/rcbo
 21 June 2012 10:59 PM
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MrOther

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Originally posted by: Fm

I assume your lighting modifications are protected by a rcd/rcbo


Soon to be. Installing 30ma RCBOs. I still want supp. bonding as I don't want to rely on an RCD/RCBO in low impedance environ.
 22 June 2012 11:01 AM
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AJJewsbury

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As Fm suggested, you don't necessarily need supplementary bonding under the 17th Ed (there are a few conditions, RCDs being one of them), but if you want to do it anyway then fair enough.

1. Only worry about pipework that can introduce a potential into the location - i.e. if pipes are bonded as they enter the bathroom it doesn't really matter what happens with short lengths of plastic or whatever inside the bathroom - there's nowhere for the metal pipework downstream of the plastic to pick up a potential from. If in doubt about metalwork entering the bathroom that might be separated from main bonding by plastic joins or even a plastic header tank then measure with an insulation tester to the MET - if below 23k Ohms, it should be bonded.

2. Yes, you can use c.p.c.s as part of the supplementary bonding scheme - it's implied nowadays as section 701 asks for bonding between all extraneous-conductive-parts and all c.p.c.s - i.e. not explicitly to exposed-conductive-parts.

3. As for the SELV lights - sorry but I've not figured out what you're measuring. If you're using a high impedance volt meter could could be picking up stray (capacitively coupled) voltages.

- Andy.
 22 June 2012 07:12 PM
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MrOther

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Originally posted by: AJJewsbury

1. Only worry about pipework that can introduce a potential into the location - i.e. if pipes are bonded as they enter the bathroom it doesn't really matter what happens with short lengths of plastic or whatever inside the bathroom - there's nowhere for the metal pipework downstream of the plastic to pick up a potential from. If in doubt about metalwork entering the bathroom that might be separated from main bonding by plastic joins or even a plastic header tank then measure with an insulation tester to the MET - if below 23k Ohms, it should be bonded.

I bond as they came in thinking along those lines. Though they have gone plastic underfloor, then a little copper for show. I can't really get to these bits one. One bit of copper is to to feed the cold bog, and can't bond as it's on show for it's length and there is no floor under it (i.e that bit of the house is bit like a balcany juttering out. Through the cracks in the boards can see my front door. LOL. Other bits of metal work are taps and shower head again all feed from plastic.

2. Yes, you can use c.p.c.s as part of the supplementary bonding scheme - it's implied nowadays as section 701 asks for bonding between all extraneous-conductive-parts and all c.p.c.s - i.e. not explicitly to exposed-conductive-parts.

Implications and the Regs are doing my head in, thank you for clearing that up for me. Say a sade is a sade and be done with it. It did seem a little stupid not including it as a bonding conductor.



3. As for the SELV lights - sorry but I've not figured out what you're measuring. If you're using a high impedance volt meter could could be picking up stray (capacitively coupled) voltages.

It were just a normal voltage tester. I tested between the two SELV conductors and got nothing (both at 12v?) and so I tested then between each conductor and the screw of a switch (believing it earthed) but when testing between the screws of three diff switches I got 2 different results (two the same, one different)

- Andy.


Thanks again Andy, you're a gentleman.
 22 June 2012 07:41 PM
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MrOther

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Originally posted by: AJJewsbury
if below 23k Ohms, it should be bonded.




Andy where does this figure come from? I'm justing writing it into my Regs book now. Have never come across it before.
 22 June 2012 07:51 PM
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OMS

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try a bit of ohms law

I = V/R

I = 230/23000 (or 23kohms)

I = 0.01A or 10mA

At 10mA you may feel it, but you won't die (not from shock anyway)

Does that help

Regards

OMS

-------------------------
Failure is always an option
 22 June 2012 07:58 PM
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hertzal123

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Hi Andy,
"If below 23K bathroom pipework should be bonded",is there not a minimum resistance reading back to the met that would prove that local bonding is not required?
Regards hz
 22 June 2012 08:12 PM
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OMS

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Originally posted by: hertzal123

Hi Andy,

"If below 23K bathroom pipework should be bonded",is there not a minimum resistance reading back to the met that would prove that local bonding is not required?

Regards hz


Generally 23kOhms - or have I misunderstood the question ?

Regards

OMS

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Failure is always an option
 22 June 2012 08:45 PM
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MrOther

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Originally posted by: OMS

try a bit of ohms law



I = V/R



I = 230/23000 (or 23kohms)



I = 0.01A or 10mA



At 10mA you may feel it, but you won't die (not from shock anyway)



Does that help



Regards



OMS


That's a face palm moment. Thanks OMS. Defo written that in my Regs. Nice little bit of knowledge.


Hertz they are saying 23k is a minimum (though not a written rule/law) as it indicates a clear earth path back to the MET.
 22 June 2012 09:04 PM
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OMS

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No drama - I've had more than my share of "face" "palm" moments over the years - they won't kill you, trust me.

Even less so if you learn something from them -

Regards

OMS

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Failure is always an option
 22 June 2012 09:08 PM
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Parsley

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I thought the pipework in the bathroom was deemed as being connected to the MET if the resistance is less than1666 ohms, if its 30ma RCD protected as per 415.2.2 and therefore supplementary bonding could be omitted.
I.E. R<50V/Ia R=1666ohms Ia =0.03 Amps (30ma) and V=50V

Regards
 22 June 2012 09:48 PM
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Parsley

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See wiring matters issue 26 launch bathroom pdf

http://electrical.theiet.org/wiring-matters/2008.cfm
 22 June 2012 10:58 PM
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MrOther

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Originally posted by: OMS

No drama - I've had more than my share of "face" "palm" moments over the years - they won't kill you, trust me.



Even less so if you learn something from them -



Regards



OMS


I have any more face palm momenst this week OMS I'll be lucky if cosmetics can save me. But you and Andy certainly have learn't me something. I will be exchanging it off as my own currency of knowledge come the morn
 25 June 2012 10:41 AM
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Parsley

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Originally posted by: Parsley

I thought the pipework in the bathroom was deemed as being connected to the MET if the resistance is less than1666 ohms, if its 30ma RCD protected as per 415.2.2 and therefore supplementary bonding could be omitted.

I.E. R<50V/Ia R=1666ohms Ia =0.03 Amps (30ma) and V=50V

Any thoughts?


Regards


 25 June 2012 11:06 AM
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OMS

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Originally posted by: Parsley

I thought the pipework in the bathroom was deemed as being connected to the MET if the resistance is less than1666 ohms, if its 30ma RCD protected as per 415.2.2 and therefore supplementary bonding could be omitted.

I.E. R<50V/Ia R=1666ohms Ia =0.03 Amps (30ma) and V=50V

Regards


Yes, Parsley - you can evaluate it's connection to the MET from R<50V/Ia when you are using the RCD, for sure

The OP, however, didn't want to make a claim on the RCD and 23kOhms would give you a sensible value to determine the extraneous part isn't connected to the MET - I think we were discussing slightly different things.

Regards

OMS

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Failure is always an option
 25 June 2012 05:59 PM
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Parsley

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Hi OMS

I was originally answering hertzals query.

If the pipework can only introduce an earth potential via a high impedance joint or connection to the water pipework (which is at earth potential) it's not actually an extraneous conductive part in it's own right. But should still be bonded, if the reading is >1666ohms and <22K ohms between the MET and the incoming bathroom pipework with all parallel paths disconnected.

I'm thinking about this under fault conditions and wondering what the likely Ut would be between the exposed conductive parts and extraneous conductive parts in the bathroom, if the supplementary bonding wasn't in place. The incoming water service pipework to the property would be main bonded and the touch voltage would depend on the Ze and R1+R2 ratios. I think; but may be wrong that the Ut in the bathroom would be further reduced due to voltage drop across the high impedance pipework joint? Depending on the value of the high resistance pipework joints and the associated voltage drop the Ut may be reduced to <50V. If we use the GN8 senario 95.8 V touch voltage with main bonding in place and a Uo 230V, Ze of 0.35ohms, Zs 2.4ohms etc would the potential touch voltage in the bathroom be 95.8V minus the VD across the joint? which is Uo/Zs x joint impedance, so if the joint impedance is above 0.53ohms would the associated voltage drop across the pipework joint prevent the potential being introduced to the bathroom pipework when the circuit parameters are passed on the GN8 example?

Best Regards

 25 June 2012 06:54 PM
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hertzal123

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Sorry if this may sound simplistic,but would a reading as high as 1666 ohms back to the met be a rather tenuous connection prone to fluctuation?
Regards hz
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