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Topic Title: Testing for extraneous conductive parts
Topic Summary: new Niceic guide
Created On: 26 February 2014 06:25 PM
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 26 February 2014 06:25 PM
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tillie

Posts: 789
Joined: 03 May 2006

Hi , the Niceic have just produced some new guides for various situations.

I have just read their guide on locations containing a bath or shower.

Now when testing whether a piece of metalwork is extraneous I have always been led to believe that if the value is above 23000 ohms then it does not require bonding.

The guide is recommending a value of 7600 ohms ie 230/0.03.

I have seen this value mentioned before but have always stuck with the higher figure.

Does anybody else use 7600 ohms ?

One more point that has been bugging me.

Reg 701.415.2 states that when testing for supplementary bonding and there is not an Rcd in circuit then you can confirm bonding is present by using the formula R> 50/In where In is the fault current to trip the OPD within 5 secs.

So why is that regulation even there if we are then told that the resistance should be < 0.05 ohms.

Advice please
 26 February 2014 07:59 PM
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geoffsd

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Have you a link, please?
 27 February 2014 07:08 AM
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zeeper

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0.05 ohms


what is the above for/from
 27 February 2014 09:02 AM
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sparkiemike

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To test if a part is extraneous, the following from the issue 172 of the NICEIC connections magazine.

The metallic part can be assumed not to be an extraneous-conductive-part if the following condition is met:
Rcp> (Uo/Ib) - Ztl
where,
Rcp is the resistance between the conductive part and the MET in ohms
Uo is the nominal voltage to Earth in volts
Ib is the value of current through the body in amperes that should not be exceeded. (The value may be taken as 30 mA for a disconnection time of up to 0.4 s, as given in DD IEC/TS 60479)
Ztl is the impedance of the human body in ohms. The value suggested in DD IEC/TS 60479 is 1000 ohms where Uo is 230 V (50 Hz) under dry or wet conditions.
Taking Ib as 30 mA and Ztl as 1000 ohms (as suggested above where the disconnection times in the installation are 0.4 s or less and Uo is 230 V), the limiting value of Rcp is given by:

Rcp > (230/0.03) - 1000
Rcp > 6667ohms


Thus, if Rcp exceeds 6, 667 ohms, the pipe may be considered not to be an extraneous-conductive-part, such that main bonding of the pipe is not required


Some people prefer to use the more onerous (or safer value) of 23000ohms (10mA) to test for extraneous conductive parts.

I also recall seeing something like this in the soon to be discontinued online essential guide

When questioned the NICEIC said it was up to you to decide what figure to use.
 27 February 2014 09:42 AM
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Parsley

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If the main protective bonding to the property meets 411.3.1.2 and 30ma rcd's are fitted to all circuits of the location. if you test between the the main protective bonding/met and any pipe work in the bathroom if the resistance between them is less than 1,667 ohms (415.2) you can omit supplementary bonding if you wish. There are risks it especially if some of the final circuits of the property aren't 30ma RCD protected. Think of an existing pre 17th property that has a bathroom re-fit the bathroom meets the 17th requirements but other non RCD protected circuits might have 5 sec disconnection times.

Regards

The 0.05 is from GN7 & GN3 I think.
 27 February 2014 09:55 AM
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Crazycolours

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Well I would use the figure of 23,000 ohms as we always have, because that limits the current to 10ma (at 230v), whereas 7600 ohms will result in a current of 30ma flowing, which is considered to be at the upper level of shock tolerance of the human body (especially when in wet conditions).

If I found anything less than 23,000 I would bond it. The 0.05 figure is the maximum resistance that your bonding conductor should be.

Rgds, C.
 27 February 2014 10:13 AM
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AJJewsbury

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The 0.05 is from GN7 & GN3 I think.

It is indeed from GN 3. There has been a lot of confusion about it though - leading some to believe that bonding conductors over a certain length (around 27m from memory) had to be bigger than 10mm2 to comply. I have been able to confirm that this was NOT the intention of the GN 3 committee. 0.05 Ohms originated as the maximum expected value for the connection of the bonding conductor to the extraneous-conductive-part - i.e. the resistance across the clamp only - NOT including the resistance of the bonding conductor itself. BS 7671 has requirements for main bonding conductor CSAs, but not for their overall resistance nor lengths - GN 3 can't impose extra constraints.

The latest edition of GN 3 attempted to clarify that point.

0.05 also makes a useful rule of thumb - if the overall resistance from the MET end of the bonding conductor to the extraneous-conductive-part tests at 0.05 Ohms or below, then you can be satisfied that it's satisfactory without further ado. But if it doesn't then you can't just say it's a fail, but have to do the calculation (based on conductor CSA & length, plus 0.05 Ohms for the connection and maybe a bit for meter accuracy) to see if your reading is as expected or not.

- Andy.
 27 February 2014 11:53 AM
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geoffsd

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Originally posted by: tillie
Reg 701.415.2 states that when testing for supplementary bonding and there is not an Rcd in circuit then you can confirm bonding is present by using the formula R> 50/In where In is the fault current to trip the OPD within 5 secs.

Just to clarify - it should be 50/Ia.
 27 February 2014 08:40 PM
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tillie

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Hi thanks for all replies.

I realised I had made a mistake but assumed quite rightly that people would know what I meant.

That is a useful piece of information regarding the 0.05 across the cable clamp.

I shall look out for it in GN3.

Concerning the test for supplementary bonding then 0.05 is a rule of thumb and does not have to be adhered to.

If in a bathroom there was only a lighting circuit and I had to test for supplementary bonding then I would measure from my lighting CPC to the metalwork in question and hopefully get a reading of 0.05.

But if I got a reading of 0.31 then regs wise this should still be fine because 701.415.2refers us to 415.2 and the formula R>50/Ia which in this case with a B6 mcb will be 50/30 =1.6 ohms.

Is this right ?

This is all hyperthetical , I like to know things thoroughly and this is bugging me.

Regards
 27 February 2014 10:59 PM
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Parsley

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Originally posted by: tillie

Hi thanks for all replies.



I realised I had made a mistake but assumed quite rightly that people would know what I meant.



That is a useful piece of information regarding the 0.05 across the cable clamp.



I shall look out for it in GN3.



Concerning the test for supplementary bonding then 0.05 is a rule of thumb and does not have to be adhered to.



If in a bathroom there was only a lighting circuit and I had to test for supplementary bonding then I would measure from my lighting CPC to the metalwork in question and hopefully get a reading of 0.05.



But if I got a reading of 0.31 then regs wise this should still be fine because 701.415.2refers us to 415.2 and the formula R>50/Ia which in this case with a B6 mcb will be 50/30 =1.6 ohms.



Is this right ?



This is all hyperthetical , I like to know things thoroughly and this is bugging me.



Regards


The lighting circuit would be 30ma rcd protected so the max reading would be 1,666ohms.

However GN7 page 20 states the following in relation to supplementary bathroom bonding.

"In practice, the resistance between bonded extraneous-conductive-parts and exposed- conductive-parts should not exceed 0.05 ohm."/I]

Andy, I remember GB highlighting the GN3 main protective bonding conductor <0.05 confusion several years ago.

Regards

Regards
 27 February 2014 11:00 PM
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geoffsd

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Yes, correct, tillie.
 27 February 2014 11:03 PM
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geoffsd

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Originally posted by: Parsley
Andy, I remember GB highlighting the GN3 main protective bonding conductor <0.05 confusion several years ago.

But that is to confirm the effectiveness of the bonding - not to determine whether it is required in the first place.
 28 February 2014 10:05 AM
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WiredScience

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Originally posted by: tillie

But if I got a reading of 0.31 then regs wise this should still be fine because 701.415.2refers us to 415.2 and the formula R>50/Ia which in this case with a B6 mcb will be 50/30 =1.6 ohms.

Is this right ?


Not quite. R <= 50/Ia, not "greater than" otherwise why bother with extra G/Y?
 28 February 2014 10:36 AM
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GeoffBlackwell

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Originally posted by: geoffsd

Originally posted by: Parsley

Andy, I remember GB highlighting the GN3 main protective bonding conductor <0.05 confusion several years ago.


But that is to confirm the effectiveness of the bonding - not to determine whether it is required in the first place.


It is just related to the resistance of the connection to a conductive-part. It is not to determine if bonding is required.

Regards

Geoff Blackwell
 28 February 2014 11:21 AM
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geoffsd

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Originally posted by: WiredScience
R <= 50/Ia, not "greater than"

Of course, missed that bit.
 28 February 2014 12:20 PM
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Legh

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Has this question from tillie been answered?

Why is there a disparity between the IET's GN3, GN5 and GN7 and the NICIEC's technical guide?

Would I want to receive a 10mA shock current or a 30mA shock current whilst standing in a shower or relaxing, hopefully, in a bath tub.? I think we know that answer and if 50mA is sufficient to send a dry body into fibrillation then I would suspect that 30mA across a wet body would have the same effect.

A wet body is definitely more susceptible to electric shock than a dry one.

Legh

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 28 February 2014 02:02 PM
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Parsley

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From the latest GN3

"Testing bonding conductors and earthing conductors To confirm the continuity of these protective conductors, test method 2 may be used.
This method can also be used to confirm a bonding connection between extraneous- conductive-parts where it is not possible to see a bonding connection, e.g. where bonding clamps have been 'built in'. The test would be done by connecting the leads of the instrument between any two points such as metallic pipes and looking for a low reading of the order of 0.05 ohm (it should be noted that not all low-resistance ohmmeters can read this low, see section 4.3).
Where metallic enclosures have been used as the protective conductors, e.g. conduit, trunking, steel-wire armouring, etc. the following procedure should be employed:
1 Inspect the enclosure along its length for soundness of construction
2Perform the standard continuity test using the appropriate test method described above. Instrument: Use a low-resistance ohmmeter for this test - section 4.3.


As Andy stated earlier the new GN3 has tried attempted to clarify see expected test results below. I think the wording in the paragraph above could have also been improved.

Expected test results
The results should first and foremost indicate no open circuit in the protective conductors. For lengths of conductor use Appendix B for resistance data. For joints across bonds by earth clamps and similar, the readings should approach 0.05 ohm taking into account both the resolution of the instrument and its accuracy at low values."

I haven't seen the NIC's new publication that Tillie refers to. Supp bonding may be omitted (see risks mentioned in my first post) if the designer wishes if the resistance between the MET and the extraneous conductive parts is equal or less than 1,666 ohms Ra<50/0.03 as 30ma RCD's will have to be installed in any new installations.
In a pre 17th installation sup bonding can be verified using 50/Ia.
which in Tillie's example is for a non rcd protected 6 amp type B MCB would be 30A so equal or less than 1.6 ohms.

Regards
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