Extraneous....Yes or No....

G

Guest123

Ripped form an NIC newsletter.....


The NICEIC are now quoting a figure of 7000ohm (based on 30mA fault current for a disconnection time of 0.4s). It was dicussed in a recent connections magazine and also discussed at one of their tech talks.

From pg 47 in the magazine issue 172 ​
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 ​
 
NICEIC may say that but if someone was hurt because you followed that you could be batting on a sticky wicket as if it is not in the regs?
 
Interesting point Ian, but i think the NIC would have responsibility if anything did happen, especially as they have published it.
 
Hi,

GN8 gives guidance on this using what they term the 'let-go' threshold, quoted as 10 mA.

Using the same equation this gives a value of 22 kohms for the threshold - above this, you would not consider the metalwork extraneous.

It is really a design issue and up to the designer which value of current he doesn't want to exceed (e.g. 10 mA).

Where the NIC get off declaring something like this as fact, I don't know.

I'm sure they help create half of the misunderstanding of the regs that exists.:D

GN8 goes on to say...."the designer would need to consider possible variations in resistance and whether a lower limit on the current flowing through the human body or livestock is necessary"

With this in mind, I think I would be tempted to err on the side of caution and use a lower figure for my current - i.e. 10 - 15 mA, and go for a higher threshold.
Anything lower than the threshold obviously should be bonded.:)
 
thought an ir test between earth and unknown part had to be less than 20k ohms to be considered extraneous, although i cant find a reference to it now
 
so the nice ppl are thinking 6. 6k, and intelligent life thinks 20-23k. i know which figure i will be using!
 
ezzzekiel said:
thought an ir test between earth and unknown part had to be less than 20k ohms to be considered extraneous, although i cant find a reference to it now
Click to expand...

I seem to remember being told by NIC that a value over 0.25 megaohms would mean that the unknown part would not be an extraneous conductive part.
 
Well I've seen the 22 and 23 but never a 25K value!
I'll stick with my 23K
 
They will have to bring out a guidance book fairly soon with the way they are going and the numerous NIc regs

Its just been on the news
The Niceic has staged a coup d"etat at the IEE building in London
The present IEE administration has been given a visual condition report and found to be out of touch with insanity and electrocuted with an extraeneous plastic pipe
Tony Cable,the spokespark for the Niceic said all rebellious other tribes will now be crushed in the next few weeks

Always knew they had a need to be the dictator
 
OK guys perhaps I wasn't totally clear in my post.
The post I was replying to was talking insulation resistance (ir) test between the MET and unknown part hence the value of 0.25 megaohms.
Hope that's now as clear as mud.
Like the news flash Des.
 
hello everyone, im new to the forum and this is my first post.... just been reading this thread and have a little thing to add....

Surely if the metal pipe has a high resistance to earth it would not be a bad idea to bond it as it would be sitting at around 230v under fault conditions?

or am i just stupid? haha.....
 
Thats the reason for the test is my understanding or we would be bonding everything.

So if the test show that is is not extraneous over 22k then no need to bond.

Hope that makes sence.
 
DETERMINING IF A PART IS EXTRANEOUS OR JUST A PIECE OF METAL​
A test should be made using an Insulation Resistance Tester set on MΩ, supplying 500 Volts.
Test procedure:
Connect one test lead to the metal part and the other lead to a known earth. (MET)

If the resistance value is 0.022MΩ (22000Ω) or greater, no supplementary bonding conductor is required.
If it is less than 0.022MΩ, supplementary bonding should be carried out.

From the Ohm’s law:

I = V / R;

I = 500V / 22000Ω = 0.023Amp

This shows that if the fault was on a single phase 230Volt supply the current that would flow between the conductive parts would of course be only 0.0115Amp (11.5 mA).

This current is unlikely to give a fatal shock.

Guidance Notes GN8 and GN5 state:
" If extraneous metalwork is above 22KΩ then it need not be supplementary bonded.

 
Dave_ said:
hello everyone, im new to the forum and this is my first post.... just been reading this thread and have a little thing to add....

Surely if the metal pipe has a high resistance to earth it would not be a bad idea to bond it as it would be sitting at around 230v under fault conditions?

or am i just stupid? haha.....
Click to expand...

This is the problem when the reasons for bonding are not understood, during a fault, the voltage on the main earth terminal rises (sometimes significantly) this can then be distributed to bonded non extraneous conductive parts, creating a danger!

That is why it's important to actually establish whether an item is actually an extraneous-conductive part before we string green&yellow everywhere.
 
gotcha.... if said pipe had a low resistance to earth and its bonded (extraneous) the dis. time would be faster (less danger), if not bonded and theres an earth fault anywhere on the property would just result in another live part to grab with no path to earth (thus no point in bonding it) ... i did know this.... living in Barcelona at the mo, of the tools, the brain is a bit slow at the mo.... i remember that equation regarding a touch voltage not exceeding 50v.... coming back now... thanks mr IQ!

Dave
 
Here's a question for you all regarding this thread..... My poor first post got me doing a little thinking (yes im sad like that, but i am drinking a beer whilst posting)

During a periodic test and inspection of the installation in a butcher’s shop, it is revealed
that the circuit supplying an electrically operated compressor does not meet the maximum
earth fault loop impedance requirements. The circuit is protected by a 16 A
Type C CB, and the unit is situated 1 m from a steel sink. Explain how, under certain
conditions, this situation may be resolved by the use of supplementary bonding.
Support your answer with calculation.

This is not my work, its Brian Scaddan's the 2391 chief examiner.

lets get the old grey old matter going

ill post the answer later..........
 
on a similar vein, used to be that all metal ( stainless steel) tables and such in a commercial kitchen were supplementary bonded, all linked together. do we now perform an IR test to see if they read > 22k, and so no longer need to be bonded?
 
Dave_ said:
Here's a question for you all regarding this thread..... My poor first post got me doing a little thinking (yes im sad like that, but i am drinking a beer whilst posting)

During a periodic test and inspection of the installation in a butcher’s shop, it is revealed
that the circuit supplying an electrically operated compressor does not meet the maximum
earth fault loop impedance requirements. The circuit is protected by a 16 A
Type C CB, and the unit is situated 1 m from a steel sink. Explain how, under certain
conditions, this situation may be resolved by the use of supplementary bonding.
Support your answer with calculation.

This is not my work, its Brian Scaddan's the 2391 chief examiner.

lets get the old grey old matter going

ill post the answer later..........
Click to expand...

sticking my neck out here as i may be way off the mark, but here goes. Zs is measured with all circuits , earths and bonding in place, thus creating parallel paths. if the machine were to be bonded to the metal sink, which we assume is in turn connected to the water pipe supplying it, this may then add another parallel path, thus reducing the measured Zs. this of course assumes that the sink is not fed by a plastic water pipe.
 
Yes, a supplementary bonding conductor between sink and compressor.

The resistance for the conductor must be not greater than 50/Ia

Remember that the rated current of the MCB must be multiplied by 10 for the 'C' curve.
 
I've enjoyed reading this post and I've asked a similar question on another thread regarding. Bonding just want to know if your testing an old installation where there are metal sinks and food prep tables that are being constantly moved for cleaning and some of the bonds are loose some are broken off and some are still in-tacked. If I was to apply the 22K test I may encounter problems due to the poor condition of the bonding. Should I reinstate all the bonding or remove it and carry out the test with everything disconnected. I would all so like to pose the question that although this is not a special location there is a lot of water used in a kitchen and in wet condition's the readings may change. father than rely on bonding would it not be a better course of action to RCD protect all circuits or would this make no difference. It just don't make sense to rely on bonding where this bonding is put under such mechanical stress.
 
Think about what you are asking:

How can a food prep table introduce a potential to the installation?

Do the sinks have metallic waste pipes heading into the ground, thereby introducing true earth potential?

You need to get bonding requirements clear in your mind BEFORE doing PIRs, do you not have a copy of BS7671:2011?
 
I don't understand your reply I am quite simply stating that the installation in is current state has lot of different metal tables and sinks at different earth potentials as some are bonded and some are not. These Item are in reach of one another so potentially if you had an item of electrical equipment on one table that is faulty and has failed to trip the CB and you then touch the other table that is bonded you have now created an earth path and could be electrocuted? I either take out all the bonding and start from fresh and determine if it is actually required or I reinstate all bonds and cross bonds as they were in the original install. Or is it me?
 
Last edited:
That's why the requirements were withdrawn, the potential-rise of bonded non-extraneous conductive parts under fault conditions created a far greater hazard than not having the metalwork bonded.

If you're doing a PIR just note that "some items are bonded together and some aren't however there is no requirement for bonding" under the current edition of BS7671.
 
I had a customer with a commercial kitchen and she received a shock as she bridged 2 tables, now both tables past the 22k to met test but a metal appliance had earthed one of the tables and a seperate fault had put a voltage into the table which had no real path through her until she bridged the tables, the fault was on a damaged fridge flex laying in a spillage which had tracked into the table - this presents a situation like previously mentioned that although the tables met requirements not to be bonded the owner had introduced an inderict connection to the MET. I find myself questioning the relaxation of the regs especially on situations where the installation isnt covered by and rcd, by the way i replaced the fuseboard and left tables unbonded as the rcd would have seen the damaged cable and i cant see this been a potential hazard in future.
 
Dear All,

I may be a bit thick here but seems to me there is quite a lot of ignorance here. Common sense is surely the thing to use. Regs is regs but guidance notes are just that.
If you have portable equipment in an environment then bonding should be carried out where a touch risk exists. The twin metal tables make no difference to the 22Kohm rule. If one table was live then a critical shock path would exist between it and an adjacent table that had a resistance to earth below the 22kohm figure. I can see no point in bonding metal parts with any resistance to earth where no electric potential can be expected. This may include many bathrooms but not those connected to a localised earth potential through for example, pipework, existing bonding etc. To my mind bonding areas where no potential is likely to exist can introduce a dangerous shock risk. The question is can the individual sparky properly understand the risks, and then justify his actions. The guy who posted the 500V potential to earth doesn't seem to understand the nature of a 3 phase supply; ie that it is 230V to earth. It all makes me worry how little some sparks seem to understand the fundamentals of the supply and associated risks.
 
A fascinating thread I am ordering GN8 we have shed loads of SS kennels at work that PIR said should be bonded, but thats for a slightly differnt reason in that they take 230v leads in to them to feed heat pads, they get damaged and could liven up the kennel it was decided after discussion that if they were bonded it assure the operation of the protecting devices, now I have my doubts..mmm
 
Hi guys
My heads in bits reading this thread.
domestic sparks, stick to the regs............
designers.......stick to the regs............
Personally, I would have my CU supplied from a 300 milli amp RCD with a breaker load of say 100 amps
Then RCBO's with a 10 milli amp trip. for every final circuit.......................
Thats just me!!
Actually i'm going off grid..and **** this leccy. I'm going 12 volts with solar panels and 400 AH batteriers
 
tricky dick said:
Dear All,

I may be a bit thick here but seems to me there is quite a lot of ignorance here. Common sense is surely the thing to use. Regs is regs but guidance notes are just that.
If you have portable equipment in an environment then bonding should be carried out where a touch risk exists. The twin metal tables make no difference to the 22Kohm rule. If one table was live then a critical shock path would exist between it and an adjacent table that had a resistance to earth below the 22kohm figure. I can see no point in bonding metal parts with any resistance to earth where no electric potential can be expected. This may include many bathrooms but not those connected to a localised earth potential through for example, pipework, existing bonding etc. To my mind bonding areas where no potential is likely to exist can introduce a dangerous shock risk. The question is can the individual sparky properly understand the risks, and then justify his actions. The guy who posted the 500V potential to earth doesn't seem to understand the nature of a 3 phase supply; ie that it is 230V to earth. It all makes me worry how little some sparks seem to understand the fundamentals of the supply and associated risks.
Click to expand...

I'm afraid the problem comes these day's with the rise of the so-called Domestic Installer, who ''at best'' only understands basic domestic installations, who then moves into working in commercial premises.

Earthing and bonding arrangements can be somewhat quite involved on multi-storey commercial buildings, as they can be, in multi-story accommodation buildings, depending on the construction method...
 
Ok, so what is your take on this situation.

PIR on a flat in a building that is a sheltered accomodation (within a large building with communal areas for the residents) . Each flat has its own meter, (TNCS) and CCU. The heating to the flat is supplied by plastic pipes, operated by a local valve & stat arrnagement, i could not find the incoming water, or stopcock, (behind kitchen cupboards) BUT the pipework to the taps at the sink in the kitchen & bathroom is copper.

There is NO bonding of any kind, main or supplementary within the flat from the CCU or MET.

Bathroom circuits are NOT on RCD, with a shaver point by the sink.

IR reading on 500v between local socket & kitchen pipework is 0.18Mohm, same reading between shaver socket & bath pipes.

All other readings etc in the PIR are good & normal, building was new fabrication in 2009.

How would you report & code it, cos i am really in two minds about it. Fortunately the flat is currently empty & off supply. But there are 50 other flats in the building.
 
Cbigfoot said:
Ok, so what is your take on this situation.

PIR on a flat in a building that is a sheltered accomodation (within a large building with communal areas for the residents) . Each flat has its own meter, (TNCS) and CCU. The heating to the flat is supplied by plastic pipes, operated by a local valve & stat arrnagement, i could not find the incoming water, or stopcock, (behind kitchen cupboards) BUT the pipework to the taps at the sink in the kitchen & bathroom is copper.

There is NO bonding of any kind, main or supplementary within the flat from the CCU or MET.

Bathroom circuits are NOT on RCD, with a shaver point by the sink.

IR reading on 500v between local socket & kitchen pipework is 0.18Mohm, same reading between shaver socket & bath pipes.

All other readings etc in the PIR are good & normal, building was new fabrication in 2009.

How would you report & code it, cos i am really in two minds about it. Fortunately the flat is currently empty & off supply. But there are 50 other flats in the building.
Click to expand...

You really need sight of the incoming water-my guess is that in a communal installation from 2009 the supply will be in Alkathene pipe and therefore there will be no main protective bonding requirement.
 
You really need sight of the incoming water-my guess is that in a communal installation from 2009 the supply will be in Alkathene pipe and therefore there will be no main protective bonding requirement

Yes i really did try, but it just disappears, my best guess is it is in poly as the heating is supplied in plastic Hep.

I have to go back next week, i am goiung to check the main buildings supply, as i think that will be Poly as well.
 

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Extraneous....Yes or No....
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