general testing questions

[just add water sparks]

i am well and truly confused now.great are 'nt they abbreviations. yesterday i drove my m.p.v to do a p.i.r on an install it was wired in p.v.c and some old v.i.r had some good Zs's tho.Not sure on some cable sizes as g.a.t.s was not taken into calcs da da da lol only joking guys.I know are my few brain cells right in telling me right that m p b is main ? bond and s b c is supplementary bond connection.Has the figure gone from 0.05 to .005 and not sure what figure i am looking for & what i am testing now.genuinely all help is appreciated this game is certainly ambiguous ain't it

 

[pushrod]

i am well and truly confused now.great are 'nt they abbreviations. yesterday i drove my m.p.v to do a p.i.r on an install it was wired in p.v.c and some old v.i.r had some good Zs's tho.Not sure on some cable sizes as g.a.t.s was not taken into calcs da da da lol only joking guys.I know are my few brain cells right in telling me right that m p b is main ? bond and s b c is supplementary bond connection.Has the figure gone from 0.05 to .005 and not sure what figure i am looking for & what i am testing now.genuinely all help is appreciated this game is certainly ambiguous ain't it

know how you feel lol - certainly keeps you on your toes with all the abbreviations especially when people make up their own (not this thread)
MPBC =main protective bonding conductor
SPBC = supplementary protective bonding conductor .

The correct figure is 0.05 ohms it is what it applies to is the problem !!!

 

[Deleted member 26818]

just to clarify i was refering to question (1 ) testing if extraneous conductive are adequate by testing with an insulation resistance tester and long leads and does not meet requirements if >22Kohms i was just using the long lead r2 method were you refering to an r1+r2 method which does not apply to this situation?

No I'm not refering r1+r2.
I was suggesting that rather than obtaining extra leads and joining them together, that you use a conductor out of the supply cable as a long lead.

 

[Chr!s]

Hi Chris - very interesting. I'm aware that brb does not specify resistances. I had this very sort of discussion with the tutor who was teaching me the 2391. I was saying so what if a mpb conductor had to be 30metres long , that would put it over 0.05Ω he said you would have to increase the csa.

It a common misconception, i remember my 2391 trying to tell us this, i explained to him he was incorrect, if you read GN3 carefully you will see that its not placing the 0.05 on the overall length. As i said above the nic also gave the same advice, so i explained and they agreed.

AFAIK the brb does not ask you to test mpb conductors if you can visually confirm continuity. However, if you can not visually confirm continuity a test would have to be done. What result would you be looking at if you were testing between a gas and water pipe (two extraneous-conductive-parts) - isn't gn3 suggesting it should be this 0.05 value?

GN3 is referring to where say the structural steel is used as a bonding conductor for example and can not be removed, testing between two points the resistance should be no more than 0.05.

What your looking to achieve is to limit the touch voltage, so you could use the equation used for effectiveness of supplementary bonding as this is in effect what your trying to achieve. That would need to bee one hell of a length of of cable between two points especially when the minimum is 6mm.

edit : reading gn3 again 2nd bottom paragraph P35 it talks about using test method 2 where the bonding clamps have been "built in" then connect across extraneous-conductive-parts and check for the 0.05 value. The extraneous conductors could be gas and water? I don't know why it doesn't say between
extraneous-conductive-part and the MET

Well its referring to any two points, this could be two of many in one given installation.

Are you and IQ talking of supplementary protective bonding conductors ? Surely they connect between an exposed-conductive-part and an extraneous-conductive-part?

Im talking about Main protective bonding though in principle your trying to achieve the same as supplementary bonding conductor limiting touch voltage.

Oh and yes the figure is 0.05 ohms my typo sorry

 

[Chr!s]

Pushrod, i dont have my GN3 to hand, but does it mention simultaneous?

 

[pushrod]

Thanks for the very detailed reply Chris - must admit i still haven't got my head around it completely - will have to give it a bit of time for it to be assimilated

[

GN3 is referring to where say the structural steel is used as a bonding conductor for example and can not be removed, testing between two points the resistance should be no more than 0.05.

Take your point and can vaguely remember something about this in the brb or a niceic guide, will have to look when i get a chance.

I must admit i interpreted gn3 differently.

I took the "eg where bonding clamps have been 'built in'. " just to mean that access to the clamps on MPBCs was restricted, ie behind panelling. Definitely see that you could be quite correct though.

Those IEE guys should get someone on board from the Plain English Campaign!

 

[pushrod]

Pushrod, i dont have my GN3 to hand, but does it mention simultaneous?

No nothing at all in this section about that, or being able to touch at same time. [That is why i was wondering about distant gas and water pipes where clamps might not be accessible] .

 

[Chr!s]

I took the "eg where bonding clamps have been 'built in'. " just to mean that access to the clamps on MPBCs was restricted, ie behind panelling. Definitely see that you could be quite correct though.

Agreed, GN3 just gives and example, its just saying where the bonding can not be removed the resistance between the two points should be no more than 0.05, this figure is only guidance too.

The BRB just requires a continuity test, 612.2.1, for PMB amd SMB, but SMB has the 50V rule which seeing as your trying to acheive the same thing through PMB makes sense to apply the same principle.

No nothing at all in this section about that, or being able to touch at same time. [That is why i was wondering about distant gas and water pipes where clamps might not be accessible] .

No nothing at all in this section about that, or being able to touch at same time. [That is why i was wondering about distant gas and water pipes where clamps might not be accessible] .

No your correct, looked earlier on, just states between two points

 

[tony mc]

Thanks

Must admit I was of the same opinion the main bonding conductor being 005 ohms or less and have just read it again and can see why.

Its amazing what you learn from the forum.

Thanks again Chr!s.

 

[Chr!s]

Well were in good company, below was ECA take on it, i hasten to add others shared the same view too.

Hi Please could you give some guidance on the maximum length of a Main protective Bonding Conductor. People mention the 0.05, though the only reference is in GN3, though this reads as a test between two points where the bonding connection is say structural and not applied to an overhaul length. The regs only mention the CSA. Now we are trying to limit touch voltage, so is there a fundamental calculation which may be used? Many thanks Chris

ECA technicals response

The length of main bonding conductors is not normally something that needs to be considered in most installations Chris. The maximum touch voltage between extraneous conductive parts for general installations is 50v (some special locations are limited to 25v). You would have to have a very long main bonding conductor to get that kind of voltage drop.

 

[pushrod]

Thanks Chris- i must admit i was always a bit sceptical about that 0.05ohm figure.

 

[pushrod]

Just playing with the numbers (must have too much time on my hands lol)

A figure in the order of 0.05Ω for a 10mm² single with a R/m of 0.00183 means that you can have a max length for your main protective bond of just over 27m.

Just randomly thinking about a circuit with the largest load that could come in contact with pipework in a house i thought "shower circuit with a 40A C type mcb"(suppose could equally could be a cooker circuit). The fault current for operation within stated time is 400A.

to restrict the Vd to 50 V

since Vd = V/A/m x Ib x L

So max length L = Vd/(Ib x V/A/m)
= 50/(400 x 0.0044)
= 28m for the length of the MPB

agreeing with a Resistance in the order of 0.05Ω
Bit of a coincidence or something in it - what do you think?

Seems like if you had a resistance that was considerably greater than 0.05 Ω you would not be guaranteeing that your bonding would be limiting touch voltage to 50V in worst case scenarios. So whether you go with that 0.05 or work it out like you do for limiting volt drop the answers are pretty similar.

(chose C type breakers as the IEE's Electrical Installation Design Guide table quotes max lengths for supplementary bonding conductors for C type MCBs)

 

[Chr!s]

Hence the equation 50/Ia

 

[Chr!s]

I think the other point is that the 50V is additional protection, where additional protection is not required touch voltages may exceed 50V.

 

[pushrod]

Hence the equation 50/Ia

TBH i don't think that does follow. The equation R = 50V/Ia is more commonly used to confirm the effectiveness of supplementary bonding and calc the max length of sup bonding. If that were used to work out the length of 10mm bonding then in the case i gave
R = 50V/400A = 0.125 ohms.
Resistance of 10mm = 0.00183Ω/m so that would correspond to a bond length of 68m.

What i did was work out the length of conductor you could have with a 400A fault current before you got more than a 50V volt drop... 28m