Testing exposed pipe work

[Lilyrose08]

Hi All, When testing exposed metal work to establish whether it needs bonding you: You test from main earth disconnected from MET to exposed metal work insulation tester @ 500V what readings should you expect if it needs bonding ? Also when testing with a continuity test what readings should you expect then ?

 

[Sintra]

The formula is:
Rcp>(Uo/Ib)-Ztl
where:
Rcp is the measured resistance between the conductive part concerned and the MET of the installation (in ohms),Uo is the nominal voltage to Earth of the installation (in volts), Ib is the value of current through the human body (or livestock) which should not be exceeded (in amperes), and Ztl is the impedance of the human body or livestock (in ohms).


Rcp>(Uo/Ib)–Ztl=(230/0.01)-1000=23000–1000=22Kohms

 

[Guitarist]

IET Guidance notes 8 will tell you all you need regarding this subject.

 

[soulman]

Supplementary Bonding. (22Kohms) You cannot use a low resistance ohm meter. Insulation resistance tester on pipework & a known earth: above the value of 0.02Mohms, just a piece of metal. Below the value of 0.02Mohms, extraneous conductive part this should be bonded.

The calculation in sintra post above, relates to 10ma as a maximum amount of current in that location to pass through the body.

Hope this helps

 

[D Skelton]

The formula is:
Rcp>(Uo/Ib)-Ztl
where:
Rcp is the measured resistance between the conductive part concerned and the MET of the installation (in ohms),Uo is the nominal voltage to Earth of the installation (in volts), Ib is the value of current through the human body (or livestock) which should not be exceeded (in amperes), and Ztl is the impedance of the human body or livestock (in ohms).


Rcp>(Uo/Ib)–Ztl=(230/0.01)-1000=23000–1000=22Kohms

Or as I do it:

(230/0.005) - 1000 = 45kohms as I was always taught that the max current allowable through the body (Ib) should not exceed 5mA when determining whether or not something is extraneous.

p.s. I'm not saying you're wrong

 

[Guitarist]

Or as I do it:

(230/0.005) - 1000 = 45kohms as I was always taught that the max current allowable through the body (Ib) should not exceed 5mA when determining whether or not something is extraneous.

p.s. I'm not saying you're wrong

And here we are full-circle...
10mA is classed as the "let-go threshold", and used to be (not sure if it still is) the rating of RCD's in hospitals. I agree 5mA is much safer as a limit, but 10mA is generally accepted as safe, for the use of calculations.

 

[D Skelton]

Haha, full circle indeed! Yes I agree, 10mA is widely regarded as the figure to use, I just personally choose to use 5mA on the back of several recommendations official and unofficial and also because like you say, it is a safer figure to use.

I wouldn't be classing an electrician as incompetent if they chose to use the figure of 10mA, although I would always push to have them use 5mA instead

 

[shagbite]

guitarist............any chance you can ask that big gay bald bloke in the silver lamè bikini top to stop staring at me? he's making me nervous.

 

[Sintra]

Guide to effects of current on human beings and livestock provides data for Ztl and Ib. For hand-to-hand contact, the value of the body impedance is given as 1000 Ohms in dry conditions where Uo is 230 V. You can then select a value of Ib between the two extremes:0.5 mA – the threshold of perception and 10 mA – the let-go threshold.

 

[telectrix]

so we could take the average, 7.5mA and use 30.667K ( - the 1k for the body) , n'est pas?

 

[D Skelton]

Or go the whole hog and look for 999Mohms IR? Lol.

 

[Guitarist]

guitarist............any chance you can ask that big gay bald bloke in the silver lamè bikini top to stop staring at me? he's making me nervous.

That made me chuckle...
You leave my twin brother, Ryback, alone you big bully.

 

[Geoffsd]

Has anyone had a measurement between 21,999Ω and 46,001Ω ?

If so, what was the cause?

 

[DJ Ovengloves]

OP, you are testing whether the pipe is extraneous (i.e. already effectively already at Earth(ish) potential)...with that in mind you leave the Earth connected at the MET when testing with an Insulation Resistance tester - you are comparing "the lump of metal in question" with a known Earth potential.

 

[D Skelton]

Remember you would need to make sure any bonding conductors or DB earthing conductors are disconnected though.

 

[DJ Ovengloves]

Well said..apologies for not stating this.

Remember you would need to make sure any bonding conductors or DB earthing conductors are disconnected though.

 

[soulman]

Excellent point d skelton!

 

[davethsparky]

Apologies if I am missing the blindingly obvious here but why do you need to disconnect the main bonds to do the tests?

Are we not looking for exposed metalwork which requires bonding after main bonding has been applied? There is surely a possibilty that you will find a bunch of exposed pipes which are continuous metal from the incoming supply that will be identified by this test as needing supplementary bonding despite the fact that these pipes will be effectively bonded when the main bonding is re-connected. You coukd end up adding extra bonding unecessarily.

What is supplementary bonding actually supplementary to? I was under the impression that it is a bond intended to supplement the main bonds? ie: a bond which is applied after the main bonding to supplement it where needed.

 

[Guitarist]

OP, you are testing whether the pipe is extraneous (i.e. already effectively already at Earth(ish) potential)...with that in mind you leave the Earth connected at the MET when testing with an Insulation Resistance tester - you are comparing "the lump of metal in question" with a known Earth potential.

You don't have to leave it connected to the MET. In fact, I usually find it easier to disconnect the main earth cable from the MET, rather than taking everything else out.

 

[Geoffsd]

I am not sure if the original question relates to main or supplementary bonding and I think some posts are referring to one and some to the other.

Apologies if I am missing the blindingly obvious here but why do you need to disconnect the main bonds to do the tests?

For main bonding - to determine if a part is extraneous in itself, obviously any connections it may have to the met and other extraneous parts should be removed.

For supplementary - all main bonding and cpcs will be left connected.

Are we not looking for exposed metalwork which requires bonding after main bonding has been applied? There is surely a possibilty that you will find a bunch of exposed pipes which are continuous metal from the incoming supply that will be identified by this test as needing supplementary bonding despite the fact that these pipes will be effectively bonded when the main bonding is re-connected. You coukd end up adding extra bonding unecessarily.

What is supplementary bonding actually supplementary to? I was under the impression that it is a bond intended to supplement the main bonds? ie: a bond which is applied after the main bonding to supplement it where needed.

Yes.

 

[davethsparky]

So basically if I had paid more attention and actually read the thread properly my question would not have been necessary!

 

[Geoffsd]

No, I'm still confused.