to bond or not to bond

[leep82]

I know its been done to death but in going to ask for your thoughts opinions anyway.

Just turned up on a job that ive been on for a while where i have had no communication with the plumber. Everything has been through the customer. The plumber has fitted a new central heating system with a boiler that is oil fed. The bund for the oil sits on a concrete pad and has been in for weeks. I was told by the customer that the pipe to feed the boiler would be surface fixed and of plastic, hence, no need for any bonding. Standing looking at it now and the pipe has been buried for 2/3 mtrs and although it has a thick plastic coating is quite clearly a copper pipe. Is the plastic coating enough to suggest that bonding may not be necessary?

 

[Richard Burns]

Sorry I am just saying that a 230V pd on 7670Ω would generate 30mA of current.

Since additional protection by an RCD at 30mA is considered generally non life threatening then can we apply this to a conductive part that may present a 30mA current to someone holding it in the case of a fault?

 

[Pete999]

If we accept 30mA is safe for RCDs, then 30mA should be safe for bonding as well, I suppose.
Though it always assumes that the protective device operates.

https://www.youtube.com/watch?v=v38PfIVy2rI

 

[yellowvanman]

A similar article was in Certsure's (NICEIC) quarterly magazine about 18 months ago, and I queried it with my asessor last year. He was unaware of it and felt it was incorrect and did say he would investigate further. When we met this year he didn't get anywhere with it as no one in the NICEIC was prepared to say it was incorrect!

I think one of the GN specifically mentions the 23k value.

 

[Leesparkykent]

Yep I wondered about that, can anyone enlighten me?

Apparently within IEC/TS 60479 it gives a value of 30mA for the value of current through the body that should not be exceeded 230V/0.03=7666 but surly this figure should even have 1000 subtracted due to impedance of the human body? The way I was taught and I think many others the current that's accepted to not be exceeded through the human body is 10mA. 230V/0.01=23000 subtract 1000 due to the impedance of the human body gives us the figure of 22000 ohms. I Know which figure I would rather stick to and its not what the NICEIC state :crazy:

 

[davesparks]

Sorry I am just saying that a 230V pd on 7670Ω would generate 30mA of current.

Since additional protection by an RCD at 30mA is considered generally non life threatening then can we apply this to a conductive part that may present a 30mA current to someone holding it in the case of a fault?

I will honestly have to put pencil to paper, and probably consult the books to clear this one up in my mind I think.
Though moving house at the moment means I haven't a clue where any of the above items are currently located

 

[Richard Burns]

I have just looked in GN8 and it specifies that it is up to the designer to choose the value of current that is acceptable
0.5mA - perception threshold
10mA - let go threshold
30mA - <300ms muscle contraction, <5s breathing difficulty, revesible heart disruption, immobilization, but generally no organic damage.

The resistance of the human body approximating to 1kΩ gives resistance values of:
459kΩ for 0.5mA
22kΩ for 10mA
6.7kΩ for 30mA

since bonding faults could be reasonably long term in some cases I would tend to aim for 10mA as at least you can get away from the fault .

 

[yellowvanman]

The other problem is how do you measure 6.7kΩ as opposed to 22kΩ. Most insulation testers have a 10kΩ resolution at these levels of resistance, my Fluke's lower limit is 0.02M, lower than that it displays 0.00!

 

[davesparks]

The other problem is how do you measure 6.7kΩ as opposed to 22kΩ. !

With a test meter which has a suitable measurement range

 

[leep82]

Wow this has escalated. If i assume it is indeed extraneous, my next problem is where to actually place my clamp. The oil feed enters the building and terminates immediately into the back of the boiler. There is no exposed metal pipe work that is accessible inside the building.

 

[davesparks]

Wow this has escalated. If i assume it is indeed extraneous, my next problem is where to actually place my clamp. The oil feed enters the building and terminates immediately into the back of the boiler. There is no exposed metal pipe work that is accessible inside the building.

At the point of entry to the installation or as near as practicable. The point of entry to the installation is presumably the point at which it passes through the external wall, so bond it either on one side of the wall or the other.

But of course that information is irrelevant if there is no exposed metal inside the building because it does not require bonding if it is not able to be touched!

 

[leep82]

Wow this has escalated. If i assume it is indeed extraneous, my next problem is where to actually place my clamp. The oil feed enters the building and terminates immediately into the back of the boiler. There is no exposed metal pipe work that is accessible inside the building.

At the point of entry to the installation or as near as practicable. The point of entry to the installation is presumably the point at which it passes through the external wall, so bond it either on one side of the wall or the other.

But of course that information is irrelevant if there is no exposed metal inside the building because it does not require bonding if it is not able to be touched!

this is what i have been asking myself. In fact there is very little exposed metal work throughout the entire run of the pipe as it is covered in a plastic coating. There are small sections where an elbow joint is used and again where it terminates into the bund. These sections arent big enough to fit a standard clamp but being as though they are outside and there is no chance that you can physically touch any of the exposed pipe and any other exposed metal part, does it actually need it?

 

[Richard Burns]

If the oil pipe joins the boiler on a non conductive point and the oil pipe inside the boiler is non conductive such that there is no continuity between the oil pipe and the boiler itself then there are no extraneous conductive parts exposed to touch within the equipotential zone and no requirement to bond.

 

[leep82]

I dont think that the pipe does join the boiler on a non conductive point, and am wondering if this is why, when i did my test to check if it was extraneous my reading was 00.0Mohms, as the water feed to the boiler is also at the same point

 

[telectrix]

basically BS7671 allows that a 10mA current through the body is safe (22k ohms. but niceic accepts 30mA. ( 6.7k ohms ). any niceic officials want to test their theory? "get hold of these while i switch on". i know where i'd sooner be if coming into contact with a live cable, considering the state of my furred up arteries.

 

[Risteard]

Apparently within IEC/TS 60479 it gives a value of 30mA for the value of current through the body that should not be exceeded 230V/0.03=7666 but surly this figure should even have 1000 subtracted due to impedance of the human body? The way I was taught and I think many others the current that's accepted to not be exceeded through the human body is 10mA. 230V/0.01=23000 subtract 1000 due to the impedance of the human body gives us the figure of 22000 ohms. I Know which figure I would rather stick to and its not what the NICEIC state :crazy:

Indeed, or you could even go a step further to limit any current to below the perception threshold where a human being would not even be aware of it.