Does this need to be bonded?

[HappyHippyDad]

This may help determining an extraneous conductive part

Hi cliffed,
These posts are definitely extraneous as they measure 101Ω to the MET. However, they are pretty much the same as the rod in the ground (66ohms). If anyone touched them under fault conditions the RCD would trip as they are below 1667ohms.

I've heard conflicting arguments about this.

Its clear to me why an Extraneous Conductive Part (ExtCP) needs bonding IF it measures between 1667Ω and 22KΩ to the MET, as between this range the current into the human (poorly worded) between a fault and the ExtCP will be >10mA.

The bit that is not clear to me is why do we still need to bond IF the ExtCP has a sufficiently low resistance to MET, meaning that the RCD will trip if we come into contact with it under fault conditions.

I have used 1667Ω as this is the known figure to trip a 30mA RCD in order to limit voltage to 50V. However, we use a figure of 200Ω for stability reasons AND we would have to be certain that the ExtCP is not going to be altered in any way (thus perhaps increasing its resistance to between 1667Ω and 22KΩ).

In my scenario, the posts are <200Ω to earth AND are not going to be altered, so why do they need bonding? What risk do they pose?

 

[cliffed]

Hi cliffed,
These posts are definitely extraneous as they measure 101Ω to the MET. However, they are pretty much the same as the rod in the ground (66ohms). If anyone touched them under fault conditions the RCD would trip as they are below 1667ohms.

I've heard conflicting arguments about this.

Its clear to me why an Extraneous Conductive Part (ExtCP) needs bonding IF it measures between 1667Ω and 22KΩ to the MET, as between this range the touch voltage between a fault and the ExtCP will be >10mA.

The bit that is not clear to me is why do we still need to bond IF the ExtCP has a sufficiently low resistance to MET, meaning that the RCD will trip if we come into contact with it under fault conditions.

I have used 1667Ω as this is the known figure to trip a 30mA RCD in order to limit voltage to 50V. However, we use a figure of 200Ω for stability reasons AND we would have to be certain that the ExtCP is not going to be altered in any way (thus perhaps increasing its resistance to between 1667Ω and 22KΩ).

In my scenario, the posts are <200Ω to earth AND are not going to be altered, so why do they need bonding? What risk do they pose?

They may need bonding for a definite earth connection reference point

 

[DPG]

Can't see the point of connecting a metal rod in the ground to another metal rod in the ground.

Or can I?

 

[HappyHippyDad]

Can't see the point of connecting a metal rod in the ground to another metal rod in the ground.

Or can I?

Covering your arse

 

[pc1966]

Can't see the point of connecting a metal rod in the ground to another metal rod in the ground.

Or can I?

Depends on the goal. Linking them lowers Ra and also under high current faults could drop the step potential gradient a bit, but really to goal is to avoid it being needed in the first place!

 

[DPG]

Depends on the goal. Linking them lowers Ra and also under high current faults could drop the step potential gradient a bit, but really to goal is to avoid it being needed in the first place!

Well, a bit. But the Ra of the proper earth rod is not going to be helped much by the resistance of the hoops I wouldn't have thought.

 

[pc1966]

Well, a bit. But the Ra of the proper earth rod is not going to be helped much by the resistance of the hoops I wouldn't have thought.

Probably not.

But ultimately for a TT system the issue of CPC being not being elevated comes down to reliable RCD protection, I would go for a dual RCD box (100mA incomer, few 30mA RCBO with DP switching per circuit), a decently reliable rod, and not bother with any bonding.

 

[LastManOnline]

This may help determining an extraneous conductive part

Am curious as to why the attachment you posted stated 7.67 Kohm, s as the cut off point for what is regarded as extraneous.? I expected to see a figure of 22 Kohm, s?

 

[Pretty Mouth]

Am curious as to why the attachment you posted stated 7.67 Kohm, s as the cut off point for what is regarded as extraneous.? I expected to see a figure of 22 Kohm, s?

If you use 10mA for the calculation, you get 22k ohms, if you use 30mA you get 6.67k ohms (not, i believe, 7.67k ohms)

 

[HappyHippyDad]

If you use 10mA for the calculation, you get 22k ohms, if you use 30mA you get 6.67k ohms (not, i believe, 7.67k ohms)

You would get 7.67k ohms with the calculation given. It's just that the calculation we generally use includes taking off 1k ohms for resistance through the body. Hence 22k rather than 23k, or in this case 6.67k rather than 7.67k.

 

[LastManOnline]

You would get 7.67k ohms with the calculation given. It's just that the calculation we generally use includes taking off 1k ohms for resistance through the body. Hence 22k rather than 23k, or in this case 6.67k rather than 7.67k.

So which of the above figures would be regarded as the cut off point for when something is regarded as extraneous?. There is quite a gap between 7.67 and 22 Kohm.

 

[cliffed]

So which of the above figures would be regarded as the cut off point for when something is regarded as extraneous?. There is quite a gap between 7.67 and 22 Kohm.

Good point thinking the 7.67 would be the obvious @ 30ma

 

[HappyHippyDad]

So which of the above figures would be regarded as the cut off point for when something is regarded as extraneous?. There is quite a gap between 7.67 and 22 Kohm.

I'm about to sit down to a cup of earl grey and a few pieces of toblerone to watch a film, so i'm afraid I haven't got the regs to hand. However, this PDF explains it quite well.
I would always class anything below 23K ohms as extraneous.

 

[LastManOnline]

I'm about to sit down to a cup of earl grey and a few pieces of toblerone to watch a film,

Thanks. And sorry to hear you are having such a stressful afternoon

 

[ImpededLoop]

Assuming that they are extraneous and that they will be extended with metallic hoops to for the structure of the polytunnel, I would say that they need to be bonded. You've mentioned a lighting installation, so it might be possible to touch a light fitting and a hoop simultaneously? Also, what about the controls for the heating? Where will they be located and do they have exposed conductive parts?

You may not need a 10mm bonding conductor. Have a look in GN8 as you may be able to use 6mm on your TT system.

Your ECA doc gives a clear explanation and I think I would want to bond it if the hoops are metallic (they may well be plastic). It's all about keeping simultaneously accessible bits of metal at the same potential (or no potential at all if they're not extraneous - ie don't bond random metal bits and bobs if they're not extraneous!). The fact it's TT makes no difference and if the RCD fails makes it all the more important.

 

[LastManOnline]

Your ECA doc gives a clear explanation

Won't download for me. What's the doc called?. I can Google it

. It's all about keeping simultaneously accessible bits of metal at the same potential

If simultaneousely accessible and class 1 accessories are used then that changes the scenario significantly. But I understand that previous posts established that the metal posts were not simultaneousely accessible.

 

[loz2754]

I'm about to sit down to a cup of earl grey and a few pieces of toblerone to watch a film, so i'm afraid I haven't got the regs to hand. However, this PDF explains it quite well.
I would always class anything below 23K ohms as extraneous.

A useful reference

 

[ImpededLoop]

Won't download for me. What's the doc called?. I can Google it

key factsheet: bs 7671: 2008 – extraneous-conductive

Free essays, homework help, flashcards, research papers, book reports, term papers, history, science, politics

studylib.net

 

[LastManOnline]

key factsheet: bs 7671: 2008 – extraneous-conductive

Free essays, homework help, flashcards, research papers, book reports, term papers, history, science, politics

studylib.net

10ma been the threshold for "no let go" is new for me. I had always being led to believe it was 16milli amp.A very Interesting read indeed.

 

[timhoward]

It's a great document and one of the clearest explanations I've ever seen.
I'm not sure it helps this situation though other than establishing a helpful target let-go current.
See the diagram below and note the bit circled. That wouldn't be there in the situation this thread is discussing.

Isn't the point here that in all cases there's an 1000 ohm path straight to earth with at least 230ma current flowing, whether you add a parallel path through a unbonded pole (roughly 1000+ 110 ohms using value to MET) or a parallel path through an bonded pole (roughly 1000 + 66 ohms)?
Am I thinking about this right or losing marbles?!