Main Bonding Gas/water Impedance

[Dal Roberts]

10mm Main Bonding measured end to end  0.20 ohms gas and water, can this stay if fitting new consumer?

 

[Sidewinder]

Why not?

Is the sizing OK?

Is it two seperate runs, or one?

Noting that if it is one run, it must be continuous.

Oh and being pedantic, it's a resistance if you are measuring it with a low resistance Ohm meter, it would only be impedance if you were using an a.c. Current in your meaurement.

 

[Dal Roberts]

two separate runs, i thought it was 0.05 ohms and yes resistance; O)

0.05 max reading ?

 

[Sidewinder]

Between adjacent bonded points.

Where did you get the 0.05 Ohms from?...

 

[Dal Roberts]

i got 0.20 ohms end to end , 2 tests consumer disconnected wander lead to earth tag croc on cable

 

[Sidewinder]

OK, I get how you measured the value, leads nulled out for the wander lead I take it.

Where did you get the idea of the maximum being 0.05 Ohms from?

 

[Manator]

I think you are getting confused.

Resistance can be measured and calculated providing you know the basics. I would doubt those readings as the length of cable would be close to 100m.

10mm cable has a resistance of 1.83miliohms/meter.

so your reading 0.20 =

(0.20ohm / 1.83milliohm) x 1000 = approx 109m

I would check your test leads

 

[Dal Roberts]

 i think i made the mistake of not zeroing the wander lead to be honest , ill try again tomorrow, thanks for your help

 

[Manator]

Sidewinder asked a very important question, where did you get the 0.05Ohm from?

Remember the resistance of any cable is dependant on its csa and length(as well as its material), so could be any given reading.

 

[Sidewinder]

Dai,

We are not criticising you, we are trying to guide you to your own enlightenment! ; )

Please come back and let us know your readings in case you did not null out for the wander lead, I ask, because I have forgotten to do that as well.

However, please think about where the 0.05 Ohms comes from, that way it can be a learning experience for many?

 

[Dal Roberts]

0.05 ohms  they should not exceed because in the event of a fault the exposed and extraneous conductive parts will rise to the same potential , so this reading is not really to ensure a good earth path , but for that reason i guess. 

 

[Sidewinder]

Yes OK, however, where did you get that figure from?

Thin air, college, a document?...

Yes bonded parts must remain at substantially the same potential across an installation, and this combined with earthing, should result in the rise in potential being negligible for the duration of the fault, IF, the disconnection time of the OCPD is correct, and the earth fault loop impedance is sufficient to allow the correct disconnection.

However, I would still like to know the exact place you got the 0.05 Ohms from.

That's all.

 

[Manator]

Stay with us Dal, I think after tonight you will be enlightened. We often take for granted what we know but we all had to learn at one point.

 

[Blue Duck]

Is there not a guide somewhere which states that with old bonding 4mm or god forbid 2.5mm cable if it reads <0.05 between the met and water/gas/whatever>? then its acceptable???

I'm sure I found it on the esc contractor guide or something (?) when I was trying to get out of re-doing  the bonding as it was a right mare, I ended up putting in 10mm reluctantly.

 

[Sidewinder]

BD,

It is the "EXACT" reference and the context, that Manator & I are after, so anything constructive you can add to the fray please do.

 

[pickleye]

It maybe a college generated figure. I saw it on a PowerPoint or two. Also stated as max variation for outlets on a ring.

No idea where it came from before that.

 

[Manator]

There is a statement Blue duck, but we are trying to get Dal to search and digest information. There is a general confusion amongst many electricians and the basic science behind all is often overlooked even by myself at times.

It is good to go back to basics from time to time.

 

[Dal Roberts]

[No message]

 

[Blue Duck]

Sidewinder / Maneater I'm sure it used to be in the Q&A guide on the ESC website but now they changed the name (and Directors) I couldn't find it on the website.

:C

 

[Sidewinder]

Dai,

Please, please understand that Manator & I are in NO WAY criticising you.

There are some "urban myths" wrt the 0.05 Ohms figure that it could be useful to get to the bottom of, now, I'm not sure where you guys have got it from, and TBH, I will have to look up references before I can comment on the exact source, and no I am not getting you to do my research, as my source will not be available to many I suspect.