Main bondage

[ultinator]

Does main bonding in every circumstance always need to be under 0.05. main bonding in a house where the main water was moved has 10mm cable but resistance of 0.25 ohms!!>>

 

[KAS1]

Sparks are still incorrectly stated that the maximum value for the resistance of a main protective bonding conductor is 0.05 ohms. This value is applicable where access to the bonding connection is not possible and a test is made between two extraneous conductive parts (GN3 Page 35). This value is not the maximum permitted resistance of the main protective bonding conductor.

 

[Geoffsd]

But it's the same value for everything.

As I keep saying - it is a value deemed 'negligible' which is what is required.

There would be even more confusion if GN3 just stated a resistance of negligible impedance between parts must be ensured.

 

[KAS1]

So where did he get the high value from, I don't think he understands what he is measuring, "Does main bonding in every circumstance always need to be under 0.05.NO IS THE ANSWER, main bonding in a house where the main water was moved has 10mm cable but resistance of 0.25 ohms!!" where did he get 0.25 ohms between where and where, does he mean 0.25 ohms between the water pipe and the main bonding cable before they are connected together?????

 

[Knobhead]

it goes in the floow at front of property, (there are 3 x 10mm and 2x6mm, one 10mm being gas) one resurfaces under the sink and terminated there but i couldnt tell you if it is continuous, i mean why would you not run it continuous whenn the extension was being built!! grind my gears this does.&nbsp;<br>It is maybe 10-15 metre run to the water pipe in kitchen. so it obviously shouldnt be that high....so considered that, it isnt acceptable as the run is so short? or is that 0.05 only ever a rough guide. I mean 0.3 isnt really that bad surely??

Could you translate that in to something like English please.

 

[KAS1]

Thanks tony that has answered my question, he doesn't know what he's measuring...............

 

[ezzzekiel]

if as stated 0.05 is used as a guide as its considered negligable then at what value does it become a prob as if that was the case then a stated value would need to be given in bs7671.

i believe voltage drop (50v safe touch) as in supp bonds plays a part in this - 0.05 ohms is good for 27m ensuring no drop +or- 50v will occur between accessible part.

always been my take on it.

 

[Geoffsd]

if as stated 0.05 is used as a guide as its considered negligable then at what value does it become a prob as if that was the case then a stated value would need to be given in bs7671.

i believe voltage drop (50v safe touch) as in supp bonds plays a part in this - 0.05 ohms is good for 27m ensuring no drop +or- 50v will occur between accessible part..

Yes, that is what serpico raised.

Then, however, as with most things you have to calculate rather than rely on the 'guides' one size fits all

R = 50/Ia - serpico made the mistake of using the In of the 100A fuse.

 

[ultinator]

So as Geoffsd and Serpico said:

for a 100A domestic supply, the Ia value would be either 1200 at 0.4s or 630 at 5s

To which the higher would make the maximum permittable resistance of the main bonding conductor is 0.041 ohms if R< 50/1200

The cable measured was directly from the conductor at the MET to the bonding clamp using croc clips?

So would a resistance of 0.25 Ohms mean that the bonding conductor is unsafe as the resistance is too high in the event of a fault?

 

[ezzzekiel]

Yes, that is what serpico raised.

.

never read the whole post - but thats my opinion also

 

[Geoffsd]

So would a resistance of 0.25 Ohms mean that the bonding conductor is unsafe as the resistance is too high in the event of a fault?

Yes, but if the conductor is continuous it cannot be that high.

Undo it from the clamp and then - if the reading is ok on the wire itself fit a new clamp.

If not - then you will have to find the problem or fit a new conductor.

 

[Chr!s]

So as Geoffsd and Serpico said:

for a 100A domestic supply, the Ia value would be either 1200 at 0.4s or 630 at 5s

To which the higher would make the maximum permittable resistance of the main bonding conductor is 0.041 ohms if R< 50/1200

The cable measured was directly from the conductor at the MET to the bonding clamp using croc clips?

So would a resistance of 0.25 Ohms mean that the bonding conductor is unsafe as the resistance is too high in the event of a fault?

Forget about resistance, select the right size job done. If the bonding conductor is in place and the right size, just test its resistance so that it relates to length and csa.

 

[Geoffsd]

Forget about resistance,

How can you do that?

select the right size job done

Someone has done that.

If the bonding conductor is in place and the right size, just test its resistance

Oh.

so that it relates to length and csa.

But it doesn't - that's the problem.

 

[Chr!s]

How can you do that?


Someone has done that.


Oh.


But it doesn't - that's the problem.

Well, it may or may not be correct, we don't know it's route, or how it was tested.

Regardless of the above a resistance of 0.05 or 0.5 is not going make much difference.

 

[Geoffsd]

Of course it is.

 

[Guitarist]

I think the point several people are trying to make here is that if it's a reasonable length of cable, correctly sized, then the reistance WILL NOT BE that high and there is an issue somewhere which needs looking into.
It may be 0.25 ohms now (still not acceptable), but what will it be in the future if the connection is poor or there is some form of corrosion at work?