0.05 Max Impedance for Main Bonding Conductor

[phawk]

Got massively ribbed on another post for not knowing that max impedance for a main bonding conductor is 0.05 ohms, so I expect some more for this post!.......

I'm going to assume there is a very simple answer for this, but despite me going blind reading GN8 I can't for the life of me work out where this figure comes from.

I'm assuming it's just ohms law, I=V/R

0.05 ohms = 50v (safe touch voltage) / 1000A

Where does the 1000A come from?

Or at least point me to the page in GN8 please!

Thanks

 

[phawk]

Wow how bad is my ohms law?

I'll try R=V/I shall I?

 

[tony mc]

Body resistance.


Where does it say max 0.05 Ohms for bonding conductors? Or are you talking about supp bonding conductors !!

My books must be different!

 

[phawk]

It was a reply to an earlier thread I posted, I asked what the max impedance on my main services bonding should be and was told 0.05 ohms. Seems to be the general consensus but I can't find a justifiable reason for the 1000A. Was told it was the fault current - but why?!

 

[phawk]

1000A body resistance? What am I missing tony mc?

 

[Joe1979]

Honky, It's the same reason that they allow that value as the tollerance when measuring R1 + R2 at socket outlets - i.e. each socket outlet should be within that range of each other.

 

[phawk]

Cheers Joe, crystal clear as usual!

 

[WayneL]

Paul, 0.05 ohms is what is considered negligible resistance - in other words it's low enough to not matter - it's not derived from ohms law. (If anything, it will be calculated using volt drop and resistivity of 10mm copper.)

That's why socket outlet R1 + R2 measurements are considered okay within this tolerance as the difference is too small to matter.

The same applies to your 'Main Bonding Coductors :- Ideally, you don't want any difference in voltage between the metal that is bonded, and the MET that it is bonded to.

As this 'ideal' situation is unlikely to be achieved, because any cable connecting the two together is going to have some sort of resistance, (and therefore 'volt-drop'), it is deemed that, again, 0.05 ohms is too small a resistance to matter.

Think of the MET being at 0 volts and the incomming water main being at 0 volts.

If there was a fault on the installation, the MET would rise in potential to 230 volts.

Without bonding, the water main remains at 0 volts, giving a PD between the two of 230 volts....dangerous!

With bonding, and a 'negligible' resistance between the two points, they would both rise, (near enough simultaneously) to 230 volts......so, (in theory), no danger.

Bonding isn't actually anything to do with fault current - that's earthing.

Hope that helps.

 

[Joe1979]

Honky,

The earthing conductor for a 100A service with 25mm tails needs to be 16mm (17th edition regs table 54.7 page 162).
The bonding should not be less than half the size of the earthing conductor, the only common size down is 10mm.
A maximum resistance of 0.05ohms for the bonding is considered satisfactory, this figure is given by the amount of current required to blow the 100A service fuse in the required time with a max touch voltage of 50v.

Large currents can flow when there is a fault, 1000A will clear a 100A BS1361 II fuse somewhere in the region of 0.4s which is good as time is important factor. If less than 1000A flows the fuse will take longer to clear the touch voltage between bonding connections will be limited to less than 50v which is good as limiting touch voltages is the important factor, for example it may take up to 5s for the fuse to disconnect a fault current of 630A.
TN-CS (PME) has other hazards i.e. if the combined neutral and earth fails upstream all bonding connections may become live at mains potential. The supply provider ie UK Power Networks will have to apply additional earthing to this type of system in an attempt to keep this voltage down.
You can get circulating currents in TN-CS (PME) if any of your metal services are common to other premises as the service will in effect be a a parallel path for
neutral current to flow.

The bonding sizing is not to ensure the service fuse blows.
The bonding sizing needs to be adequate to ensure that it does not become damaged should fault current flow through it and also that the resistance of the conductor keeps connections at or about the same potential during a fault.
It is the job of the Earthing Conductor to provide a path for fault current to flow to ensure the service fuse blows should a fault of sufficient magnitude occur. Some current may still flow though the bonding should this happen.

 

[tony mc]

1000A body resistance? What am I missing tony mc?

Sorry mis read the 1000A as 1000 ohms.

Have a look at page 129 on GN8 re body resistance etc.

As far as safe touch voltage is concerned you have to remember that bonding minimises the magnitude of touch voltage within a premises when a earth fault occurrs.
So when a earth fault develops in a premises and as this current flows to earth touch voltages can be generated on exposed conductive parts etc etc! So with the aid of main bonding you are attemping to reduce the touch voltage that can flow in the exposed parts!

 

[WayneL]

I'm sorry, but the sizing of 'Main Protective Bonding Conductors' has nothing to do with flowing 'fault' currents.........if you disagree, perhaps you could explain, (with calcs), the likely currents you will expect to get down your average water main bonding?.......I think you'll find they are relatively small.

 

[lauriehurman]

Well, better than your algebra

Wow how bad is my ohms law?

I'll try R=V/I shall I?

 

[dagrat]

I'm sorry, but the sizing of 'Main Protective Bonding Conductors' has nothing to do with flowing 'fault' currents.........if you disagree, perhaps you could explain, (with calcs), the likely currents you will expect to get down your average water main bonding?.......I think you'll find they are relatively small.

Quite right, the continuity test of protective bonding conductors is purely to ensure that all main bonding conductors are unbroken and that all ECP,s will be at the same potential in the event of a fault, and nothing to do with the adequacy of earth fault paths.
To this end a figure of 0.05 ohms is given to confirm compliance.

 

[ezzzekiel]

0.05 applied to 10mm for 27m length, over that then 16mm would be required

As i understood it the resistance was limited by the length to keep volt drop below 50 v

 

[dagrat]

Agreed, the readings taken in the test should always correspond to the CSA of the conductor and length of run, which is given in table B1.
The 0.05 ohms is a maximum for each conductor size based on maximum length of run.
There is a table somewhere which uses this maximum 0.05ohms to give maximum lengths of run for each main bonding conductor size, but I can't remember where it is just now.

 

[loop3]

Here's a good one for you, 18.1 is the resistance milli ohm meter of a copper 1mm csa conductor. Divide this by you earth csa and multiply by 50 for maximum length within the 0.05 ohm restriction eg (18.1/10)x50 for 10mm bonding . Answer is maximum length

 

[loop3]

Ignore the last equation, I got it wrong. Lol

 

[dagrat]

10mm. 27m
16mm. 43m
25mm. 68m
35mm. 95m

These are all maximum lengths of bonding conductors before the 0.05 ohms is exceeded.

 

[loop3]

That's it, 0.0181 divided by your csa and then 0.05 divided by that answer. Got there in the end. Answer is in meters. Or you could look at the table for max lengths :S

 

[loop3]

A good book to read if you're interested in how many of the figures in the wire regs cam about is 'commentary on the iee wiring regs by Paul cook'