Bit quick advice please

[hightower]

They are for earthing not bonding.

Why should they be different though?

 

[Ian1981]

tables will give you the minimum CSA of steel deemed to comply with table 54.7 for armour to be used as a circuit protective conductor which is not the same as a protective bonding conductor.
Like lee says you would need the armour to be at least 85mm steel to offer the same conductivity of 10mm copper using the 8.5 ratio.

 

[hightower]

Thanks but why are they different. Why is a steel to copper equivalency that's good for a CPC not good enough for bonding?

 

[Ian1981]

Thanks but why are they different. Why is a steel to copper equivalency that's good for a CPC not good enough for bonding?

Because for a cpc the Adiabatic equation can also be used.
You often find table 54.7 gives an over sized cpc than what is needed.
Bonding requirements have no wiggle room I'm afraid

 

[Leesparkykent]

Why should they be different though?

544.1.1 and the * under table 54.8 both say about the bonding conductor being copper or a cross-sectional area affording equivilant conductance in other metals. GN8 has a section on it but I don't have it around me ATM. Probably to do with limiting touch voltages under fault conditions etc.

 

[Wilko]

Why should they be different though?

Man did I struggle with this one ... The SWA table gives k numbers suitable for use in the adiabatic equation. I haven't got it open, but I recall the factor is about 2.2. For bonding it's about voltage rise so it's the resistivity that's important and as LSK has said the factor is about 8.5. So armour is normally ok for CPC but rarely enough for bonding.

 

[Flanders]

There was a site i was on the other day with metal porta cabin's, but the earth had been exported from the pme , i was looking at using the structural steel as the earth sorce to the cabins , i spoke to nic tech , and said they could not see a problem doing that way ,not sure if that somthing you can do with your job.

 

[hightower]

Cheers guys. I've just had a read of the relevant section of GN8 but it didn't offer any reasoning behind the 8.5 factor and the difference between a CPC and bonding conductor in terms of copper equivalency.

@Wilko thanks, that answer makes a lot of sense (I think). Appreciate you all taking the effort to explain it, not being one to accept 'cos that's he way it's always been'

 

[westward10]

For the armour to be adequate for a 10.0 bonding conductor the minimum four core cable size would be 50.0. Are 10.0 bonds the required size for the factory supply.

 

[davesparks]

So what are the copper equivalent columns in that link referring to then? Reason we've used 4-core is because it's what the factory owner had lying around and he's on a cost saving exercise. Will have to have a look at pulling in a separate conductor.

They are for situations where you need to know the copper equivalent CSA, this is not used for bonding.

For bonding you need equivalent conductance, this is a different physical property. A factor of 8.5 is generally accepted as being the ratio of conductance of copper versus steel. So for a 10mm copper bond you need 85mm steel.

 

[davesparks]

Cheers guys. I've just had a read of the relevant section of GN8 but it didn't offer any reasoning behind the 8.5 factor and the difference between a CPC and bonding conductor in terms of copper equivalency.

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A CPC's size is based on the temperature rise in the time it takes the ocpd to operate when a particular fault current flow. So this is looking at the ability of the conductor to survive a high current for a short length of time.

Bonding is sized considering its ability to safely handle a sustained flow of current.

Also it's worth noting that the table you linked to above is incorrect and has been for many many years.

 

[Leesparkykent]

They are for situations where you need to know the copper equivalent CSA, this is not used for bonding.

For bonding you need equivalent conductance, this is a different physical property. A factor of 8.5 is generally accepted as being the ratio of conductance of copper versus steel. So for a 10mm copper bond you need 85mm steel.

Theres an echo in here .......

 

[Ian1981]

Gn8 also gives warning to using the armour as a combined cpc/protective conductor as with tncs diverted neutral currents can raise the temperature of the cable meaning a increased sized conductors maybe needed for your ccc.
It's up to the designer whether this may happen or not.

 

[Ian1981]

Gn8 also gives warning to using the armour as a combined cpc/protective conductor as with tncs diverted neutral currents can raise the temperature of the cable meaning a increased sized conductors maybe needed for your ccc.
It's up to the designer whether this may happen or not.

Or rather raise the temp of the armour I should say