Cu change in a flat earth size

Tetris1985 · Apr 22, 2022

In my flat the main earth is the cable pictured below it has 7 strands any idea what size this is?

And if it would need to be upgraded for a cu change

The bonding is in 6mm but is easy to change to 10mm but I guess that's irelavent if the earth is 6mm?

The conduit system the 16mm tails come up in with the main earth gives a better ze than the main earth cable 0.15 ohm on

[ElectriciansForums.net] Cu change in a flat earth size

conduit

60 amp fuse in rifled board

westward10 · Apr 22, 2022

Probably 7/.036 or 7/.044 imperial what size is the green/yellow conductor.

Tetris1985 · Apr 22, 2022

westward10 said:
Probably 7/.036 or 7/.044 imperial what size is the green/yellow conductor.

Gas bond 6mm

westward10 · Apr 22, 2022

I assume this is TN-S.

Tetris1985 · Apr 22, 2022

westward10 said:
I assume this is TN-S.

it is a TN-S

westward10 · Apr 22, 2022

From a picture it is hard to gauge but from the imperial sizes I stated in terms of metric it is a bit over 4.0 to a bit over 6.0 so the latter should suffice for a TN-S system.

Tetris1985 · Apr 22, 2022

westward10 said:
From a picture it is hard to gauge but from the imperial sizes I stated in terms of metric it is a bit over 4.0 to a bit over 6.0 so the latter should suffice for a TN-S system.

6mm is okay as a main earthing conductor on a TN-S I guess that's subject to the adiabatic?

Does that mean 6mm gas and water bonds from the cu earth bar would be sufficient

pc1966 · Apr 23, 2022

Tetris1985 said:
6mm is okay as a main earthing conductor on a TN-S I guess that's subject to the adiabatic?

Does that mean 6mm gas and water bonds from the cu earth bar would be sufficient

Also remember if it is unsheathed (and not a flammable environment) then the adiabatic limit is significantly higher.

The on-site guide tables B3-B5 has the Zs limits for various fuse ratings and conductor sizes, but they are for 70C insulation.

Tetris1985 · Apr 23, 2022

pc1966 said:
Also remember if it is unsheathed (and not a flammable environment) then the adiabatic limit is significantly higher.

The on-site guide tables B3-B5 has the Zs limits for various fuse ratings and conductor sizes, but they are for 70C insulation.

Its unsheathed in the metal conduit, yes.

The original water bond is also unsheathed
Why does that make its limit higher?

pc1966 · Apr 23, 2022

Tetris1985 said:
Why does that make its limit higher?

The adiabatic limit is based on the assumption the heating is short-lived so essentially no significant portion heat escapes during the event (the definition of "adiabatic").

So the conductor starts at its normal temperature (which might be 20-30C for a separate CPC, or around 70C for a conductor in a set that is running at limit, etc) and during the fault the I2R term is heating the conductor for 't' seconds causing the temperature to rise until the fuse/MCB/etc disconnects. At that point the conductor is much hotter, but provided it is not so hot that the insulation is permanently damaged by such a short and occasional event the cable is OK. Depending on the type of insulation that is usually in the range of 140C to 250C.

If you know the conductor material then you can convert that change in temperature in to an I2t limit for the fault, with a bigger temperature difference allowing more heating and so a higher I2t limit.

So if you go from typical PVC upper limit of 160C (18th edition regs Table 54.2 first copper example) to bare metal with, say, 500C upper limit (Table 54.6 bare copper in "visible and restricted area" case) then your 'k' in the adiabatic equation goes from 143 to 228 and so the resulting minimum cross-sectional area goes down to 63% of the normal case (so roughly one standard cable step down, e.g. 10mm -> 6mm or 6mm -> 4mm, etc).

But that adiabatic limit is only for short lived faults like a short leading to a fuse/MCB opening. The other concern for earth conductors is the 'open PEN' fault in TN-C-S systems where you could have the local faulted segment of the network's neutral current returning via bonded service pipes, steel foundations, etc. In that case you could see tens of amps flowing for hours and the limit for conductors then is the maximum safe running temperature (basically the usual current carrying capacity).

pc1966 · Apr 23, 2022

Looking at table B5 from the OSG, for a 60A fuse with Zs of 0.2 ohms or below it is meeting the adiabatic for 2.5mm PVC and above 6mm it is basically limited by the 5s disconnection time, and not the adiabatic limit of the cable.

So if your earth is around 6mm for your Zs is fine.

loz2754 · Apr 23, 2022

pc1966 said:
The adiabatic limit is based on the assumption the heating is short-lived so essentially no significant portion heat escapes during the event (the definition of "adiabatic").

So the conductor starts at its normal temperature (which might be 20-30C for a separate CPC, or around 70C for a conductor in a set that is running at limit, etc) and during the fault the I2R term is heating the conductor for 't' seconds causing the temperature to rise until the fuse/MCB/etc disconnects. At that point the conductor is much hotter, but provided it is not so hot that the insulation is permanently damaged by such a short and occasional event the cable is OK. Depending on the type of insulation that is usually in the range of 140C to 250C.

If you know the conductor material then you can convert that change in temperature in to an I2t limit for the fault, with a bigger temperature difference allowing more heating and so a higher I2t limit.

So if you go from typical PVC upper limit of 160C (18th edition regs Table 54.2 first copper example) to bare metal with, say, 500C upper limit (Table 54.6 bare copper in "visible and restricted area" case) then your 'k' in the adiabatic equation goes from 143 to 228 and so the resulting minimum cross-sectional area goes down to 63% of the normal case (so roughly one standard cable step down, e.g. 10mm -> 6mm or 6mm -> 4mm, etc).

But that adiabatic limit is only for short lived faults like a short leading to a fuse/MCB opening. The other concern for earth conductors is the 'open PEN' fault in TN-C-S systems where you could have the local faulted segment of the network's neutral current returning via bonded service pipes, steel foundations, etc. In that case you could see tens of amps flowing for hours and the limit for conductors then is the maximum safe running temperature (basically the usual current carrying capacity).

This possibly one of the clearest explanations of the use of the adiabatic equation I've ever read. Thank you.