adiabatic equation, help / advice

[Tapouchoke]

Hi folk’s adiabatic equation, help / advice

S = Size of earth bonding conductor required.
I = pfc in amps.
t = disconnection time in seconds.
K = values in table 43.1.

I am trying to work out if a 6mm main earth is adequate on a TN-C-S system
Is the 6mm cpc in the 16mm T&E allowed?
Have been asked remove and upgrade / a DB in a flat supplied by a 16mm T&E on a 60A switch fuse BS1361.
Already present from flat DB 10ml to gas and water
or would you go with table 54 .7 s = mm2 if the protective conductor is of the same as the line conductor ?
but it must have been able to carry the fault current as acpc???
Zs @flat 0.16
Pfc 1.85ka
Thanks in advance guys

 

[Sav]

Hi,
I thought that the MINIMUM size of conductor for main equipotential bonding is 6mm !!

Also was that ZS 0.16 or ZE 0.16??

Thanks ,
Sav

 

[Tapouchoke]

appologges , for not being to clear

i have a 16ml T&E supplying to the flat and the cpc is 6ml this would in effect be the main earthing conductor

the 0.16 is at the flat wich would be zs

thanks

 

[Sintra]

Is the meter in the flat or elsewhere?
Is this 16mm t&e a sub main?

 

[pushrod]

On a tn-c-s main earth must be a minimum of 16mm and main protective bonding must be a minimum of 10mm ( see table 54.8 of the regs book)

Suspect you need to upgrade the supply cable to the flat. Earthing arrangements other than tn-c-s seem to be more flexible with regard to main earth and bonding.

 

[bugsy]

Would I be right in thinking your trying to find find the calcs to allow compliance?

 

[scotsparky]

Hi folk’s adiabatic equation, help / advice

S = Size of earth bonding conductor required.
I = pfc in amps.
t = disconnection time in seconds.
K = values in table 43.1.

I am trying to work out if a 6mm main earth is adequate on a TN-C-S system
Is the 6mm cpc in the 16mm T&E allowed?
Have been asked remove and upgrade / a DB in a flat supplied by a 16mm T&E on a 60A switch fuse BS1361.
Already present from flat DB 10ml to gas and water
or would you go with table 54 .7 s = mm2 if the protective conductor is of the same as the line conductor ?
but it must have been able to carry the fault current as acpc???
Zs @flat 0.16
Pfc 1.85ka
Thanks in advance guys

From the time current caracterisitic curves in appendix4 what is the true disconnection time of overcurrent device ? this is the time you use on the adiobatic.

in section 534 of the regs it does give a tabe the sizes for main earths but on the page before it does say use the The chart or cablesizes can be calculated using the adiobatic. Give it a shot and see what the answer is.

have a look at there regs,

542.3.1
Table 54.7
table 54.1
Table 54.8

Remember in TN-C-S systems the Main earthing conductor shall be no less then the Main Protectinve Bonding Conductor.

 

[WayneL]

I don't think you are talking about an 'earthing conductor', unless I've misunderstood you.

There is only one 'earthing conductor' in an installation - connected from the means of earthing to the MET (also only one per installation).

What you have is a 16mm distribution cable with 6mm CPC feeding a remote distribution board.

It is, in effect, a circuit, just like any other circuit, fed from your main DB at the supply intake.

You wouldn't question the size of CPC in a 2.5mm T&E cable, and roll out the adiabatic equation, so why are you questioning it in a 16mm T&E. The cable will be protected at the main DB by it's (hopefully) correctly selected protective device.

Any extraneous conductive parts in the flat should be bonded in at least 10mm and connected to the MET (at the supply intake) - not to the earth bar in the flat CU.

 

[Sintra]

That's what i was thinking of.

 

[WayneL]

That's what i was thinking of.

I noticed that with your question about where the meter was

 

[Tapouchoke]

I don't think you are talking about an 'earthing conductor', unless I've misunderstood you.

There is only one 'earthing conductor' in an installation - connected from the means of earthing to the MET (also only one per installation).

What you have is a 16mm distribution cable with 6mm CPC feeding a remote distribution board.

It is, in effect, a circuit, just like any other circuit, fed from your main DB at the supply intake.

You wouldn't question the size of CPC in a 2.5mm T&E cable, and roll out the adiabatic equation, so why are you questioning it in a 16mm T&E. The cable will be protected at the main DB by it's (hopefully) correctly selected protective device.

Any extraneous conductive parts in the flat should be bonded in at least 10mm and connected to the MET (at the supply intake) - not to the earth bar in the flat CU.

Cheers guys for your feed back

 

[Guest123]

Services should be bonded at point of entry to the overall building AND point of entry to each flat as each flat is an electrical installation in it's own right.

Either it's done at the flat CU which means your 6mm CPC is insufficient OR a 10mm is run for the MET in the basement (or wherever it is) to each service in each flat.

 

[acvc]

So does this mean that, e.g. if there are three flats in one building, fed from the common mains intake, it would be acceptable to have the main earth conductor as 16mm from MET to common c/u, then have each flat fed from there by a 16mm T&E to each flat CU using the 6mm cpc as the (effective sub main earth).

Sorry Lenny! I understood the first part of your last post but not the second.

 

[WayneL]

So does this mean that, e.g. if there are three flats in one building, fed from the common mains intake, it would be acceptable to have the main earth conductor as 16mm from MET to common c/u, then have each flat fed from there by a 16mm T&E to each flat CU using the 6mm cpc as the (effective sub main earth).

Sorry Lenny! I understood the first part of your last post but not the second.

Hi mate,

Sorry, I don't know if you're asking me...or Lenny

I'll give my answer anyway.

Yes, what you are saying is fine - but you must remember that the 6mm CPC is an earth, not a bonding conductor.
If there are extraneous conductive parts in the flats, then they would require Protective Bonding Conductors (10mm minimum) connected back to the MET, not the flat CU.

Alternatively, you could run a combined CPC and Protective bonding conductor from the MET to each flat, connected to what is termed an 'earth marshalling terminal' (basically another MET).
You would probably then need to use the Adiabatic Equation to calculate the correct CSA of this conductor, dependant on the system and live conductor CSA.

Each flats CU and bonding conductors can then be connected to this EMT.

I'm sure if I've got this wrong someone will correct me soon.

 

[Guest123]

What Wayne says is correct, if the services are to be bonded at the flat CU then the 6mm CPC in the 16mm T+E is not big enough to act as CPC and MEBC. So a bigger earthing conductor would need to be installed between the main intake and the CU in each flat to compensate for this.

 

[claret73]

How would this be done in a practical sense? Taking the example of 3 flats and each flat had Gas & Water, would you terminate 6x Bonding conductors back to a Multiple earth block at the Intake and feed your 16mm MET to the Main CU from that also?

Just done a testing exam at College & the sub board was protected at the Main CU by an RCBO. The Sub CU had MCB's no RCD Protection. I would think in a situation of the flat's CU it would require RCD Protection under current Regs...

 

[Tapouchoke]

Hi guys what we did was would you all agree ?????

As the bonding had been previously installed in the flat,
Therefore, the 6ml cpc would then have been too small as a MET
As a result, We Run a 16ml earth from the MET @the incomer in the basement to the flat.as per table 54-7

Alternatively
The adiabatic equation worked out as 6ml would be enough
However, we then could have run x2 MEBC for gas and water from the MET to the flat

Nevertheless the 16ml t&e should have been rcd protected it was listed as a comment ont the cert we would have given it a code 4

Thanks for posts

 

[Guest123]

Spot on mate.