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nrbspark

Hi Guys my question is with relation to the adiabatic equation, I am aware that the following is true.

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

If I am trying to work out if a 6mm gas and water bond is adequate on a TN-C-S system where in the regs do I find the following.

1. disconnection time (I presume it is for the main incomer fuse) which in this case is 80 amp BS1361.
2. What cable am I wanting the K value for and how do I find its characteristics if it was not installed by me.

Any help appreciated guys :)

If someone could do a working calculation it would be fantastic. pfc on this job is 2.4 KA.

Just to add I always upgrade the gas and water bonds if I am asked to change a consumer unit but in this case it is almost impossible without major disruption which the customer wants to avoid.

Also am I right in thinking that if it was a TN-S system then I would have to upgrade the gas and water earths to 10mm.
 
Hi there.

You're right but you've got it mixed up....as you have a TN-C-S system, all main bonding conductors must be 10mm.

For TN-S you can use the adiabatic to check conformity of a 6mm bonding conductor by confirming the minimum CSA of main protective conductor.



EDIT.....Works out at about 6.5mm.;)
 
Last edited by a moderator:
Thanks Lenny

But on a consumer unit change am I right in thinking the gas and water earth being 6mm would only gain a recommendation code 4 if there are no signs of deterioration of the insulation, and therefore I can happily perform the consumer unit change if all else is fine.

Also I would appreciate it if you could lead me in the right direction for the adiabatic please. :)
 
I squared you have there.


(T) comes from APP 3 of ol red. (K) comes from table 43.1 of ol red. (S) must be equal to or greater than the result of the sum.

(S) = the square root of ((I) squared x (T)) divide by (K).



2400 x 2400 = 5760000

5760000 x 0.1 = 576000

Square root of 576000 = 758.9

758.9 / 115 = 6.59mm
 
Last edited by a moderator:
2400 x 2400 = 5760000

5760000 x 0.1 = 576000

Square root of 576000 = 758.9

758.9 / 115 = 6.59mm


Hmmmmmmmm

ermmmmmmm

mmmmmmmm

Hmmmmmmmmmmmmmm

Hmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm

Erm Lenny, mmmmmmmmm

Hmmmmmmmmmmmm

Lenny,

Hate to ask thissssssssssssss ...........

but was it you who nicked my calculator? :D
 
Thanks again fellas:confused:

But with regards to a consumer unit change can it be performed if the gas and water earths are left at 6mm but the main earth is upgraded to 16mm ? There are mitigating circumstances due to disturbance that would be caused.

PS how did you arrive at the K factor and disconnection time please.

Thanks again.

I see the tables you noted Lenny but is the cable K factor for the Electric board supply cable or tails?
 
Last edited by a moderator:
The K factor is for the bonding cable i.e 70 degrees c thermoplastic, copper conductor, less than 300mm CSA.

Disconnection time is from the time/current graphs in appendix 3.
 
Thanks guys it finally hits home :D

What would be your feelings on changing the consumer unit if the gas and water earths were not changed ? Could it be done with a recommendation code 4 as it must have been fit for purpose when installed.
 
I squared you have there.


(T) comes from APP 3 of ol red. (K) comes from table 43.1 of ol red. (S) must be equal to or greater than the result of the sum.

(S) = the square root of ((I) squared x (T)) divide by (K).



2400 x 2400 = 5760000

5760000 x 0.1 = 576000

Square root of 576000 = 758.9

758.9 / 115 = 6.59mm

That's the main earthing conductor, the bonding conductors have to be at least 1/2 this and a minimum of 6mm, so his 6mm gas and water bonding is fine if t-n-s.
 
Thanks guys it finally hits home :D

What would be your feelings on changing the consumer unit if the gas and water earths were not changed ? Could it be done with a recommendation code 4 as it must have been fit for purpose when installed.


If changing the consumer unit then it is needs a EIC so where can you put a code 4 on a EIC?
 
I thought I could put the note in comments on existing installation on the almost bottom right of page 1.

Does anyone else have a view on the 6mm gas and water earths being left as they were fit for purpose on installation, and surely I would be improving the installation?

I spoke to the NICEIC tech helpline today about the matter and the guy on there asked me why I was carrying out a PIR for a consumer unit change so I had to give him advice can you believe. :eek:

PS The supply characteristic is TN-C-S Andy so with this in mind are your thoughts the same mate ?
 
Last edited by a moderator:
That's the main earthing conductor, the bonding conductors have to be at least 1/2 this and a minimum of 6mm, so his 6mm gas and water bonding is fine if t-n-s.

Sorry, should have made that clear.:eek:

With regard to TN-C-S 6mm is too small as it states that main earthing conductors should be size for size upto 16mm and main protective bonding conductors must be a minimum of 10mm.
 

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