Adiabatic- bonding

[Frosty69]

Hi people,
I'm not the best at math but I'm trying to figure out what size cpc for water bonding I can calculate.
Tns system
Ze 0.18
PFC 1.52kA
t=0.1 sec
K=143
8 metres in length
Main fuse= 80a, type 2, 1361.
It is currently a 6mm cpc but I can't replace it due to floors and new kitchen! I'll show you how I've tried calculating but I know I'm doing something wrong?
230/0.18=1277
1277 sq x0.1=163,072.9
Sq route of 163,072.9= 403.8
403.8/143=2.8mm?
As I say Im not the best when it comes to math so please go easy. I know there have been loads of posts on this but I'm still struggling.
Thanks

 

[topquark]

There's a good example here: http://www.electriciansforums.net/i...xample-use-adiabatic-equation.html#post217091

Have a read and see if it clears things up.

 

[Sparky Ninja]

A 6mm bonding CPC?

Are we trying to work out the size required for supplementary protective bonding or a circuit protective conductor?

 

[Frosty69]

Supplementary bonding to water. It's a 6mm and in an ideal world it would be changed to 10mm. So on calculation is 6mm large enough?

 

[WayneL]

Frosty69,

The adiabatic equation is used for calculating CSA of Circuit Protective Conductors(CPCs) - not 'Bonding' conductors.

Your 'Bonding' conductors on a TN-S are required to be not less than half the CSA of the 'required' 'Earthing Conductor' and not less than 6mm.

BRB - Pg 134 544.1.1

It won't be 'supplementary', I don't think - more likely 'Main Protective Bonding'

 

[Frosty69]

Thanks Wayne I was reading that in the regs before but I'm in a sticky position where the 6mm cannot be changed. Its for a db change so can I just note it on exiting installation comments on eic?

 

[WayneL]

Thanks Wayne I was reading that in the regs before but I'm in a sticky position where the 6mm cannot be changed. Its for a db change so can I just note it on exiting installation comments on eic?

I don't think you can, mate.

One of the things you have to do on a CU change is confirm that the 'Earthing and Bonding' arrangements are satisfactory.

Don't get me wrong - if you confirm that 6mm is okay, then happy days.

Use the adiabatic equation to confirm the required size of your 'Earthing Conductor'.

If this comes in at 10mm, then 6mm 'Main Bonding' will be adequate.

If it turns out that you require a 16mm 'Earthing Conductor', then your 6mm 'Bonding' would have to be upgraded to 10mm.

 

[Deleted member 26818]

You've basically got the adiabtic equation nailed, however I believe your values are inconsistant.
Your calculated PEFC, will produce an operating time less than that provided in the time/current characteristics graphs in Appendix 3 of BS7671. Most likely the operating time will be 0.01s, not 0.1s.
To determine the correct operating time, you would have to consult the fuse manufacturer's data.
If you can't be asked to obtain the manufacturer's data, then (again I believe) you should really use the current values as listed in Appendix 3. In this case 1100A instead of 2700A.
The outcome is most likely irrelevant in the majority of scenarios, and I may in fact be quite wrong in my belief. It's just that's how I've always understood it to be.

 

[Deleted member 26818]

I don't think you can, mate.

One of the things you have to do on a CU change is confirm that the 'Earthing and Bonding' arrangements are satisfactory.

Don't get me wrong - if you confirm that 6mm is okay, then happy days.

Use the adiabatic equation to confirm the required size of your 'Earthing Conductor'.

If this comes in at 10mm, then 6mm 'Main Bonding' will be adequate.

If it turns out that you require a 16mm 'Earthing Conductor', then your 6mm 'Bonding' would have to be upgraded to 10mm.

He has efectivly determined that 6mm² is satisfactory.
The value he has obtained from his calculation indicates an CSA of 2.8mm² for the earthing conductor. As such his bonding conductor need be no larger than 1.4mm², obviously well below the 6mm² minimum allowed.

 

[CCRISP]

why not us e the table in the regs 54.7 page130

 

[malcolmsanford]

Look at post #1 and #6 which is why he wanted to work it out. By doing the equation he will prove that the 6mm bonding conductor is capable of taking any fault currents that might occur.

Also Table 54.7 is for protective conductor sizes not bonding conductors, I think you mean he should look at table 54.8 page 134 for a PME bonding conductor size. Again though if a risk assement is taken on the existing 6mm conductor and it is deemed suitable, ie no burning or damge to it, then 6mm IMO would be adequate after doing the adbiatic equation.