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[malcolmsanford]

So for a TT system that that can if mechanically protected have a main earthing conductor of 2.5mm, no let's be generous say it is not mechanically protected and therefore you work it out by Adiabiatic to be in fact 4mm.................your bonding to your calculation would be what half of 4mm.............

Are you being serious here ....................... or are you going to tell me now that a bonding conductor can not be less than 6mm, so where does the adiabiatic equation come in there

Your just making this up as you go along ..................

 

[1983]

Little update there is a 10mm bonding on gas and water which goes directly to intake met must do had no sigh of it in ccu and no sigh of met in communal cabent where main supply for the flats is. There's a 16mm earth coming to the fused isolator from there a 16mm twin and earth goes to flat and the 6mm earth in the flat is connected to the 16mm earth shall I run a 10mm earth from the isolator to the flats ccu tomake it 16mm? Also I got a reading of 0.07 ohms from the gas and water to the ccu met

 

[Deleted member 26818]

Are the flats part of the overall installation, or separate installations?
If you feel that they are part of the overall installation, then the bonding will have to be sized according to the main incomer.
If you feel that the flats are separate, then the main earth is sized by use of the adiabatic equation.
If the 6mm is adequate as per the adiabatic equation, and there are no visible signs of deterioration or heat damage, then a code C3 would to my mind be applicable.

Love the way that in one post the bonding is sized by use of the adiabatic equation, then in another it is sized according to the supply neutral.

 

[1983]

Thank you for your feedback appreciated I guess there all separate as each flat got there own meters own fused isolators but all bonded back to the main met. Can't take a ze reading as can't disconnect main earth got a good zs reading from submain flats ccu. pfc was 1.5 something ka no sign of heat damage on 6mm at ccu.gaz and water bonded back to main met at communal cabinet somewhere no sight of it got 0.07ohms reading from water and gas to flats ccu met even tho it doesn't go there.

Are the flats part of the overall installation, or separate installations?
If you feel that they are part of the overall installation, then the bonding will have to be sized according to the main incomer.
If you feel that the flats are separate, then the main earth is sized by use of the adiabatic equation.
If the 6mm is adequate as per the adiabatic equation, and there are no visible signs of deterioration or heat damage, then a code C3 would to my mind be applicable.

Love the way that in one post the bonding is sized by use of the adiabatic equation, then in another it is sized according to the supply neutral.

 

[rattlehead85]

The earthing conductor in a tn-c-s should be as a minimum equally sized with the protective bonding conductors which are sized in relation to the neutral of the supply.
Minimum sizing of earthing and bonding in a TN-C-S is 10mm2 END OF!!

 

[rattlehead85]

How can you apply the adiabatic equation to size an earthing conductor on a tn-c-s when it is supposed to be sized in accordance with the sizing of the main protective bonding. The adiabatic equation is used on tn-s & t.t installations and normally is used to confirm adequacy of existing earthing conductors on said existing installations. For new work the relevant tables in BS7671 for sizing of protective conductors in tn-s / t.t installations should be used.

 

[rattlehead85]

Section 544 on page 171 of bs7671 clarifies the requirements for tn-c-s installations.

 

[rattlehead85]

Table 54.7 should be used and applies to tn-s earthing conductor sizing.

 

[rattlehead85]

End of debate its there in black and white!

 

[rattlehead85]

Simple way to decide the action if any you need to take. Firstly determine what type of earthing system you are dealing with, then apply the required tables and regs baring in mind if its a tn-s you can apply the adiabatic equation to determine if your earthing conductor is sufficient (thats as long as you know what type of protective device you have to apply the adiabatic in the first place)

 

[rattlehead85]

Also see Best Practice Guide 4 on how to code undersized earthing and bonding. Some situations will require a c2 others c3 it depends what system you have and the condition of the cables regarding thermal damage.

 

[andyb]

How can you apply the adiabatic equation to size an earthing conductor on a tn-c-s when it is supposed to be sized in accordance with the sizing of the main protective bonding. The adiabatic equation is used on tn-s & t.t installations and normally is used to confirm adequacy of existing earthing conductors on said existing installations. For new work the relevant tables in BS7671 for sizing of protective conductors in tn-s / t.t installations should be used.

You use it exactly as you would for t-n-s.
You do not size the earth in a t-n-c-s in accordance with the the size of the main protective bond. You do it the same as you would a t-n-s or t-t, but it must 'also meet the requirements of the main protective bonding conductor.

So it could be larger than 10mm but not less.

 

[davesparks]

Table 54.7 should be used and applies to tn-s earthing conductor sizing.

That table is an optional alternative to the use of the adiabatic equation.

 

[rattlehead85]

That table is an optional alternative to the use of the adiabatic equation.

Agreed. I was just taught to use the table over adiabatic on new work but perfectly acceptable if the calcs stack up.

 

[davesparks]

Section 544 on page 171 of bs7671 clarifies the requirements for tn-c-s installations.

That only gives guideline requirements for PME supplies. Whilst all PME supplies are TNCS, not all TNCS supplies are PME