What size earthing cable?

[MEGGER?]

Been to price up a kitchen today,customer has had DB replaced,new 10mm to gas and water,but electrician has left in what appears to be 6mm green cable from distributors earthing terminal to new DB.What if any regs does this contravene and if so what should the cable size be? (Still and always learning)

 

[slocm3105]

sounds like main earthing conductor. Are you sure its one 6mm, sometimes they were paired up or trippled but looks like this needs to upgraded if thats not the case to 16mm or 10mm if its going to an earth stake

And if it goes to an earth electrode it should be 25mm.

 

[MEGGER?]

Definately just one cable TN-C-S. we have to form housing around DB

 

[slocm3105]

Yup should be 16mm

 

[MEGGER?]

Many Thanks slocm3105

 

[mrloy99]

And if it goes to an earth electrode it should be 25mm.

sorry matey not quite right there.If its going to an earth rod,its probably TT and all you need is 6mm!

 

[Rajan Shakti]

if its going to the earthbar protective earthbar what should be the earth wire size

 

[johnboy6083]

right, a few myths are abounding here.

If your fed from a TNC-S system, then your minimum main earth is a 10mm, and this is to satisfy the bonding requirements of table 54.8 of BS7671.

with a TT system , you earthing copnductor should be sized to the lowest posssible resistance of your earth rod. The resistance of an earth rod is called Ra. Howveer your must also comply with table 54.1, which says that if your cable mechanically protected, then the min size is 2.5mm2, and if your cable is not mechanically protected, then the min size is 16mm.

On a TN system, and this method can also be used to size the earh to an earth rod, you need to use the adiabatic equation, or table 54.7 of BS7671.

the adiabatic is S= (square root(i2t))/k

i is the current that will flow in the conductor in an earth fault. This can be calculated from Uo/Ze, or your measured PEFC can be used.
2= squared (cant get the symbols on here)
t is the time taken for the protective device to tripwith the current above. this is found by looking at the time/current curves either from BS7671, or manufacturers info.
k is a factor that is listed in BS7671.

The adiabatic is the most economical way of sizing earthing, and circuit protective conductors.

Read chapter 543 onwards in the BS7671, page 160 onwards.

 

[Elmer BeFuddled]

Personally, I'd just whack in 16mm². Mind, that's probably because I've a reel of the stuff in t'van!

 

[Rajan Shakti]

right, a few myths are abounding here.

If your fed from a TNC-S system, then your minimum main earth is a 10mm, and this is to satisfy the bonding requirements of table 54.8 of BS7671.

with a TT system , you earthing copnductor should be sized to the lowest posssible resistance of your earth rod. The resistance of an earth rod is called Ra. Howveer your must also comply with table 54.1, which says that if your cable mechanically protected, then the min size is 2.5mm2, and if your cable is not mechanically protected, then the min size is 16mm.

On a TN system, and this method can also be used to size the earh to an earth rod, you need to use the adiabatic equation, or table 54.7 of BS7671.

the adiabatic is S= (square root(i2t))/k

i is the current that will flow in the conductor in an earth fault. This can be calculated from Uo/Ze, or your measured PEFC can be used.
2= squared (cant get the symbols on here)
t is the time taken for the protective device to tripwith the current above. this is found by looking at the time/current curves either from BS7671, or manufacturers info.
k is a factor that is listed in BS7671.

The adiabatic is the most economical way of sizing earthing, and circuit protective conductors.

Read chapter 543 onwards in the BS7671, page 160 onwards.

where can i get the above stated standards

 

[lauriehurman]

Addditionally;
514.4.5 the single colour green shall not be used.

 

[trev]

where can i get the above stated standards

You can google the adiabatic equation, try to get hold of an official copy of BS7671 but bear in mind that your electrical regulations will most likely be different from ours

 

[Engineer54]

Did anyone notice this thread is nearly 4 Years old!!!!

 

[trev]

Rajan getsthe award for the oldest resurrected thread then

 

[Brightspark2]

Rajan getsthe award for the oldest resurrected thread then

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