Cable size to Earth Rod in TT instalation with 30mA RCD ?

[nickwhittaker]

My installation has an TT supply (no earth provided by supplier). I have provided an earth rod but am not sure as to the correct size of the cable to connect betwen the earth rod outside and main earth block next to the CU. My CU has a 80 Amp 30mA RCD. I have seen 16mm and 25 mm recommended-> What should I use ?

 

[Spark123]

How will the earthing conductor cable going be installed? Will it be burried? Will it be protected against mechanical damage and / or corrosion? What is the impedance of the earth rod?

 

[ban-all-sheds]

Look at Table 10C in the On-Site Guide.

 

[mapj1]

The resistance of the rod will probably be around 30 to 150 ohms, so the dominant factor, as stated is not the current handling of the cable, but its mechanical suitability, and long life. Thick enough not to be damaged, is usually 16mm, but if it is buried un-protected, more my be needed.
remeber to use the proper outdoor clamps, and avoid any joins outdoors.

 

[HDRW]

Mike,

Thick enough not to be damaged, is usually 16mm, but if it is buried un-protected, more my be needed.

Are you saying that you can direct-bury 16mm² earth cable?

If not, what extra protection would be needed - would flexible plastic conduit be enough, or would it need something like this:

http://www.tlc-direct.co.uk/Products/KX20.html

Cheers,

Howard

 

[ban-all-sheds]

16mm² is fine for burying, if it's protected against corrosion. i.e. not bare cable.

No harm in using thicker and no harm in providing more mechanical protection if you want. If you do use that conduit you could get away with 2.5mm²...

 

[mapj1]

Sorry, another dopy post -I'm not getting enough sleep!
in essence B-A-S has it. However, all connections need to be inspectable (in an earth pit if at the rod) and weather proof. Burying crimped joins isn't really on- so no joins outdoors that are not at the rod (or rods) in proper pit(s) please (unless you do thermite of course - normally to much hassle). Also if multiple rods in such a way that disconnecting one doesn't mean the other is cut off too, so loop through unbroken cable or crimp.

If in your engineering judgement the cable might be damaged in normal use (ploughed field anyone?) then clearly it isn't OK to bury it unprotected. Otherwise yes it is. As said, it can be ridiculously thin if protected, as actually it carries no current to speak of, and the possible failure is being chopped or rotting through, not melting.

 

[plugwash]

out of interest whats the minimum size if using a bare copper wire as the electrode?

 

[ban-all-sheds]

Presumably 25mm²

 

[nickwhittaker]

I have a proper earth rod that is buried in the ground (as it must be)within 3 inches of the wall so the earth wire is protected as it goes straight into the wall then to the earth block. So if the wire is protected what size should it be ? Thanks

 

[plugwash]

well its not burried but it doesn't have seperate protection either so that would be min 4mm

having said that call me paranoid but i'd like to see somthing a bit bigger than 4mm for a main earth.

 

[Big_Spark]

out of interest whats the minimum size if using a bare copper wire as the electrode?

You cannot, only Electrodes that meet the relevent BS can be used, they also need to meet the requirements of the Electricity Supply Regulations Act, as they are actually part of the supply not the installation per se.

Standard Copper cable is no hard drawn annealed, which provides mechanical protection whilst burried in the ground, further Earty Rods are not 100% pure Cu, to protect against an Oxidisation layer forming on the surface, thus insulating the rod from the ground. Again Copper Cable does have this in-built protection and would, over a period of time, form an Oxide Layer that would reduce the effectivness of the earthing.