Wiring in Consumer Unit in garage with 2 way lighting source

[ban-all-sheds]

Yes I'm aware I need Building Regs approval

Which you have to get before you start any work.

and a Part P qualified, registered sparky will be checking everything once complete.

It doesn't work like that.

 

[bernardgreen]

You're absolutely right that the maximum earth current is 30mA from the garage, but the earth spike is also connected to the earth of the armoured cable which also feeds the house (which doesn't have an RCD) adding additional earth capacity to the house (as I'm not convinced how good the earth in the house is). I do have a tendency to slightly over-engineer though

Please get accurate advice about the earth rod being connected to the earth system of the house. In some areas, depending on the type of supply and earthing arrangements connecting the "earth" at the meter box to a ground rod is "not recommended". In my property I have to make sure that my "earth" cannot make contact with the earth of the attached property which is earthed by an earth rod.

How will you test the impedance of the earth rods is less than 1666 ohms which is the maximum permitted ? It should be a lot lower, less than 100 is recommended.

The earth rod impedance determines the voltage on the earth wires in the system when there is a leakage of current through the earth rod to ground. If the RCD allows 30 mA before tripping then with a rod with a 1666 ohm impedance will create a voltage of 50 volts between ground and the earth wiring. That is the maximum safe permitted voltage.

 

[bernardgreen]

WHY?? The maximum earth current will be 30 milliamperes. A bit of telephone wire would be more than enough.....

The maximum current is only limited to 30 mA if the leaking current comes via the sense coil of the RCD from Live or Neutral.

The telephone wire could possibly melt if the current to ground was not limited by the RCD. Connecting a neutral derived "earth" to a real ground earth will create a current into the rod that does not go through the RCD and is therefore not limited to 30 mA but is determined by the voltage of the neutral relative to local ground and the impedance of the earth rod and cabling. Not a problem until there is a fault in the local network that means the neutral bounces significantly above local ground.

 

[Taylortwocities]

Bernard. Ok I was exaggerating about the phone wire!

Benzas. Get an electrician on board before you install another bit of copper. Per the above, exporting the house earth, via the armoured sheath, to a remote earth spike is fraught with serious technical issues.

Firstly, you'd need to determine the Prospective Fault Current that could arise. Let's say you are close to the substation and its 2,000 amps.
You'll need to use the adeabatic equation (see here http://niceic.com/en/account/media/17thpg15.pdf)..

But, in some cases you are just not allowed to export the DNO's earth. It depends on your supply type. Do you even know what yours is?
Have you tested the EFLI & PFC.?

Basically the remote installation will be a TT. So the only earth will be from that spike.
And that is another thing. How many rods do you think you will need? Have you tested the impedance of the one you've installed? How? Does it meet the requirements of BS7671? Will your RCD trip in time through your bit of copper?

You talk about over-engineering, but some of what you have in mind would introduce more danger, not reduce it..

Time to call in an expert. IMO.

 

[SimonH2]

Leaving aside the building regs and earthing issues, I can see a couple of ways to make this work. But the first thing I think of is ... 4 core SWA cable won't have been cheap if it's large enough for the size of supply.

First option is to take the live off in the house, fuse it down to suit the switch and light fitting ratings, and then feed the circuit down two strappers as now*. To allow for effective isolation in the garage, replace the 2 pole main switch in the CU with a 4 pole one (if one can be found that fits) and feed the strappers through that. Then take the neutral for the light to the supply side of the RCD.
That would give you a system that allows all garage circuits to be isolated with one switch.

The second that comes to mind is to take a live off at the house end, fuse it to suit the switch, and take one switched live down the cable to the garage. In the garage, use this supply to operate a C/O relay that forms one half of a standard two-way switching circuit. The neutral for the relay coil goes to the supply side of the RCD, the lighting circuit comes off the load side of the CU as normal and so is isolated by turning off the main switch.
The relay coil can only be isolated at the house, but this is the same as the cabling and supply side of the CU isolator.
This latter method also frees up a core to be used for earth.

* Well actually I;d connect the live feed to C on the house end switch and take L1 and L2 down the cable. While I suspect the cable is "somewhat larger" than needed for the light circuit, one setting of the switches results in the live going down the cable to the garage, back to the house, and back down to the garage again (so 3 trips) with part of the circuit sharing the live for any other loads - thus slightly increasing volt drops.

WHY?? The maximum earth current will be 30 milliamperes. A bit of telephone wire would be more than enough.....

The maximum current is only limited to 30 mA if the leaking current comes via the sense coil of the RCD from Live or Neutral.

Not even then. The RCD does not limit the earth current to 30mA - it only limits the duration of an earth fault in excess of that. The actual earth current could potentially be in the order of hundreds of amps or more (for a L-E short) - for a very short time.