Earth rod required

[Mark McGee]

Hi

I'm running some cable to a consumer unit in some outbuildings, and I
vaguely recall reading something about needing an earthing rod.

Should I earth the consumer unit to an earth rod? If so, what do I do
with the earth on the supply cable?

Do I just bash the earth rod in to the ground outside, and run some
(6mm?) earth wire in to the consumer unit from there?

Thanks
Mark

 

[Lurch]

Hi

I'm running some cable to a consumer unit in some outbuildings, and I
vaguely recall reading something about needing an earthing rod.

You don't 'need' an earth rod, but you can use one, recommended if the
outbuildings are some distance from the house.

Should I earth the consumer unit to an earth rod?

Yes, run an earth wire of 4mm cable if mechanically protected or the
same size as the supply cable live conductors if not. Connect it to
the earth bar in the consumer unit.

If so, what do I do
with the earth on the supply cable?

Terminate it in a plastic adaptable box if it is an SWA and earth it
at the house end. If it is T&E make the end safe within the CU.

Do I just bash the earth rod in to the ground outside, and run some
(6mm?) earth wire in to the consumer unit from there?

Pretty much, you'll need an earth loop impedance tester to test the
effectiveness of the rod and add more or extend the one that's there
until the Ze is low enough to comply with the relevant regulations.

 

[John Rumm]

It is a good idea to make your outbuildings a TT system which as you say
requires an earth electrode but use at least 16mm. As it is a TT system your

You probably won't need 16mm for the earth conductor (the earth loop
impedance of the spike is unlikely to be low enough to pass that much
current through it).

soket circuits need to be RCD protected to limit fault current although it is
not necsarry to rcd protect your lighting circuit. You need to test your spike
and it must be below 200 ohms, if not stick 2 in and retest. ( 200 ohms earth
fault loop impedence can result in high fault current before a mcb interupted
supply hence an RCD ) >Should I earth the consumer unit to an earth rod? If
so, what do I do

I think you have your "high fault current" and "high earth impedance
confused".

A high earth loop impedance (i.e. a "poor" earth) will result in LOW
fault currents - that is the problem. In a setup with a good earth
(typically one provided by the electricity supplier), should a fault
occur that allows the exposed metalwork of an appliance come into
contact with live, the current that flows to earth will be massive -
this is a "good thing" because it will cause the protective device
(fuse, MCB etc) to operate and cut off the current.

With a TT system that has a high earth loop impedance you can not rely
on this form of protection because the earth impedance may limit the
fault current to a level where it will not activate the protective device.

For example with a fault loop impedance of 100 ohms, your maximum fault
current on a live to earth short will be 2.3A - not even enough to trip
a typical lighting circuit breaker. This is why TT systems will need an
RCD in addition to fuses / breakers.

If you are using SWA just gland off the armour at each end and earth the cu to
the rod.

or use an insulated (i.e. plastic) consumer unit.

 

[Mark McGee]

Here's a bit more info (and some more questions!);

I'm actually running two separate cables, one to an outbuilding about
20 feet from the home CU (cable runs above ground, along outside of
house - this cable was already installed - 6mm T&E, I think). The
other is a 10mm 3-core SWA to a workshop, which I just laid, the cable
run is about 100-120 ft from the home CU, and runs underground for the
majority of the run.

I've obtained some earth rods for both installations.

I'm still not 100% sure what to do with the earth wire on the cables.
Do I earth them at the house end, and just ignore them at the w/s or
outbuilding end, and just hook-up the workshop CU earth to the earth
rod?

Currently, in a special (and very expensive!) junction box, I've
joined the earth core wire with the outer cable shielding. It's not
live yet.

Can I not test the earth impedance with a multimeter?

Thanks,
Mark

 

[John Rumm]

I'm still not 100% sure what to do with the earth wire on the cables.
Do I earth them at the house end, and just ignore them at the w/s or
outbuilding end, and just hook-up the workshop CU earth to the earth
rod?

Yup that is about it. In many cases you would use 2 core SWA and just
earth the screen. Terminating in an insulated CU at the workshop end
would then neatly drop the connection to house earth leaving you free to
connect your earth rod.

(the setup in my house (came with the house - not installed by me)
actually does the reverse - connects the screen of the SWA to the
workshop earth rod via a metal clad CU and then isolates it from the
house at the house end of the cable)

Currently, in a special (and very expensive!) junction box, I've
joined the earth core wire with the outer cable shielding. It's not
live yet.

You can either earth the third wire or ignore it. The main purpose of
the earth in the SWA is to give a fault current return path should
someone manage to get a spade through it.

Can I not test the earth impedance with a multimeter?

Alas no. The test needs to check the impedance when connected to AC
mains voltage to get a true picture of how it will behave in real life.

 

[John Rumm]

Sorry , let me clarify my thoughts ( I am at moment doing year 2 of 2361, so am
trying to get my head around a lot of issues ) I understand the maths and
recently done earth loop testing and with a high earth fault loop impedance on
TT system if a fault occurs as you say with 100 ohms impedance 2-3 amps will
flow and protection will not trip, but as a 15 mA shock across the chest could
be fatal I am basing a 3A fault on metalwork or pipes as a dangerous current
flow.

Not sure I follow that. The current flowing through exposed metalwork is
not in itself relevant from a shock point of view. The voltage present
on it however may pose a risk. To mitigate this there is a requirement
that your RCD trip current is low enough to ensure that a voltage of no
more that 50V can appear on an earth conductor. So in the case of a
100mA RCD with your 100 ohm fault loop impedance, you could get a
maximum voltage drop of 10V drop. Put it in other words, the earth
conductor could rise to 10V above ground potential before the breaker
tripped. Hence that setup would be safe. However a 330mA RCD with a 190
ohm earth fault loop would give a voltage on exposed metal work of over 60V

It is a dangerous current flow in the sense that it is too small and
will not trip a breaker - not that it is a shock hazard.

As for the 16 mmsq earth cable i thought the earth cable on TT system had to be
a min of half the supply size cable but I may be confused by main equipotential
bonding cable to pipes. But thanks for the reply.

Have a look at table 10C in the on site guide. That seems to suggest a
conductor size of as little as 2.5mm sq for situations where the wire is
not burried and is protected against mechanical damage, but upto 25mm sq
for a burried unprotected conductor.

 

[Mark McGee]

Can I not test the earth impedance with a multimeter?

Alas no. The test needs to check the impedance when connected to AC
mains voltage to get a true picture of how it will behave in real life.

Thanks John. I have an electrician coming over soon to install a new
CU indoors and a few other jobs. I'll get him to do the earth loop
impedence test.

Thanks again
Mark

 

[John Rumm]

A 330mA RCD seems quite high to me are you refering to the old ELCB that used
to fitted ?

I was using 330mA to exagerate the point that either a high leakage
current or a high earth impedance could result in a dangerous voltage
being present on the earthed metalwork. Having said that you can get
300mA+ RCDs:-

http://www.tlc-direct.co.uk/Products/MK5860.html