Multiple Electrodes, distributed TT system

[EastAnglian]

I've just started work to "upgrade" an old installation, which also involves taking out some earlier sub-standard work. Basic description:

Rural farm house & outbuildings - overhead single phase supply - 3 phase transformer on pole in adjacent field.

Supply arrives in an outbuilding, from which then run sub-mains to the farm house and barns. TT system, Ze of earth electrode at the outbuilding with the service head & meter 21 ohms, luvlly jubbly. Sub-mains are wired in 3 core SWA with one core acting as CPC.

At the buildings being served some have an additional earth electrode, some don't. I want to achieve some consistency in the system and its my intent to have an earth electrode for each building, suitable installed close to the DB etc.

The question then, simply is should I use the CPC (and the armour) to connect together the multiple earth electrodes, thereby creating a single equipotential zone across the installation or should each building be an equipotential zone in its own right, without the earthing terminals being connected to each other, but simply connected to their own local earth electrode?

Intuition is saying to me that multiple earth electrodes provide a degree of redundancy, so long as the CPCs are adequately sized to carry the potential earth fault current.

 

[Deleted member 26818]

Is there any livestock?
I'd probably link the electrodes.

 

[jedisparks]

I would suggest linking the electrodes throughout. The intake is TT, so there is no reason to have seperate equipotential zones. Also consider discrimination of RCDs.

 

[EastAnglian]

Hi guys, thanks for confirming my intuition! No livestock, only children

Yes indeed re discrimination, the next task is to replace the old black voltage operated RCD with a 100mA Type S at the origin and then use 30mA RCDs at each DB

 

[Deleted member 26818]

Obviously, I don't know the installation, but you may find that you will provide better discrimination if you use a 300mA RCD at the origin, then 100mA at the boards with 30mA where required.

 

[Engineer54]

Agree, link all the electrodes, via the SWA or by a separate core in the SWA. Don't position rods too close to the outbuildings walls, that's where all the builders rubble is going to be!! Try and go a metre or so from any wall. Use of a 6mm earth conductor to connect the DB/CU boards to your electrodes. Are you providing Earth pit/boxes over the electrode positions for protection??

I would suggest 2 coupled rods to give you depth and stability for each of these new electrode positions. ...You never know, with the generally good soil conditions of East Anglia, you may even approach TN-S levels on your final Ra values. Which is good news for a farm environment!! lol!!!

 

[johnboy6083]

i was carrying out an isolation the other day, on a borehole site, and noticed an EIC on the wall. I had a qucik look, and noticed it was TT, but saw no RCD. I was just about to ring the engineer from the water board, and i noticed that all of the Zs values were well within max allowed. And i have little doubt that it will be stable, as they have probably used the site pipework as the electrode.

 

[EastAnglian]

Engineer54 - Yes to all your points there ... you must know our sandy soil well! I assume you meant 16mm earth conductor!

 

[ackbarthestar]

Engineer54 - Yes to all your points there ... you must know our sandy soil well! I assume you meant 16mm earth conductor!

for a standard 23kVA TT supply 6.0mm2 would be sufficient as long as its protected from mechanical damage and corrosion.
You could go down to 2.5mm2 !

 

[Engineer54]

Engineer54 - Yes to all your points there ... you must know our sandy soil well! I assume you meant 16mm earth conductor!

I know there are some very rich good soil types that are almost permanently damp throughout the year 2 or 3m down, and around the salt marshes you have in abundance...lol!!

As has been stated, you rarely if ever need 16mm earth conductors on TT systems of this type. But if you get that Ra down to under an ohm it may be worth it then...lol!! If you so wished, nothing stopping you using 10mm for these very short runs to the new electrodes. Down to you and cost at the end of the day...lol!!