TT installs

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Got a job on the go which is TT (stables), they have several sub CU's.

Now, I feel that I have 3 choices as to the earthing-

1. put a rod in at the incommer only!
2. put a rod in at every CU with all being linked by a common earth
3. put a rod in at every CU (leaving incomming earths to CUs disconnected

Any of these options is better that they had, as someone had connected to an earth block on the side of the cut out which was not connected! I had to knock in some 22mm pipe that I had in the van as a temporary measure (got 5.57ohms which suprised me!)

P.S. there is a 100mA 'S' type RCD at the incommer and all socket circuits will have 30mA RCD protection
 
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Theres probably some logic to each of those!, just depends on the actual situation

1. put a rod in at the incommer only!
Is the incommer in a separate building in which there are no animals kept? if so then it could be a good one to minimise the risk of step voltage where there is a voltage gradient on on ground nearby to the electrode, and as animals don't normally wear rubber soled shoes its not good for them!, (also make sure electrode ends up in a proper pit eventually, etc)


2. put a rod in at every CU with all being linked by a common earth
Would bring overall impedance down a bit and thus reduce touch voltage which would be useful I suppose if you were getting a ze of nearer 100, or would be useful if the earthing systems would end up commoned by something else (ground of some audio system, etc), as the latter is very unlikely in a stable and Ze doesn't seem to be a problem then I'm not sure it'll give you many advantages


3. put a rod in at every CU (leaving incomming earths to CUs disconnected
Would make sure that the earthing system at each outbuilding was connected to the ground locally rather than far away and keeps them independant, but then you've got the possible issue of step voltage around each one.


I'd probably go for option one, unless there was a DNO transformer right next to the incommer (and thus a HV electrode right next to where you want to put an LV one!)

Allowed touch voltage is 25v instead of 50v as well AFAIK, but it shouldn't cause you a problem with an impedance of 5.57 ohm and a 100mA RCD


P.S. Not sure it would have been much better if the earth terminal was connected!, TNCS is probably not a good idea here (which is possibly why the DNO haven't actually connected it to the neutral!)[/quote]
 
Interesting topic, Adam, for my benefit could you elaborate on a couple I points I'm struggling t with, i.e.


1. put a rod in at the incommer only!
Is the incommer in a separate building in which there are no animals kept? if so then it could be a good one to minimise the risk of step voltage where there is a voltage gradient on on ground nearby to the electrode, and as animals don't normally wear rubber soled shoes its not good for them!, (also make sure electrode ends up in a proper pit eventually, etc)

What is a voltage gradient in the ground ?

P.S. Not sure it would have been much better if the earth terminal was connected!, TNCS is probably not a good idea here (which is possibly why the DNO haven't actually connected it to the neutral!)

Why would a TN-C-S system not be advisable in this situation ?
 
If you imagine that most of the impedance of the electrode is not in the main body of the earth (being made up of many many parrallel paths), but rather the 'bottleneck' in the few metres of ground surrounding the electrode, then if you were to measure the voltage between true earth and a point right next to the electrode while a current is flowing, you'll get almost the full value of touch voltage, move it to 6" measure again and it'll be less, 12" less again, and it'll slope down to zero like a gradient, its called step volatage, because if you are standing near the electrode the volatage between one foot could be different to the other foot!

I'll be limited in this situation to 25v with that being the max allowable touch voltage, and its not going to be a problem for me in my rubber soled shoes and relativly close together feet, but if it was a cow with its feet much further apart and stood in wet ground... I'm not sure whether or not it'll feel a tingle or anything, 25v is deemed to be safe for them, as to whether it'd notice it? :confused:

EDIT: Our regs don't consider step voltage (some others do I'm told), instead limiting touch voltage to an accepted 'safe' value... doesn't hurt to consider it when positioning your electrode though :)

The TNCS thing is simply that the TNCS earth is likely to be a few volts off true earth and while its not a problem for us in our cosy equipotential zones, this is a load of outbuildings spread around a field, no equipotential zone as such and there could be distances involved, possibly metal structural elements of barns, etc, and of course theres animals who are more sensitive to voltage than we are
 
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I recall cattle refusing to feed from a feeding trough after the shed was re-wired. During the re-wire the trough was bonded to the earth in the supply to the automatic filling system and this earth was a few volts above local true earth. Bonding the trough to an earth rod solved the problem.
 
Cheers guy's,

Should have said in my original post that the incommer is in a roadside box which then feeds a db about 30m away, this feeds two other db's (one for the stable lights [8m away] and another in a mobile home [adjacent]). The db at the stables then feeds another db in a shed and then another db in an office.

While there are lots of problems with this installation (damage) I am mostly concerned with the earthing and voltage drop in the existing cables.
 

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