two installations, one earth rod

[vxwestie]

A customer of mine has two installations (self contained barn coversions) connected to the same earth electrode.
Each consumer unit has a 16mm cable connected to the same rod. I cant help thinking this is poor practice, but what regs does it break, if any?

 

[MDJ]

A means of earthing is the arrangement through which a connection is made to the general mass of Earth. For an installation forming part of a TT system, the means of earthing is an installation earth electrode. As for any earth electrode, that for a TT system has to meet certain requirements. For example Regulation 542.2.4 requires the type and embedded depth of an earth electrode to be such that soil drying or freezing will not increase its resistance above the required value. Furthermore, the design used, and the construction of, an earth electrode is required by Regulation 542.2.1 to be such as to withstand damage and to take account of possible increase in resistance due to corrosion. Details of these requirements for earth electrodes are given in Section E5 of the Essential Guide. Reference should also be made to item 4 of this topic, which addresses the relevance of the earth electrode resistance with respect to ADS requirements for TT systems. In a TT system, every exposed-conductive-part protected by a single protective device has to be connected to a common earth electrode via the MET. However, if two or more protective devices are in series, the exposed-conductive-parts may be connected to separate earth electrodes corresponding to each protective device (Regulation 411.5.1). Refer to Fig 2.
	Exposed-conductive-parts connected to separate installation earth electrodes Fig 2 A requirement which must not be overlooked, however, in situations where there is more than one means of earthing (such as that shown in Fig 2), is that of Regulation 411.3.1.2 that simultaneously accessible exposed-conductive-parts have to be connected to the same earthing arrangement to prevent such parts attaining substantially different voltages with respect to one another. This is further explained in Topic E45-65.

 

[telectrix]

but here, to OP has 2 installations using a common rod. not sure if it's against regs, but gut instinct would make me separate them. that way, any mucking about with installation #1 would not compromise the earthing arrangements of installation#2. and vice versa.

 

[vxwestie]

Telectrix, thats my arguement, but the customer isnt keen. Its a new customer, so along as there's now actually break in the regs he'll probably not go for it. (read, tightass)
Instint too is that it just aint right, but trying to find facts before I make an issue of it.

 

[telectrix]

Ze or Ra readings from each CU. if over 100Ω, then hit him with the regs.

 

[Silly Sausage]

200Ω Tel!

Where's Eng54 when you need him?

 

[telectrix]

can get 200 on a piece of wet string tied to a tree. same as 6mm cable will not melt on a 9.5kW shower. we're in the real world here, not the IET silver cloud.

 

[PEG]

Also,to properly test earth electrode,both units will need to be disconnected which may be impractical without "team" meeting,there is always someone whose slow cooker can't be turned off....

 

[Engineer54]

If these 2 supplies are separately DNO metered, then each building will require a separate TT system, end of!!

 

[Deleted member 9648]

If these 2 supplies are separately DNO metered, then each building will require a separate TT system, end of!!

Agree in practice....however the OP is after a reg to hit the client with,and I cant find one. Seems extraordinary though that with the overall cost of two barn conversions they would scrimp on the earthing.

 

[telectrix]

If these 2 supplies are separately DNO metered, then each building will require a separate TT system, end of!!

we all agree, eng54, that they should be separate, but nobody can give a definitive answer as to why.

 

[Edd]

If its a reg about the practicality of testing and inconvenience then this will only get a C3, whats needed is a reg where danger could occur. Like if its not clear that the earth is shared then any sparks isolating property A to do an Ra test will introduce a danger to property B therefor getting a C2. Shame my regs books in the van, Im way too lazy this morning.

 

[Engineer54]

I don't think you'll find anything in BS7671 or it's OSG's. Though you'll not find a similar example being put forward as being compliant. Examples in the relevant OSG, will always be shown as separate electrodes being required!! So you could try actually showing the client these diagrams to explain what is generally required in such circumstances. lol!!

It's not too dissimilar to an old DNO supply that feeds 2 houses via a single TN-S cable looped between them, when you think about it!!.

In this case, commonsense would tell you to provide separate TT systems, for two completely separate supplies though. As wirepuller states, why anyone would scrimp on a simple earthing arrangement given the overall costs on converting these barns is beyond me. I put the blame squarely on the electrician responsible for wiring these two barn conversions. Even if both buildings were fed from a single supply point, having just a single electrode for both buildings would still make little to no sense. Another case of relying on RCDs and the single electrode TT acting only as a reference earth!! ...What's the betting it's a twig electrode too?? lol!!

 

[vxwestie]

Thanks fr all the replies guys.
I think everyone has been here though, I think the reaction Im going to get from the client is, "well, if it does'nt break a regulation and your not going to fail it on a test, I'm going to leave it" ..... fools...
Glad Im not the only one who cant find a reg against it, but at the end of the day, all I can do is recommend an additional rod.
(it is actually a decent size)