British gas TT earthing and rcds

[JD6400]

I see !
That is the trouble with not seeing it first hand , I had just pictured a standard farm set up ( all be it on a large scale ) , is that the one that you part dug some of the rods in as well ?
I know you are aware of this but for others that are not , you just mentioned the livestock areas , but as I understand it the reg is to do with at least partly , fire protection . So it should cover all circuits within the holding that is classed as agricultural .
What do others know of it as ?

 

[JD6400]

On a side note , I have often pondered where to draw the line as to what is classed as agricultural and what is not on a site of mixed use ( all the more common now days as farmers diversify into all sorts , it seems )!
We used to test the councils stock of farms ( until they were mostly sold off ) and we have always been told that if the origin of supply was in the farm house then the whole site falls under part Pish , if applicable that is .
What do others think of that ?

 

[Geordie Spark]

Segregation a wonderful thing as long as it's not "Apartheid"!

Would that apply to Geordies and Mackems???

 

[richy3333]

The OP did say he was going to put a rod in, what more could it possibly need?

Don't you have to **** on the rod too, Tony?

I always urinate on my installed rods. It's cheaper than cracking a bottle of champagne!

 

[D Skelton]

I see !
That is the trouble with not seeing it first hand , I had just pictured a standard farm set up ( all be it on a large scale ) , is that the one that you part dug some of the rods in as well ?
I know you are aware of this but for others that are not , you just mentioned the livestock areas , but as I understand it the reg is to do with at least partly , fire protection . So it should cover all circuits within the holding that is classed as agricultural .
What do others know of it as ?

300mA RCDs are used for fire protection and are required on all circuits within an agricultural or horticultural premises. If no livestock is present, no grain or hay is being stored and nothing is growing in a particular area how can it be deemed an agricultural premises? The word agricultural can be used to loosely describe the use of land, but not every part of it has to be specifically used for agriculture.

As an example, the new build block we were pulling in a new submain for was going to be used as office space for local businesses to rent. Miles away from any cows lol.

 

[JD6400]

Exactly Damian , I know you already know this but It was for the benefit of others .
I see it with nearly every new agricultural client we take on , either no TT on livestock holdings where required and without fail no RCD protection on the arable ones .

 

[D Skelton]

Exactly Damian , I know you already know this but It was for the benefit of others .
I see it with nearly every new agricultural client we take on , either no TT on livestock holdings where required and without fail no RCD protection on the arable ones .

Yeah, this one we worked on had no RCDs anywhere other than on final circuits and no supplementary bonding anywhere. The Ze was tested before we started work as we needed to design the system and I can't remember the exact value, but I can tell you it wasn't a TN value. Essentially, the option I had was to RCD protect the new submain at great cost leaving the rest of the system potentially dangerous, or, upgrade the earthing system to enable adequate protection of the whole lot at not a huge cost. It was a no brainer really lol

 

[davesparks]

They would be too close together then!

Would you care to expand on that comment? All of the guidance I have read on the subject has the minimum separation distance as being equal to the length of the longest rod, ie. for 2x 8' rods seperation should be >8'.

The following is copied from a copper development association publication titled 'earthing practice',

6.1.6 Proximity effect.
If two earth electrodes are installed close together, then their zones of influence will overlap
and the full benefit possible will not be achieved. In fact, if two rods or horizontal electrodes
are close together, the combined earth impedance of the two can be virtually the same as for
one, meaning the second is redundant. Spacing, position and soil characteristics are the
dominant factors in this. Figure 6-6 shows how the overall resistance of two five metre
vertical rods changes as the distance between them is increased. From this it can be seen that
the rods should be more than 4m apart in uniform soil. Calculations of this type are the basis
for the established practice of installing electrodes at least the same distance apart as their length.



My use of 8' and 10' originates in an entry in Stubbs' electrical encyclopaedia, which although it is a bit out of date seems to concur with modern information I have read on the subject.

 

[ImpededLoop]

Would you care to expand on that comment? All of the guidance I have read on the subject has the minimum separation distance as being equal to the length of the longest rod, ie. for 2x 8' rods separation should be >8'.

That's my understanding as well.

 

[davesparks]

The above graph, whilst not very clear on a detail level shows a definite trend that once separation is more than the depth of the longest rod then there is very little advantage to going any further.

 

[netblindpaul]

Rods should be a minimum of 2x depth apart from memory from 7430, though I would need to check, that is what I was taught and have always worked to.

Can't look it up now go to go, wife has just been ill.

 

[ImpededLoop]

Just checked Guidance Note 8 which says the spacing should be at least equal to the length of the rods used.

Edit: Page 28 GN8

 

[davesparks]

Rods should be a minimum of 2x depth apart from memory from 7430, though I would need to check, that is what I was taught and have always worked to.

Can't look it up now go to go, wife has just been ill.

That's a new one on me, I shall try and get some more research into the subject done

 

[Engineer54]

Just checked Guidance Note 8 which says the spacing should be at least equal to the length of the rods used.

Edit: Page 28 GN8

All very much depends on the the resistivity of the soil the electrodes are being used in. With a good soil resistivity the spacing between the earth electrodes need to increase to avoid electrode zones of influence overlapping.

My general rule of thumb for small installations is 1.5 to 2 X the buried rod length when soil resistivity isn't known....

 

[Knobhead]

My use of 8' and 10' originates in an entry in Stubbs' electrical encyclopaedia, which although it is a bit out of date seems to concur with modern information I have read on the subject.

Stubbs may be old but it’s a mine of information that’s still relevant. Electrical theory doesn’t change.