Multiple earth spikes

[Ianhaycox]

Are there any potential (pun intended) problems using multiple earth spikes

within a single installation ?

I ask because I have a barn separate from the house supplied by an overhead

cable into a local fuse box that carries phase, neutral and earth. An extra earth

spike has been added for the barn to augment the earth supplieed from the house.

Is this prudent or could it cause problems with potential differences/resistances

etc.

I'd like to run a supply to another outbuilding, should I add a third earth

spike ?

[Punch]

Multiple earth spikes can often be the solution to low resistances obtained with just one spike. The general rule is that they should be a distance of a least the length of the rod apart from each other. If you needed more than two to get a reading of 100 ohms or less, I would seriously consider using another method of earthing ie; plates or a "boucle" (circle of bare 25mm copper wire in a deep trench around the house), terminating in a rod or plate.

The rods of 1metre length sold in all the brico's are not long enough, I would suggest rods of minimum 1.5M

Paul

[Ianhaycox]

I'm fairly sure the current earth is OK, but was concerned that adding a spike might 'upset' the 'resistive balance' <- insert proper electrical terms here.

The spikes are abut 30m apart and are most certainly not 30m long.

Thanks,

[Punch]

Ian, the only way to know if the earth resistance is OK is to measure it with an appropriate tester. Otherwise you will only ever find out when there is a fault!

Paul

[Ian Hoare<br>All the best<br>Ian<br>La Souvigne Corrèze<br>http:www.souvigne.com]

Hi Ian,

What Punch didn't say, but meant to, was that his reply implied that all the earth spikes are bound together by a single earth cable. There was also a small typo in his answer in that he should have said that multiple spikes are the answer to high earth resistance.

If your different buildings are fed with electricity entirely separately, and there's no chance of any cross connestion ever taking place, then a spike for each building wouldn't be a problem though I'm not sure it's within the rules.

Remember that in the event of a nearby lightning strike, the earth under your barn might be at a competely different potential to that of the earth near your house for a milli second or two. ALL electical apparatus will then be at the potentials of the earths. If anything's tied together in any way, then there is a risk of a heck of a bang as the earth itself tried to equalize its potential through the link.

So, what you're supposed to do in the case of a "single installation" where - say - you may have phones in different places, or a TV and satellite dish, is to tie all your spikes together with a heavy duty earth cable (10mm² is minimum I think) and then feed this cable into each junction box. That way, the earth cable is the path of least resistance, and so the junction boxes are all at the same potential, which keeps things neutral between the buildings.

Hope that helps.

[Ianhaycox]

I have a single 3-phase supply coming into the house that then spurs to each outbuilding, one via a 10G5 cable and the other a 16G5 cable. The earths are all connected together and each of the three spikes (house, + 2 outbuildings) connect to the common earth, so it sound like I'm OK.

Thanks everybody.

[Punch]

[quote]Hi Ian, What Punch didn't say, but meant to, was that his reply implied that all the earth spikes are bound together by a single earth cable. There was also a small typo in his answer in that he shou...[/quote]

Hi Ian,

Thanks for picking that up, it should have course read high and not low resistance. The truth is that people rarely know the resistance of the earth which should be measured with an appropriate tester. Just banging an earth rod into the ground is rarely enough unless you are lucky to have very damp soil, and even then a 1.5m rod is often needed to obtain the minimum 100 ohms required.

Paul 

 

 

 

[Ian Hoare<br>All the best<br>Ian<br>La Souvigne Corrèze<br>http:www.souvigne.com]

[quote]Hi Ian, Thanks for picking that up, it should have course read high and not low resistance. The truth is that people rarely know the resistance of the earth which should be measured with an appro...[/quote]

Hi Paul,

No probs about the typo. I do them all the time! I call them "having a senior moment".

As for testing the earth, yes, you're absolutely right. The only problem afaiac is to know how to make sure your tester makes a good enoigh connection with the earth to be able to test the resistance. :-)

So, Ian OP. make sure you have your earth properly tested and make sure you have enough spikes going deep enough.

If ever you want to put in proper barrier lightning protection, you'll need an earth way better than 100 ohms, by the way.