New consumer unit on TT installation - earthing and SPD queries

[l4urence]

I believe we should consider a big difference between a lightning strike and over voltage, since they are not the same. An SPD is for over voltage and will not protect against a direct strike or one very close by. I recently attended a course for SPD’s and have an interest on TT (there are many locally). The recommendation was a type 1 for incoming overhead lines. I’ll dig out the detail and put it here.

 

[davesparks]

Why only one rod? Personally I aim to get at least two 2.4m rods in suitably spaced for a TT system.

 

[l4urence]

Why only one rod? Personally I aim to get at least two 2.4m rods in suitably spaced for a TT system.

Really? I'm interested in your thinking? Do you fit the second rod because the first reading was too high or just your standard practice. I had considered doing this but depending on the Ze measurement alone. I'm going back to a couple of properties in the summer to test again - my measurements where sub 40 ohms recently.

 

[davesparks]

Really? I'm interested in your thinking?
Do you fit the second rod because the first readingwas too high or just yourstandard practice.

The general idea is to get the Ra as low as possible. As you increase depth the reduction in Ra is not linear, so 2x 2.4m rods will give a lower Ra than 1x 4.8m rod.
It has become my standard practice, the first reading is always too high in my opinion because I know it can be improved by adding a second rod.

 

[Nigel]

Why only one rod? Personally I aim to get at least two 2.4m rods in suitably spaced for a TT system.

probably because mostly one rod is plenty.

 

[Nigel]

Really? I'm interested in your thinking? Do you fit the second rod because the first reading was too high or just your standard practice. I had considered doing this but depending on the Ze measurement alone. I'm going back to a couple of properties in the summer to test again - my measurements where sub 40 ohms recently.

The correct way to do it would be a cost v benefit analysis. Is another rod viable? Is another rod worth the additional cost in terms of improved earth loop impedance.

 

[Nigel]

The general idea is to get the Ra as low as possible. As you increase depth the reduction in Ra is not linear, so 2x 2.4m rods will give a lower Ra than 1x 4.8m rod.
It has become my standard practice, the first reading is always too high in my opinion because I know it can be improved by adding a second rod.

Interesting. Do you connect the rods together then measure Ra or do you measure each rod separately and record the highest reading?

 

[davesparks]

The correct way to do it would be a cost v benefit analysis. Is another rod viable? Is another rod worth the additional cost in terms of improved earth loop impedance.

The 'correct' way to do it is carry out soil resistivity testing and chemical analysis then design an earth electrode system from that.

The Ops situation appears to be domestic, so the cost of putting in two rods over one is relatively small, the benefit is chiefly a reduced Ra (approx half) with greater stability and redundancy.

Even greater stability could be achieved by driving a single rod to 4.8m, but the Ra will not be reduced by a lot less than half.

 

[davesparks]

Interesting. Do you connect the rods together then measure Ra or do you measure each rod separately and record the highest reading?

I measure each rod seperately and record the result and then measure the complete network of rods together to establish Ra.

 

[Strima]

I would either measure both rods together using the Ze method, or measure separately, either using Ze or rod tester, then calculate resistance in parallel.

 

[Nigel]

I would either measure both rods together using the Ze method, or measure separately, either using Ze or rod tester, then calculate resistance in parallel.

Agree. I am still trying to work out Ra with multiple rods. I would just do Ze.

 

[davesparks]

I would either measure both rods together using the Ze method, or measure separately, either using Ze or rod tester, then calculate resistance in parallel.

Resistance in parallel calculation doesn't necessarily yield the correct answer, the interaction of the multiple electrides has an effect.

 

[davesparks]

Agree. I am still trying to work out Ra with multiple rods. I would just do Ze.

I realise this is going beyond the realms of the OP's domestic installation but an efli test isn't always possible or acceptable for testing earth electrodes.

 

[davesparks]

Here's a couple of PDFs on the subject:

 

[Nigel]

I realise this is going beyond the realms of the OP's domestic installation but an efli test isn't always possible or acceptable for testing earth electrodes.

Agree. Like when no supply is available.