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Discuss New consumer unit on TT installation - earthing and SPD queries in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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I'm going to replace the consumer unit for a customer in rural Oxfordshire. The supply comes in overhead, drops down a telegraph-type pole into the ground, and runs about 2m to a recessed box in the wall (perhaps coming up in the cavity). I can't find the earth rod - probably entirely buried, but am getting Ze of 0.25 ohms, which is encouraging. Because it is hidden, I've put a new rod in, 2.4m deep. I tested it (GN3) and got results of about 95 ohms for the ground resistance, which is 60% of the maximum for a 300mA main switch RCD, and 20% for 100 mA. For discrimination, I'm keen to use the higher rate (delayed) RCD if it is reasonable to do so. The ground was damp ( it hadn't rained that day, but there had been a lot of weather about in the preceding days), and my question is; what order of variation can I expect between damp and dry soil, all other things being equal, and where can I find a quotable source for that information? If it is wildly variable, I may have to wait for a heatwave, so I can retest (which won't suit me or my customer). And question 2 (if I may); can anyone help with a source for information on the dissipation (via the short underground section) of a lightning strike to the incomer which can be expected given the description above. Or should I just include surge protection? Many thanks, Roy
 
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N

Nigel

At 2.4m deep I would not expect much of a change between winter and summer. You won’t get a much more stable Ze than that.

Can you confirm your testing method for your new rod? Was it Ra or Ze?

For an overhead supply I would install a combined Type 1/2 SPD at the mains.
 

Wilko

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I have had far lower results on a TT.
Good to know
 
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  • #9
Hi Roy - you lost me back there with Ze of 0.25 Ohms on a TT ... Sounds like TNCS?
It surprised me too - but there is no MET. A 10mm2 g/y comes in to the existing board through the masonry, but I can't see where from. My thinking is that I have to have a visible, testable earth to comply. I don't see any obstacle to leaving the existing one connected.
 
  • Thread Starter Thread Starter
  • #10
At 2.4m deep I would not expect much of a change between winter and summer. You won’t get a much more stable Ze than that.

Can you confirm your testing method for your new rod? Was it Ra or Ze?

For an overhead supply I would install a combined Type 1/2 SPD at the mains.
Thanks - Ra. Intuitively, I think 2m underground won't be enough to dissipate a lightning strike. I'd just be more comfortable if I can provide evidence to justify the spend. I expect there's a rule of thumb somewhere.
 

Strima

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If you're installing a rod for an LP system then you need more than one. The combined resistances of all rods must be below 10 ohms.

As there is an increased likelihood of a lightning strike on the incoming overhead supply then more than likely your risk assessment will require you to install SPDs. This is to reduce the effects of transient over-voltages damaging the electrical installation.

The installation of SPDs are there to limit the effects of the above over-voltages, the rod you're installing is part of the electrical system and not part of a lightning protection system. Unless you're going to start using rolling spheres, fixing tape and finials?

As you're installing an electrode for a TT system then just try and get it as low as possible, if you need to install an extra rod then place it three times the depth of your first rod away to prevent the spheres of influence impacting on each other.
 

Ian1981

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Do the regulations actually say that if you have a TT system then a SPD should be installed?
Obviously an overhead line is more at risk of a lightning strike but bs7671 Section 443 does not deal with direct strikes on buildings and structures.
Forget the risk assessment and the 4 indents of 443.4 I’m just curious about the TT part and in particular why would you install a type 1 if you do not have a LPS?
Asking for a friend ;)
 
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  • #15
It surprised me too - but there is no MET. A 10mm2 g/y comes in to the existing board through the masonry, but I can't see where from. My thinking is that I have to have a visible, testable earth to comply. I don't see any obstacle to leaving the existing one connected.
I misled you - apologies all round. On checking my notes; the existing Ze is 24.7 Ohms, which is more normal for TT. I must stop relying on faculties which have diminished over time!
Embarrassed of London
 
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

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

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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.
 
N

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.
 
N

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

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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

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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

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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.
 
N

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

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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

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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.
 
N

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.
 
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