New consumer unit on TT installation - earthing and SPD queries

[Roy Thomson]

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

 

[Wilko]

Hi Roy - you lost me back there with Ze of 0.25 Ohms on a TT ... Sounds like TNCS?

 

[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.

 

[Nigel]

Hi Roy - you lost me back there with Ze of 0.25 Ohms on a TT ... Sounds like TNCS?

I have had far lower results on a TT.

 

[l4urence]

Lost me too! 0.25!!! Did you mean 25, which is still pretty low!

 

[Wilko]

I have had far lower results on a TT.

Good to know

 

[l4urence]

I have had far lower results on a TT.

I don’t doubt but crikey! Never had lower than 10’s of Ohms myself.

 

[Nigel]

I don’t doubt but crikey! Never had lower than 10’a of Ohms myself.

Had it only once sub 1 ohm but it is possible.

 

[Roy Thomson]

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.

 

[Roy Thomson]

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.

 

[westward10]

You would need to have a soil resistivity survey carried out.

 

[Strima]

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.

 

[buzzlightyear]

Keep hitting it ,till it comes out in Australia.

 

[Ian1981]

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

 

[Roy Thomson]

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