TT system advice

[Robin Spark]

I've added a couple of extra sockets in today for a friend as he uses a lot of adaptors plugged into more adaptors for his tv and stereo!

Anyway Zs were only 0.68 ohms and as I have not much experience with TT systems I was astounded to get such a low reading.

I checked the Ra which came out at 291 ohms and therefore reckon that the low reading is due to the water supply pipe. This is main bonded under the sink and then the water pipes go back underground in a concrete floor before reappearing at the downstairs WC.

The earth rod is supplied by 6mm 6491x enclosed in pvc conduit but the top of the rod and connector is buried about 4-6" in the soil.

I know we shouldnt rely on the water pipes for earthing but as Zs are so low then is there any need to replace the earth rod?

 

[Zeespark]

Is the earth rod needing to be replaced due to environmental effects Robin? if not, and you want to replace to get the reading down, why not attach a second rod onto the top of the old one? I could be wrong in My advice though.

AndyGuinness

 

[Robin Spark]

The earth rod does look quite old and at first when i went outside to look it there seemed like nothing was under the conduit box which was broken. However I found the buried cable and rod a few inches away.

I was thinking about putting a new rod in near to the existing one which I would disconnect. But need advice if you think this is going to be necessary as Zs are low at the moment.

 

[Zeespark]

I would remove the old rod and install a new one, but I wouldnt install it in the same hole.

AndyGuinness

 

[Robin Spark]

Ok cheers mate, ignore the amended post as we must been have typing at the same time

 

[steptoe]

Zs is imo irrelevant, Ra is important, but its not bad.

The one issue is that it has to be accessible for inspection.

Is it possible to add a rod to existing? As has been suggested.

Oh, and upgrade the 6mm.

 

[Robin Spark]

Hi Steps, as the rod was buried in the soil i couldnt really see the condition of the rod too well or the cable connector. However the cable is connected firmly but i think the rod is probably too corroded to screw on another rod to drive in deeper.

Access from the consumer unit to the present rod is really easy and only a couple of metres away so it wouldnt be a pain to fit a new earth rod and cable if you think that might do the trick.

 

[NozSpark]

If you're going to install a new rod and earth cable then make sure that you stick a 5/8" rod in and put it in a pit in as well

 

[Zeespark]

Do you know what your PFC is? from that work out your adiabatic equation, that will give you the cable size.

AndyGuinness

 

[steptoe]

Deffo new cable, 10mm or 16mm, you havent mentioned current set up so I dont know what size you should use,

And a new rod, or 2 preferably, you dont know if it is already doubled up, but if its not, then you doubling will more than likely get well below 200.!

And make sure your new rod is accessible for inspection!

 

[NozSpark]

Zee,

The PEFC is going to be really low 100ohms gives 2.3a so theoretically you could get away with 2.5mm cable,, but there is a table in the regs about this and that it should be selected with

 

[Zeespark]

OK, Lesson learnt

AndyGuinness

 

[Robin Spark]

Ok thanks for the advice, When I go back I 'll use 10mm 6491x and get a couple of 5/8 rods plus one of those inspection pits.

I'll let you all know how I get on with the new readings.

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I thought that table was regarding the cable being protected against mechanical and corrosive damage?

Oh and not by the fault current on a TT system.

Zee,The PEFC is going to be really low 100ohms gives 2.3a so theoretically you could get away with 2.5mm cable,, but there is a table in the regs about this and that it should be selected with

 

[NozSpark]

Yes it is,,, it tells you what size cable you should be using in different circumstances,,, I've not go my regs to hand and I've had a look at GN1 which refers you to table 54.1,,,, but also regs 543.1.3 (stating that calculation is permitted) & 543.1.4

 

[Robin Spark]

TBH even if i could get away with 4mm or even 2.5mm I dont think I would be happy doing that for the sake of saving a few pennies, plus i've got plenty of 10mm anyway which will last a lot longer.

Oh, thanks for having a look at your GN1, its too late for me now to get BRB out.

 

[NozSpark]

TBH even if i could get away with 4mm or even 2.5mm I dont think I would be happy doing that for the sake of saving a few pennies, plus i've got plenty of 10mm anyway which will last a lot longer.

You're not wrong there!