DNO's Ze?

[JohnW2]

As a matter of interest, does anyone know roughly what is the typical 'Ze' at the supply end - i.e. the impedance to earth of the DNO's earthing at the transformer?

It's a very long time since I 'properly' measured the Ze of my TT earth electrode. For many years I've relied on measuring the loop impedance at the (isolated) earth electrode - which is, after all, what really matters to me. I usually get an answer of around 70Ω (except when ground is exceptionally dry) - but that, of course, is the sum of my electrode's Ze and the DNO's one (plus the impedance of the L path, which is negligible compared with the Ze figures). If I had some clue as to the likely impedance of the DNO's path to earth, I'd be able to estimate how much of the loop impedance was down to my electrode.

Thanks.

Kind Regards, John.

 

[PrenticeBoyofDerry]

They will tell you 21 ohms.

This will include resistance of neutral to earth plus the impedance of the trany windings and line conductor but not the resistance of the consumers electrode.

 

[JohnW2]

They will tell you 21 ohms.

Better than the 49Ω or so that leaves for my earth electrode, then!

Do you have any idea as to how true/typical that 21Ω is? - I guess one of those here from that side of the industry will probably know, because I imagine that they have to measure it sometimes.

Kind Regards, John

 

[PrenticeBoyofDerry]

I can only say that is what the DNO are likely to give you as a value.
As they would TN-C-S @ 0.35 and TN-S @ 0.8.
Maybe someone on here that does work a little more closely with DNOs will know.

 

[JohnW2]

I can only say that is what the DNO are likely to give you as a value.
As they would TN-C-S @ 0.35 and TN-S @ 0.8.

Fair enough. Thanks.

Maybe someone on here that does work a little more closely with DNOs will know.

Indeed.

P.S. Is this only happening to me? That last sentence I've quoted above was not visible when I viewed the message, but magically appears in the reply window when I click on 'quote'. That's happened to me several times in the last few days - anyone else experiencing this?

Kind Regards, John.

 

[PrenticeBoyofDerry]

I blame it on the mushrooms

 

[westie101]

Hmmm
Ignoring all other elements we work to the following
Max value of LV (230V) earth electrode = 20 ohm
Max value of HV (6.6 or 11 kV) earth electrode = 40 ohm, this only connects the steel work of equipment to earth
This is usually the case for Pole Mounted transformers

If however if we wish to connect the two earths together (usually on underground networks) the value is not allowed to exceed 1 ohm. Obviously when we connect all the HV cable and other substations to an underground system the 1 ohm value will be rather lower!

The values at the cut-out position I dealt with on another thread

 

[JohnW2]

Ignoring all other elements we work to the following
Max value of LV (230V) earth electrode = 20 ohm

Thanks. I was hoping you might show yourself!

So, that is the maximum value. Are you allowed to tell me what 'typical' values might be?

Kind Regards,John.

 

[westie101]

Fair to say that as soon as we get it to within those values we stop hammering rods in.
(we will often have 4 rods connected together to make one, if ground conditions permit, as well as running bare 70mm2 copper between them) rods spaced the same distance as their depth.

This may be useful to some

The resistance to earth of a length of buried uninsulated cable sheath (or bare copper
conductor) depends upon the soil resistivity. The length L required to achieve 10Ω is given
by: L = 30 x (R/100) where L is the length in metres, R is the soil resistivity in
Ωm (eg if R = 1000Ωm, L = 300m).
Typical soil resistivity values are given in Table 1 below:


Table 1
Ground Type Resistivity (Ωm)

Loams, garden soils etc 5 to 50
Clays 10 to 100
Chalk 30 to 100
Clay, sand & gravel mixture 40 to 250
Marsh, peat 150 to 300
Sand 250 to 500
Slates and slatey shales 300 to 3000
Rock 1000 to 10000

I was hoping you might show yourself!

New computer system going in at work so couldn't use it for most of the day! At one point my new laptop needed about 60Gb of data loaded onto it (all the cable records for the whole company!)

 

[JohnW2]

Fair to say that as soon as we get it to within those values we stop hammering rods in.

That makes sense!

(we will often have 4 rods connected together to make one ....

That also seems roughly consistent with my observations, and suggests that your rods are probably fairly similar to mine, performance-wise - if you are achieving 20Ω as compared with my one's 50Ω.

This may be useful to some

Thanks- that's interesting.

Kind Regards, John.

 

[securespark]

When you say 4 rods connected together, you don't mean in leangth, do you?

Grouped I guess!

 

[westie101]

When you say 4 rods connected together, you don't mean in leangth, do you

Yes, in length (you can't hammer them in by hand but can get adaptors to use on hydraulic breakers or similar)

No actually I have seen them hammered in by hand!

 

[JohnW2]

When you say 4 rods connected together, you don't mean in leangth, do you

Yes, in length (you can't hammer them in by hand but can get adaptors to use on hydraulic breakers or similar)

westie, I don't really get this. You previously said of the 4 rods

"rods spaced the same distance as their depth.

... which doesn't seem to make sense if you meant joined together end-to-end. Am I missing something?

Kind Regards, John.

 

[westie101]

OK first rod four coupled together is hammered into the ground (I think the rods are 1,2m in length) so 4.4m long.
The next rod must be 4.4m (or more) from the first.

Obviously if only using 1 x 1.2m rod the next need only be >1.2m away.

In part the depth means less concern of changing resistance value in dry conditions

 

[JohnW2]

OK first rod four coupled together is hammered into the ground (I think the rods are 1,2m in length) so 4.4m long.
The next rod must be 4.4m (or more) from the first.

Testing me with 'deliberate mistakes' again, are you That would be 4.8m, I presume?!

...so, we're actually talking about 16 rods, are we (4 x 4)? If so,no wonder you're doing better than my humble electrode!

Kind Regards, John.