Max Ze

[m4tty]

Hi,

Reading some notes and ive got max Ze's being 0.35 ohms TNCS, 0.8 ohms TNS & 21 ohms for TT but have read on forum and elsewhere that max Ze for TT is 200 ohms so where does the 21 ohms come from?

Thanks

Matt

 

[Badger]

Hi,Reading some notes and ive got max Ze's being 0.35 ohms TNCS, 0.8 ohms TNS & 21 ohms for TT but have read on forum and elsewhere that max Ze for TT is 200 ohms so where does the 21 ohms come from?

Thanks

Matt

ive never heard of tt being max of 21 ohms before, always been under 200 ohms for TT supplies.

 

[wozz]

around here you will be very lucky to get 21ohm with a single rod.

There is a reg regarding swimming pools which comes into play with hot tubs that requires the max Ra to be below 20ohm (cant remember exact details). you will have to check that section.

 

[Bez]

I thimk the 21ohms is at the DNO end, but don't quote me on it.

 

[Colonel Hathi]

I believe the regs state that ZE on TT should be as low as possible - but that anything above 200 ohms may not be stable (due to ground freezing or drying out).

The 21 ohms is sometimes recommended by the DNO as per the previous post.

Hope this helps

CH

 

[mojorebel]

according to the OSG note (page 11) 21Ohm is the stated max of the earth electroode at the dno transformer . The resistance of the consumers installation electrode should be as lows as possible . A value exceeding 200ohms may not be stable - refer to table 41.5 note 2 and reg 542.2.2

 

[niceguy]

But it would still pass at <1667ohms.

 

[Andy™]

21 ohms applies to DNO rod at subsation - no point having a rod at your house if DNO havent stuck one in the ground at their end

for 7671, the rod should be considered unstable at over 200 ohms.

if using a 30mA RCD, then your max Zs will be 1667 ohms, but if you do have this, then something is wrong somewhere

 

[jalaner06]

For future refrence, on a TT installation I always work out my maximum Ze to comply with maximum disconection times like this.

50 (touch voltage)

----

0.03 (mili volt amp rating of RCD, in this case 30miliamp)

= 1667ohms

another example

50 (touch voltage)

----

0.1 (100mili amp RCD)

= 500ohms

If you get above 200ohms ZE value on a TT instalation, the value can be reduced by adding another earth rod next to the original earth rod, the new earth rod obviously would be linked to the first rod with a continual copper conductor, try get the distance between the two rods about the same as the length of a earth rod itself, if that makes sense.

 

[Andy™]

or you could use extendable rods

 

[SPECIAL LOCATION]

But it would still pass at <1667ohms.

:|

Not according to page 48 of Guidance note 3!

as has already been pointed out

 

[steptoe]

For future refrence, on a TT installation I always work out my maximum Ze to comply with maximum disconection times like this.50 (touch voltage)

----

0.03 (mili volt amp rating of RCD, in this case 30miliamp)

= 1667ohms

another example

50 (touch voltage)

----

0.1 (100mili amp RCD)

= 500ohms

If you get above 200ohms ZE value on a TT instalation, the value can be reduced by adding another earth rod next to the original earth rod, the new earth rod obviously would be linked to the first rod with a continual copper conductor, try get the distance between the two rods about the same as the length of a earth rod itself, if that makes sense.

you sure about that? check it out, there is a minimum distance for this.

 

[jalaner06]

you sure about that? check it out, there is a minimum distance for this.

Hey,

Below is a diagram taken from the IET regarding multiple earth electrodes on a TT system.

 

[wozz]

i always thought there was a guide the the minimum separation between the rods, i cant really remember the thinking behind it, is it they are only effective if a certain distance apart? or something to with voltages appearing on the rods?

 

[steptoe]

Hey,Below is a diagram taken from the IET regarding multiple earth electrodes on a TT system.

and?

S and D have no definitions

 

[jalaner06]

and?S and D have no definitions

Ok,

I've managed to find the IET article regarding multiple earth electrodes for you to review, please open the PDF found at the bottom of this page: http://www.voltimum.co.uk/news/7036/cm/multiple-earth-electrodes-in-a-tt-system-for-a-dwelling.html

But if you cant be botherd to read there article, they word it as follows:

"Generally the resitance area is deemed to be fulfilled by A seperation distance equal to the driven depth of the rod. A seperation distance in excess of twice the driven depth offers little benefit.".

 

[jalaner06]

But I have no idea why it becomes uneffective if you exceed twice the driven depth of the earth rod?, any ideas from the forums scientists?. Pray

 

[steptoe]

as far as my own experience of TT goes, the driven depth is both rods,

ie, 2m rod, and a second 2 m rod, the driven depth is now 4m.

the site says in excess ,

its your own decision, but due to a lot of factors, I always install multiple rods at twice the driven depth, IMHO if the first one is bad, putting another beside it in poor soil wont be much better, but placing it away will improve the strength as such.

in much the same principle as wavelengths I suppose,

ie, 1/4 wave, 1/2 wave, wavelength, gain.

its similar to how hi gain antenna workie, 27MHz = 11metres, so you want your antenna to be in multiples of this, 5&1/2m, 11m, 22m etc.

 

[steptoe]

But I have no idea why it becomes uneffective if you exceed twice the driven depth of the earth rod?, any ideas from the forums scientists?. Pray

it doesnt become ineffective,

it simply means that the benefits you get upto placing the rod further away from the first rod level out at twice the driven depth, (all soil being equal that is.)

 

[Lurch]

its similar to how hi gain antenna workie, 27MHz = 11metres, so you want your antenna to be in multiples of this, 5&1/2m, 11m, 22m etc.

Someone else with a shed full of CB's?