Measuring the Earth Electroded resistance

[Podooser]

Yes below 25V for agricultural is correct. 133ohm with a 30ma RCD will be below 25V.[/quote]

 

[nozspark]

It's just that you said <50v..

Even though 133ohms will comply it is still above what the NICEIC require and from what I have heard the 17th regs will require <100oms for TT Ra.

And while the NICEIC think they run the industry, the regs are written by the IEE!!

 

[Podooser]

What would one do or change in this instance to make it comply

 

[nozspark]

If you wanted to get your Ra down below 100 ohms then you could either extend the rod that you have (knock it in further) or add a secondary rod several meters away from the existing rod.

However at 133 ohms you rod easily passes (according to the regs) - you are allowed up to 833 1/3 ohms, though the reason 100ohms is quoted is to ensure that you have a reliable Ze - as this can vary due to wet, dry, hot or cold conditions

 

[Podooser]

Thanks for the help Nozspark.
I will try a second earth rod. Where does the resistance of the SWA fit in?

 

[plugwash]

it doesn't really, the resistance of its live conductor will become part of the value you measure just as the suppliers live conductor will but it will be pretty tiny compared to the resistance of the rod.

 

[securespark]

Isn't the max touch voltage <25v for a stable/agricultural environment??

What about an unstable one?

 

[securespark]

you are allowed up to 833 1/3 ohms

Where do you get this figure from, bearing in mind GN5 (I think) lists the max on TT as 200?

123 - I thought the 200 figure was maximum Zs, ie the total loop figure anywhere on a TT install should not exceed 200 Ohms, not the maximum Ze? Surely if the limit was 200 on the Ze, then the Zs will exceed this figure, contrary to the guidance?

 

[Podooser]

It is 50v but for construction sites/agricultural sites it is 25volt.
25/0.03 = 833 1/3 ohms

 

[nozspark]

TT systes are governed by touch voltage. The max you can have in a domestic install is 50v, construction sites/agricultural/horticultural are 25v as I suspect are some other special locations (not checked regs).

With a 100mA RCD (domestic)
50/0.1=500ohms

With a 30mA RCD (domestic)
50/0.03=1666ohms

100mA (site/agricultural etc)
25/0.1=250ohms

30mA (site/agricultural etc)
25/0.03=833ohms

These are the maximum allowed, however 200ohms is the max in the OSG and 100ohms fo the NICEIC. You do have to remember that your rod resistance will vary due to weather conditions so that is why lower values are recommended.

As for this being you max Zs. your R1+R2 will make very little difference to these values, so if you are well below then you will be ok. Also you should ensure that your R1+Rn is such that should you have a direct short (live - neutral) that the OCD will operate

 

[Podooser]

nozspark wrote

If you wanted to get your Ra down below 100 ohms then you could either extend the rod that you have (knock it in further) or add a secondary rod several meters away from the existing rod.

I added a second earth rod and the new reading is 73.7 ohm. Sorted.