Would you accept a Ze of 199Ω on a TT Installation which also has an RCD.

Deleted member 9648 · Feb 3, 2015

Re: Would you accept a Ze of 200Ω on a TT Installation which also has an RCD.

baldsparkies said:
"In the mean time I will continue to go with 100 ohms purely because logic dictates its a safer bet than 200 ohms".

In context with the rest of the quote it seems logical to me !!
Where a TT system is used I will always attempt to get sub 100 ohm readings as opposed to 200 ohms because its a safer bet.
Is not the whole point to reduce the resistance of the earth return path to a point where stability issues are minimised.
Technically we can often go much higher than that, but I prefer not to always rely on what's technically correct.
Otherwise we can bring into play your own analogy of 1667 or even 1000 which I agree would lack logic, but on paper !!
BS 7671 are looking at values below 200 ohms.
NICEIC have quoted below 100 ohms, as does BS 7430.

Actually BS 7430 is a very interesting read so here's a link,

http://jack.nazwa.pl/doc/UK.17th/BS/BS 7430 Earthing.pdf

With reference to Skelts 200 mph vs 190 mph,
If its a Ferrari, with decent brakes, on a traffic free autobahn !!

GO FOR IT unk:

Wasn't having a go mate,the whole Ra value thing is a lottery. As I've argued before,i just cant get my head around a view that pitches ANY high Ra value as 'better' than any other high Ra value.The end result will be the same whether it's 20 ohms or 1000 ohms. Tn values or you may as well go twiggy IMO lol.
You there Eng?

D Skelton · Feb 3, 2015

Another thing worth pointing out is that stability has absolutely nothing to do with ohmic values. A 10 ohm Ra only suggests that the rod is more stable than one with a 100 ohm Ra. Depth is the key to stability, and I've had rods with Ra values far higher than 200 ohms that are infinitely more stable than any low Ra value obtained from the use of a four foot twig!

baldsparkies · Feb 3, 2015

Re: Would you accept a Ze of 200Ω on a TT Installation which also has an RCD.

wirepuller said:
Wasn't having a go mate,the whole Ra value thing is a lottery. As I've argued before,i just cant get my head around a view that pitches ANY high Ra value as 'better' than any other high Ra value.The end result will be the same whether it's 20 ohms or 1000 ohms. Tn values or you may as well go twiggy IMO lol.
You there Eng?

I totally see your point, and in agreement with all you and Skelts have said.
In fact Skelts has put the point very well in his last post.
The integrity and methods employed to achieve stability are not disputed, nor should they be.
Its the way I tell em !! (sorry)
Problem is when dealing with "as you say a lottery" You have to start somewhere, and this thread asks the question sub 200 vs sub 100.
I simply have never liked the thought of walking away from a TT system with a value over 100 ohms.
With petrol filling stations employing TT, the mass of steel and tanks would often give lower values than rods, once bondings were connected.
lower ohmic values in most cases will give an indication of stability.
And its that alongside installation methods, materials used, soil conditions within seasonal changes and type that help to take the lottery equation out of things.
As said I don't think we are disagreeing here, just putting viewpoints over from a different perspective.

Still fancy the 200 mph Ferrari though.

GEOFF S · Feb 3, 2015

Its quite simple, if the soil is a poor then a 200 ohm reading may be considered stable, if you have good soil then you would consider it to be unstable.

Now unless your going to survey the ground resistance, your hitting and hoping. If this is the case use good methods and remember the first meter is open to seasonal variation so drive deep if you can.

Cheers

baldsparkies · Feb 3, 2015

GEOFF S said:
Its quite simple, if the soil is a poor then a 200 ohm reading may be considered stable, if you have good soil then you would consider it to be unstable.

Now unless your going to survey the ground resistance, your hitting and hoping. If this is the case use good methods and remember the first meter is open to seasonal variation so drive deep if you can.

Cheers

And don't forget your cat 4 :icon12:

Deleted member 9648 · Feb 3, 2015

Despite appearances it's actually an awful long time since I used a twig,SOP these days is two coupled rods,perhaps a result of the discussions on this forum!....although TT's are an increasingly rare job. I recall using two coupled rods for a small fairground supply and barely hitting the fabled 200 ohm mark,but mostly in my area anywhere between 12 and 150 ohms is the expected result. Cant help wondering whether a twig would do the same job though!!!

123 · Feb 3, 2015

I think the twigs are for gas installers to bond the gas line :38:

Knobhead · Feb 4, 2015

Jonspark said:
well in theory these do have the joule/watt energy to ignite some types of flammable vapours. And you don't want to be around when a 100M3 vessel goes up.This has happened in the past, a few been in the US, but there was one explosion in the north of England can't remember the sites name at the moment.
Flixborough in 74

The main cause of Flixborough was an incorrect repair on a pipe bridge. The wrong grade of steel pipe was used to try and keep the plant running. The pipe fractured due to the substance being carried.

Jonspark · Feb 4, 2015

Tony said:
The main cause of Flixborough was an incorrect repair on a pipe bridge. The wrong grade of steel pipe was used to try and keep the plant running. The pipe fractured due to the substance being carried.

A quick Google search will give you an in depth report and changes in management of change at hazardous area sites.

Engineer54 · Feb 4, 2015

wirepuller said:
Despite appearances it's actually an awful long time since I used a twig,SOP these days is two coupled rods,perhaps a result of the discussions on this forum!....although TT's are an increasingly rare job. I recall using two coupled rods for a small fairground supply and barely hitting the fabled 200 ohm mark,but mostly in my area anywhere between 12 and 150 ohms is the expected result. Cant help wondering whether a twig would do the same job though!!!

Glad to hear it...lol!!

If the soil type is that bad, where you only managed to get a 200 ohms value from a coupled 2.4 m rodded electrode, then NO, a twig would not, and certainly wouldn't be stable. It's soil conditions such as this, where the use of soil conditioning and/or electrode enhancers should be considered.

As i've said many times here, there's a lot more to creating a rodded (or any other type of electrode) TT system than just bunging a rod or two in the ground and hoping for the best...