Earthing or Grounding

[Colinjohnson]

Hello,

I work in the water industry where the standard earth nest resistance for an LV transformer is usually specified as 1 ohm.
We have a site which where the ground conditions make it very difficult to get anywhere near that figure which has prompted a debate about where the requirement for 1 ohm originated. Is it contained in any national standards, is it dictated by the prevailing fault level, the magnitude of a lightning stroke, does it matter if it is 10 ohms etc ?
Our earthing contractor has suggested a borehole with the earth rod embedded in bentonite however bentonite is not suitable for use at a water treatment works and again, is there a stable alternative on the market ?

Looking forward to hearing from you,

Colin J

 

[isquaredr]

I work for an electricity distribution company in the uk, for distribution plant (11kv)we use the following guidelines

HV earths <40-ohms
lv earths <20-ohms
If a seperate HV & lv earth mat is used they must be seperated by 10m
Using a combined earth mat for both HV & lv <1-ohm

 

[Colinjohnson]

Thank you Isquardr,

Are you aware of a document which relates to requirements for earth nests ?

CJ

 

[sslobodan]

Well for start before you can ask your self what is the maximum value of the grounding resistance you must ask why do you put it at all? Most of the people meet grounding as another boring requirement from the law, but it has a great value. By definition grounding is put to make shortest path for current to flow into ground, because that is the reference potential for all electric systems. Now if you would draft a electric system to represent the current flow you would draft an electric circuit with it's own impedance for every part of the system. as you know currents trough such system are proportional to a impedance in that circuit what means that smaller the omh value the greater the current is. In a case of short circuit of any kind system would try to balance it self now let us take a look a grounded metallic pipe in your bathroom. You are opening the water in a moment of a big short circuit if your ohm value is smaller than the value of the system that will mean that most of the current will pass trough you and most probably kill you, if the grounding system is done as it should be you would feel small current going trough you and that would be all. Now as you can see it is not the question what is the smallest allowable current bu network but how big are resistances of other circuits to the ground in your case (because no matter how big ohm value of resistance are there will be a current trough all circuits). That is why the value is set to a certain amount (usually tops the 10 ohm) by a power company.

There is one interesting issue when you have mentioned lightning. Different than the philosophy stated above it is good to have a high voltage grounding in such case U=R*I in a case of a lightning you have at least 10KV (to few hundreds of KV) because the current is spreading in that time by a same law from text above you would have some 10 KA on your grounding outlet if your ohm value of the grounding is 1 ohm and 1 KA if it is 10 ohm. Now you can imagine why is that most of the house equipment is not suitable for such high ratings. Most of the sockets in your house would not be able to withstand not even 1 KA from ground to phase (or neutral) not to mention 10 KA or more
I hope that this helped all the people out there that did not know what is ground protection for.

 

[Prospero]

Hello Colin,

I too work for a water company in the UK, and I too have wondered where this often quoted 1ohm comes from.

Recently I was told that it originates from Engineering Recommendation G12 (PME installations)and, as Isquared states, is applicable where the HV and LV systems share a common earth nest

 

[Colinjohnson]

Gentlemen,

Thank you for your responses but I think I have answered my own question.
The reason that an arbitary number has been abandoned is that no matter what that number is it is no subsitute for good design up front.
It is a bit like the old 500 lux on the working surface in an office space. This has gone now in favour of a holistic approach which attempts to light the whole of the space and environment whilst enabling the visual actuity of the occupiers to accomplish their tasks. In other words a qualitative approach over a quantitative one. A similar revolution has taken place within health and safety where the designer risk assessment has triumphed over the mandatory safety measure (where appropriate)

Thanks again.