3.6 Mt Earth Electrode

[Electroglow]

hello, 

been on a job today and they had a lighting test to done. it failed with a high readng… they have suggested installing at earth electrode 3.6mt. This needs to bring the reading down to below 10 ohms. 

my Q is what does 3.6mt mean? as i have not heard of this before and can't seem to find it on the net ??

Cheers

 

[m4tty]

Its not 3.6 metres is it ?

 

[Electroglow]

never seen a 3.6 mt earth rod before?

 

[Sharpend]

Leave 4x1m rods joined together, as by the time you have finished driving it into the ground the fourth will be 600mm long!!!!!!

 

[batty]

Why does it need to be below 10 ohms its still going to be TT anyway even if it is low enough for tripping a circuit breaker on that day what happens in a hot summer when it goes over that reading and they havn't got any RCD protection?

 

[Marvo]

Maybe the sub10 ohm earth impedance is required for functional reasons rather than protective.

 

[Evans Electric]

As Batty says , where did you get the criteria of 10 ohms from ?   And the figure of 3.6mtr .   Nobody knows how long a rod needs to be in my opinion , its all down to the type of ground etc .   Trial and error basically.

Is the installation TT ?

 

[m4tty]

Isnt he talking about lightening conductors

 

[Doc Hudson]

Isnt he talking about lightening conductors

+1, although the original questions does say "lighting" I would think it is referring to Lightening conductors. There is no BS7671 standard test on "light" circuits that would involve earth rods. That would come under the supply characteristics for all circuits, where sockets would be far more of an issue than lights. However Lightening conductors is a very different beast. In which case I do hope Electroglows insurance is sufficient as this is outside of the scope of BS7671 and from previous post as well as this one, this job sounds like it could be a case of the incompetent leading the blind.

Doc H.

 

[NozSpark]

If this is indeed regarding lightening conductors then I'd leave it well alone.. the only connection from the lightening conductors to the electrical earth should be a bonding conductor

 

[Evans Electric]

Ooops  I assumed loop impedance test on some "lighting " was showing a high reading .

The stuff that flashes down from the sky needs to done by a specialist IMHO

 

[batty]

I still read it as testing a lighting circuit, A circuit breakers maximum loop is around 7 ohms and most other protective devices are around this. I read it somebody probably a 5WW tester has failed an installation and suggested putting a longer earth rod in. I would think  a lightning electrode would need to be miles under 10 ohms and wouldn't even be mentioned when talking about lightning protection and surely as this is specialist work no company would ask a spark to put something right. I could be wrong generally I am.

3.6 mtr will be three earth rods screwed together I certainly wouldn't want to be knocking them into the ground by hand.

Although possibly is for lightning system that a customer wants putting right on the cheap.

 

[SPECIAL LOCATION]

Whether its LIGHTENING or LIGHTING...

Where does the 10ohms come from???????

Its not a Zs,  max or rule of thumb for a common lighting circuit....

As for Ra..... 

would be nice to have 10ohms...

But not compulsory...

I would be interested to see what the actual readings are that have "failed" the installtion...

can you scan a copy of the report for us to look at..?

opcorn

probably a 5WW tester has failed an installation and

another 5WW trying to fix an unknown problem.....

How can you offer to charge to do any work that you have to ask on the internet what you should be doing!

:shakehead

 

[Marvo]

The sub 10 ohm is likely a functional requirement. There's many systems that require a low Ra or Ze, could be for effective operation of surge arrestors or for an earth referenced comms system to name just two.

 

[Steve3948]

Marvo may have a point there, many years ago while working on a mod base, one of buildings where they fixed all radio equiment for the planes had work done, and part of the job was putting additional rods and mats to get a very low reading, the reading I cannot remember too long ago but at the time we had no idea why it had to be so low, it was all very hush hush.

 

[steptoe]

I done a laser cutter once that required a Ze <0.05 ohms ,

TNCS supply and DNO had to come out and sort it.