CK Tools :) The professionals choice when it comes to Electrical Tools
This official sponsor may provide discounts for members

Discuss Multiple Earth Electrodes Test Results in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

Strima

-
Arms
Esteemed
Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:  American Electrical Advice Forum

I need to clarify something that's rattling around in my head.

I was taught when testing lightning protection systems that the maximum permissible combined electrode readings shouldn't be over 10 ohms. So we would test all the electrodes individually then work out resistance in parallel.

We now have to test street cabinets and part of the DNO requirement is that supplementary electrodes have to be installed and the resistance must be below a certain amount dependant upon the cabinet loading if TNC-S earthing is used. Normally if the reading is over the maximum permitted we would convert to TT as per our customers policy, easy enough.

However we are now having to install additional electrodes, normally using conductive concrete as opposed to rods, to try and get the resistance down to maintain the TNC-S earthing arrangement. This removes the need for RCD protection on telecoms equipment and takes away the risk of nuisance tripping.

Here's where I'm getting confused. If I tested a site with two electrodes and got 48 and 52 ohms on each and my maximum permitted is 47 then the site would be classed as a fail as the lowest results is still above the maximum. This is using test method 3 from GN3, EFLI test, the tester would only show the path of least resistance.

However if I work out resistance in parallel my result is a pass: 1/48 plus 1/52 = 1/0.03 = 33.33 ohms.

So which is the correct result?
 
SuperlecDirect - ElectriciansForums.net Electrical Suppliers
This official sponsor may provide discounts for members
When installing and testing electrical earthing I've always just connected all the rods together with the tape and tested the lot together using the fall of potential method. That's how I was shown anyway.

The only time I have used the calculation is for an overall reading on a lightning protection system as you said but I don't see why the result would change in this instance...

Once connected together they are essentially one rod in terms of the overall resistance? That's how I've always understood it anyway
 
D

Deleted member 26818

The whole point of adding rods, increasing the length of a rod, using a mat etc, is to reduce the resistance.
To then turn around and say you haven’t achieved that because you shouldn’t combine the resistances is silly.
 

Wilko

-
Arms
Esteemed
Hi - I would test with both rods connected. If they are both about 50 Ohms I'm pretty sure you'll be well below 47. But taking a result for each rod independently and using parallel resistance is not always correct as the "sphere of influence" of the rods may not be independent. So two 50s may only get you 30 or some such. Just my thoughts.
 

Strima

-
Arms
Esteemed
  • Thread Starter Thread Starter
  • #6
Just a quick math check - 48 Ohms in parallel with 52 Ohms is 24.96 Ohms.
Well done for spotting my deliberate mistake... :cool::D
 
Did you drink alot of tea before starting your test. This can prove beneficial in said situation if not at least amusing for yourself.

** And technically it would be Part Pee.

Cheers
 

Strima

-
Arms
Esteemed
  • Thread Starter Thread Starter
  • #8
Did you drink alot of tea before starting your test. This can prove beneficial in said situation if not at least amusing for yourself.

** And technically it would be Part Pee.

Cheers
I find coffee gives better results...
 

Strima

-
Arms
Esteemed
  • Thread Starter Thread Starter
  • #9
Using method 3 from GN3 may not give us an accurate result.

We can't use method 1 due to most footways being covered in tarmac and I've only got a couple of clamp meters for method 2, funnily enough all the lads have MFTs so method 3 it is..
 

davesparks

-
Mentor
Arms
Esteemed
Using method 3 from GN3 may not give us an accurate result.

We can't use method 1 due to most footways being covered in tarmac and I've only got a couple of clamp meters for method 2, funnily enough all the lads have MFTs so method 3 it is..
Is method 1 the three wire fall of potential method? This can be carried out on solid ground by using contact plates in place of test electrodes, one recommendation i have seen is a small pile of wet bare metal chain used as a test electrode.
 

Strima

-
Arms
Esteemed
  • Thread Starter Thread Starter
  • #11
Is method 1 the three wire fall of potential method? This can be carried out on solid ground by using contact plates in place of test electrodes, one recommendation i have seen is a small pile of wet bare metal chain used as a test electrode.
It is but trying to do this on a footway with pedestrians wandering around is a bit of a mare...
 
If it's a built up area I would normally get a long test lead and clip it to something electrical generally find AC units or lamp posts are easy enough to come by. Then just plug this lead into both the c&p sockets on your tester then connect other lead to the rod.

Failing that just ask in a shop if you can use a plug socket get a plug with a wire coming out from earth pin so you can connect long lead to it.
 
Scolmore Electrical Products
This official sponsor may provide discounts for members

Reply to Multiple Earth Electrodes Test Results in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

CK Tools :) The professionals choice when it comes to Electrical Tools
This official sponsor may provide discounts for members
Top Bottom