Earth electrode testing

[Traineeboy]

My boss was testing an earth electrode he installed. He just took the ZE reading like you would do an a TN-S or TN-C-S. i through to test a TT system you had to have an earth electrode tester etc. 
 

is that right that to test an earth electrode you just need to disconnect from installation and clip to the main earth conductor and live conductor ? I though there was more to it when testing TT. 

 

[SPECIAL LOCATION]

What you are thinking of is the test shown on Pg 113 of the current OSG..  "Earth Electrode Test"....

i.e. establishing the resistance to earth of the rod itself  (Ra).

What you boss was doing, as you quite rightly state, was an External earth loop test..

i.e. The local rod and the suppliers earth..

Which by definition the Ze must be greater than the Ra as Ze on a TT includes your Ra + Suppliers earth electrode..

(Suppliers earth is stated as max 21ohms) 

So providing the Ze is value is acceptable, (lower than 200 and nearer to 21 as possible)..

There isn't much need to measure Ra itself.

See note on bottom of page 11 and top of page 12 OSG..

These are all based on the 411.5 section of the big book!

Basically in the real world time is money...  

doing an Ra takes longer than a quick Ze test!

If there were concerns about the ground conditions then further investigation by doing proper Ra test may be beneficial..

Or may be just bang another (or deeper) rod in!!

Guinness

 

[Traineeboy]

So basically after a rod has been installed as long as it is not less that 21 ohms and not greater then 200 ohms the just doing a Ze is fine. 
 

if you didn’t achieve these readings then another rod should be banged into the ground. 
 

Thanks 

 

[Andy™]

it can be less than 21 and greater than 200... lower than 21 not an issue. greater than 200 and it could be unstable, although in some areas you could have a stable reading thats over 200. at least around here, greater than 100 is rare, even from a single 1mtr rod

 

[SPECIAL LOCATION]

This is one of those typical situations where you need to apply more than a 5WW training concept of X=good and Y=bad....

When looking at the results of a Ze or Ra test on a TT installation,

a bit of understanding of the ground conditions your rod(s) is/are sunk into needs to be considered..

Some things BS7671 related do NOT fit into easy tick-box, or idiot guide flow charts...

Earth rods is one of them IMHO..

you need to be able to apply your full understanding of the reg's and what safety considerations you are aiming to achieve..

and the practical limitations of the geography of your installations.. 

In certain situations on three different site locations 56ohm,  134ohm,  205ohm, may all be perfectly reasonable and good Ze values..

that are all impossible to achieve at the other sites, or different time of year..

due to the soil/ground types..

Single, Extended or Multiple rods / Sandy, Clay, Rocky ground etc..  

tested during a really dry or damp period of weather can have significant effects of what you can or can't achieve for your Ra & thus Ze...

So it you don't take full account of the conditions of your earth arrangements when assessing if your test readings are acceptable or not.. 

you could end up tying yourself in knots trying to achieve the impossible..

I know some places round here that are very sandy and you can push a 1m rod in by hand , others where after 4inches you need to use a mallet or hammer drill to bang a rod in..

If you want a bit of bedtime reading...

https://www.weschler.com/wp-content/uploads/2020/01/gettingdowntoearth.pdf

When you have read that....

Come back and we will test you...

correction...

come back and you can test us!!!   

opcorn

Guinness

:coat  

 

[Sidney]

Carrying out a Ze test on an electrode is an acceptable method as listed in guidance note 3.

There may be circumstances where you're unable to carry out an Ra test, such as street cabinets surrounded by tarmac and people wandering around. 

Due to the amount of TT systems in my area, not many, it's not financially viable for me to buy a meter capable of testing electrodes when a Ze one using my MFT will suffice.

 

[Traineeboy]

Great thanks guys I will go and do some reading.