Testing Lightning Conductors and Earth Electrodes.

Hmmm interesting. Don`t know. I would not try the fly a kite method though

I have previously been responsible for organising the inspections of the lightning conductor at our local church of a similar vintage and also with a tall tower though due to work commitments I have never managed to witness the work. From what I understand they put one reference electrode in the earth near, but separated from, the lightning conductor electrode. As the lightning conductor is electrically separate from other conductors the downleads do not affect the measurement so it does not matter if you disconnect them or not as they do not give an alternative electrical path.. The only advantage to having a disconnection point is that it makes it easier to replace the downlead or (as is more likely to be the case) the earth electrode.

If you have any queries I would suggest talking to the person carrying out the testing. It is quite specialised and uses specific testing equipment so the person testing should be able to explain things - and if they can't then perhaps they are not the right person to do the job.

Alasdair

When testing well spaced electrodes in parallel, without opening the loops, what you can do is use a thing that looks like a clamp meter,  and has a similar  current transformer inside, but it injects a known signal and looks at the voltage to current ratio to deduce the impedance, so if you clamp it in a shorted loop of thick cable  it reads near zero, but if you had a loop with a resistor in it would measure that - and so when you clip it round the lead to one electrode, it pushes current into the ground, and sucks it back up all the other earthed items connected to the other side to the wire t is clipped to. 

This sort of test needs a bit of thought, for example metering between 2 electrodes close together it may under-read, but it is usually good enough for 'has it rotted off at the roots' testing, which s normally the main concern, not accuracy to the nearest half a percent.

Also occasionally used for verifying the legs of antenna masts have not rusted out....

The test requirement is to ascertain the overall ohmic resistance of the system - ie less than 10 Ohms. You can test each electrode individually (and you need test clamps for that purpose) and calculate the overall resistance  - or you can test the overall system resistance (but you need to be clear that any bonding to the MET has been isolated (and any other fortuitous connections to say roof air handling plant, cold water tank up or down feeds, lift rails etc)


Generally, you would expect to see simple test clamps on a system that has no further below ground interconnections between simple driven electrodes (perimeter ring tape for example - or connection to the facility foundation slab)


In most cases, it would be usual to test on a fall of potential method or Wenner method- some systems may have a deliberate remote electrode position (up to 90m away) from interconnected systems to assist with this (usually several electrodes to allow a near, middle and far condition on a 4 wire instrument)


Loop (or clamp) testing may tell you something, but not necessarily what you want to know


Regards


OMS

Am I right in thinking the clamp test will only work on multiple interconnected electrodes, so for a single earth electrode you need to use long test leads to temporary stakes?


 Andy

It sort of depends what you need to know -there has to be another electrode somewhere, but ideally far enough away for distance not to alter the reading too much- If for example you have already a TNx supply earth you can push against then you can compare your local earthing with that - not for lightning perhaps, but OK for verifying radio earths and so on.

Sparkingchip:

Am I right in thinking the clamp test will only work on multiple interconnected electrodes, so for a single earth electrode you need to use long test leads to temporary stakes?


 Andy 

The test clamp only isolates the electrode under test - generally where at the end of every down conductor, there is an electrode - and a test clamp to just separate the connection (above ground rather than scrabbling about in the electrode pit).


Once you have the electrode isolated, then typically a fall of potential or wenner test will give you a value if you have a suitable reference electrode (and usually you also need a "moveable" electrode for the so called 4 wire test) 


Regards


OMS

Thanks all. The church has been quoted about £65 for the testing, but only if four churches are tested at one time on one day and are close together, which ain't difficult in Norfolk, as the churches are spaced at about two mile intervals. The cost seems very low to me.


Z.