Testing Lightning Conductors and Earth Electrodes.

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....

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....