Fluke 1662. Earth spike test

[wriggs]

Hi all. First post.
Earth rod question. All work I've done, as a recently qualified DEI has been on tncs. Have a new job with TT. Old earth rod is buried under concrete steps. So I have installed additional visible 2 m rod and run new 10mm earthing conductor. I have a fluke 1662 and recently bought a earth spike kit. 1662 doesn't have Re option. Any ideas on how to check rod resistance with kit I've got. Or can it not be done?
TiA.

 

[LewisM]

You can just obtain the reading by doing a ZE

 

[loz2754]

Follow the procedure in the IET On-Site Guide page 112 for the loop impedance method.

 

[pc1966]

By far the easiest option is to measure Ze to your new rod (basically Zs test but with the rod alone as the earth point) as @loz2754 describes. You should take care as the rod will get energised during the test (probably to 50V-ish) so make sure people and animals can't get in contact with it. Normally the plastic cover-box is enough for that protection.

The Ze test though is not always that accurate if it is close to an existing rod or metal structure that is part of the installation you are testing. Now in reality that is probably not something to worry about as:

• Probably they are not too close (unless you hammered in a rod adjacent to an existing one)
• The buried metalwork ought to be bonded as "extraneous conductive parts" anyway, so it enhances the system earth during operations.
• The usual requirement for Ra to be < 200 ohms and < 50V/In for the RCD gives you a bit of wriggle room should something really go wrong.

You are right to look in to properly testing the rod Ra value on its own, as a system Zs might look very low today but then someone replaces old lead water pipes with less toxic plastic and suddenly that means of earthing has gone.

(Which is why the have not allowed them to be used for donkey's years, unless they are under specific control of the property, etc)