Clamp-on Ground resistance Testers 1

[Deansharafi]

Dear all,
Anyone has any experience with clamp-on ground resistance testers?
I am looking into replacing old ground resistance testers which operate on the basis of "Fall of Potential" method with clamp-on testers, which are quicker and more convenient. I appreciate if you could share your experience with me. Thanks.

 

[digitrex]

We have 2 units and it works. These are our experiences:

1. It only works correctly on an earth electrode which is interconnected with many other earth electrodes.

2. If multiple earth cables are clamped onto the earth electrode, you need to pass all these earth cables through the clamp jaws.

3. The jaw diameter is small (only 25mm for my testers). If the multiple earth cables are big eg 70sq.mm. each and stiff, it is impossible to clamp the jaw onto all the cables.

4. If you need to disconnect the multiple earth cables from the earth electrode and then link a temporary jumper cable to it for measurement, you need to reconnect back later on. You have to re-check the contact resistance between the earth cable and earth electrode. Normally old electrodes are dirty or corroded on the surface and you need a bit of patience to clean the contact surface.

 

[digitrex]

5. Further to Point 3 above. When the earth electrode is big (50mm galvanized pipes for ours), we always need to do Point No. 4 above.

 

[busbar]

 
When the red head clamp-on ground-resistance testers first hit the streets, one was sent out to my facility for evaluation as an “effortless upsell” where repetitive fall-of-potential testing on institutional-building ground rings for lightning protection was almost a religion.

We got a big laugh opening the box containing the tester, for we immediately realized the uselessness of the instrument for our system.

The measurement head had a signal-injection core stacked over a sensing core so it needed around 1½ inches of completely straight 1-inch OD-max (ground) conductor with full jaw closing and ±12-inch perpendicular clearance positioned for adequate display visibility.

In our case we had almost exclusively 4/0AWG 7-strand conductor looped through heavy copper-alloy u-bolt clamps to ¾-inch driven rods at the bottom of concrete valve boxes.

Even if the clamp tester could have been used, the four-lead (Kelvin) set was still needed for splice-continuity tests within facilities.

 

[alehman]

Assuming the conductor will fit in the device, I think it has it's uses. It must be applied with knowledge of it's limitations with regard to digitrex's point 1.

For results to be meaningful, you must understand exactly what it is you are measuring. The instructions that come with these devices typically allude to using the "multiple grounds" of the utility system as the distant electrode. In this case, obviously it is important that the utility system is sound and there is one and only one direct connection from the utiltiy grounds to the electrode under test. Also, the utility electrodes should be relatively distant from the electrode under test.

 

[digitrex]

The clamp-on earth resistance tester is quite useful to us eventhough we hope for further improvements in design. There are more other experiences I can tell:

1. All the ground surface in our plant are concrete paved (150mm thick) there is no open ground for driving the test probes of the conventional tester. The first commissioning test results of the few hundred earth electrodes in the plant were taken before the concrete paving were completed. Subsequent measurement of earth resistances were almost imposssible until we get this clamp-on testers.

2. The clamp-on is bulky and big (it is about 200mm long). If the original earth pit enclosure were not designed to suit this dimensions, you cannot bring the testers into the pit and clamp onto the vertical earth electrode, evenif your electrode has a small diameter. The easy way out is to clamp onto the flexible part(if the earth cable is flexible enough) of the earth cable.