Can you really test electrical bonds adequately using a multimeter?


An active electrical system may have present many small currents and voltages, both dc and ac, as a result of leakages, unbalances, etc.

An ordinary digital multimeter uses dc, at very low voltage and current levels, in order to perform resistance tests. That is done in order to maximize battery life.

Unfortunately, any residual dc in a system may cause a digital multimeter resistance reading to be higher or lower than its actual value. And relatively small amounts of ac can render the readings unstable.

While there are techniques that may minimize these effects, adding to the difficultly is the fact that the resistances we're looking for are often likely to be less than one Ohm, and you want a solid stable reading of that value.

So, how do you test bonds? Well, you use a tester designed for the job.

That means using a meter that uses higher test currents for dc tests, and/or ac tests, again with higher current. In the case of ac testing, it's important that the test frequency be such that it won't produce a ‘beat' frequency with the line voltage frequency or 50 or 60 Hz. A test frequency of 128 Hz is often used, and more capable testers also offer lower test frequencies, some under 100 Hz. For entry-level meters in this category, consider the Fluke 1621 Basic Earth Ground Tester. For full capacity, consider the Fluke 1625 Geo Earth Ground Testers.

Are there any bonding tests that can be performed with a digital multimeter?

The answer is yes. For example, you can determine if the ground pin in a duplex receptacle has been incorrectly bonded to neutral at that point. You will need the digital multimeter, of course, and a high wattage load such as a hair blow dryer. The procedure is as follows:

With nothing plugged into either outlet, measure the ac voltage between the neutral (wide-slot) and the ground (D-shaped socket.) You should read 0.0 or maybe a few millivolts. Now, plug the hair dryer into the other socket and turn it on high, placing a 1kW load on the circuit. You should read an increase in the millivolt reading, because of the voltage drop due to the current flowing in the neutral. Since there should be a bond of the neutral to ground at the service entrance (and there only), and since no current should be flowing in the ground lead to the receptacle, you should read the voltage drop in the neutral lead using the ground lead as an extension of your test lead.

If you see no change, then it's likely that an improper bond has been placed in the receptacle box, or one near it, on the same circuit.

Such tests require careful thought to ensure that you're really testing what you think you are.

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