The Effects of Test Lead Length on Measurement Accuracy

When circumstances require longer test leads, can you rely on the results?

Does extra test lead length introduce a variance in the results of your test measurements? Laboratory or field tests on electrical equipment often require test leads to be as much as three or four times longer than the standard length. For example, long test leads may be necessary for adding extra distance between field personnel and the high-voltage piece of equipment they may be testing, thereby increasing safety. In another example, monitoring a variety of electrical points on a large piece of equipment may require several test leads, a few of which may be longer than usual.


The extra length is useful in these situations, but can it alter the results of a test? Because much of the testing it conducts in the laboratory and in the field relates to power quality, EPRI's compliance specialists designed an experiment to determine the effects of test lead length when measuring voltage sags, voltage swells, and steady-state voltages.

Setting up the experiment.

The experimental setup comprised three basic elements, as shown in the Figure above. For simplicity, only one test meter and two pair of test leads are shown.

  • Four pairs of test leads, each of a different length.

  • One electrical disturbance generator.

  • Two voltmeters.

An electrical disturbance generator is a programmable unit capable of producing steady-state voltage and voltage sag and swells. In this experiment, a second meter was necessary for repeat testing to confirm the accuracy of the first. If the first meter registered an unexpected measurement, the other meter could confirm or deny the reading.

The Table shows the lengths of the four different sets of test leads used in the experiment. All test leads were made with 17 AWG stranded wire and included banana plugs on each end.

The experiment.

The test plan consisted of first performing baseline tests for the three voltage conditions: steady state, voltage sag, and voltage swell. The tests were conducted with 14-in. long test leads because the effect of lead length (as measured in voltage loss across the lead) should be negligible with such short leads.

Then tests were performed for the same conditions with the three longer lead lengths. The voltage readings for these tests were noted and compared to the voltage readings from the baseline readings to note the differences.

Five readings were taken with the first meter for each combination of voltage condition and length of test lead to ensure good lead contact at the point of measurement and a consistently working meter. The tests were then repeated with the second meter.

Results.

Little or no difference was noted between measurements of the same voltage condition using either meter. In fact, all five measurements at each of the three voltage conditions were in perfect agreement. In other words, for test lead lengths up to 693 in., the length of the test leads had no effect on the accuracy of the measurement. So don't hesitate to use that extra-long test lead set to take measurements from a safe distance or when unusual test setups require it. They'll be right on the mark.

Hubbard is a PQ technician at EPRI PEAC Corp., Knoxville, Tenn.