METERS and TESTERS
ELECTRICAL METERS and TESTERS
Modern, high-tech electrical measuring instruments do not always produce better, more accurate results than the low-tech instruments of yesteryear. What would be best depends on the application. The electrical measuring device one chooses to use must be correct for the situation and measurement needed. The wrong type of meter could give incorrect results.
RESISTANCE MEASUREMENT
An old fashion ohmmeter applies a voltage (generally 1.5 volts or less) to the resistance being measured. This pushes a relatively small current through the resistance. The principle employed is that the smaller the resistance, the larger the current that will flow through the resistance. The needle on the meter swings over in proportion to the magnitude of the resultant current through the resistance. Thus, the meter can show the value of the resistance directly by simply having the scale marked not to indicate current, but to indicate values of resistance.
The continuity checker found on many voltage testers carried by electricians often uses a small watch battery and an LED (Light Emitting Diode) to indicate continuity. The result is a tester that indicates continuity when there may only be a slight continuity. This could lead to an error. For example, once when checking to see which wire at one end of a long conduit corresponded to which at the other end, an LED type tester indicated continuity between all wires (due to leakage through the insulation of the wires). However, a simple light bulb and battery type continuity tester clearly showed which wire was which. An ohmmeter would have worked because it would have shown the difference between a slight leakage (a high resistance) and a direct connection (a low resistance).
Similar errors and difficulties have been encountered while checking for a blown fuse with an LED continuity tester. The test showed the fuse was still good when it was not. The light bulb and battery type continuity tester showed the fuse was bad.
THE MEGGER
There is an electrical resistance measuring device commonly known as a “megger.” It is essentially a high voltage ohmmeter. Originally, its sole purpose was to test insulation at high voltages in order to detect insulation breakdowns and failures. Good insulation resistance values should measure in the hundreds of meg-ohms range (hundreds of millions of ohms). Early meggers were of the hand crank generator type. Electronic models are most common now, and are able to measure resistance at selectable voltages ranging up to 5,000 volts or even higher.
Resistance measurements at elevated voltages can give quite different results than would be found using a low voltage ohmmeter. At higher voltages, insulation can show evidence of breakdown and even the dust or dirt on an insulator may conduct excessively. Detection of insulation breakdown or deterioration in large motors or other equipment can warn of impending failure.
When measuring the resistance to earth of a grounding rod using a low voltage ohmmeter, the readings will fluctuate all about due to the various low voltage battery type actions that occur within the soil. These effects become insignificant as the voltage used to measure the resistance is increased.
VOLTAGE MEASUREMENT
When testing for power, beware of the cheap little neon light type testers. They place very little load on the circuit. The chirping little voltage sensors that sound when near a live wire only tell that one might get bit if one were to get too close. It does not provide a valid test for power. It can sound when near a dead wire that only runs sufficiently near other wires that are live.
The magnetic solenoid type of voltage tester, having a plunger that is pulled down to indicate the magnitude of the voltage, is most dependable for electricians. A voltmeter may be used to measure voltage at a load, to see if power arrives to the load, but voltmeters have very high input impedance and draw virtually no current when connected. As a result, even a slight leakage of current through a bad connection could read as full voltage. Nonetheless, the lines may not be capable of supplying more than only that slight leakage current through the bad connection. This could even be a blown fuse that still conducts, though only very slightly.
Magnetic solenoid type voltage testers place a significant load on the supply lines (Typically 1 or 2 tenths of an amp at 120 volts AC). Thus at least, a limited capacity to deliver current to the load must exist for full voltage to be measured. This is not something one would want to use on sensitive electronic circuits, but it is exactly what is needed when an electrician is testing power circuits. I have seen more than one instance of a blown fuse passing enough current to cause a voltmeter to read full voltage. However, under load it was incapable of delivering power. The old fashion voltage tester said the line was dead and the fuse was blown. The fancy voltmeter said “no problem here!”
I have since used some fairly sophisticated testers, both digital and analog, made for electricians. I like them. I have been using a BEHA 600 since the early 80’s. I’ve learned its limitations. I carry other testers in my truck. And I don’t get fooled easily.
The bottom line is this: One must learn and understand the properties and limitations of the testing equipment one chooses to use.
Joe Duncanson
Copyright © 1987,2009 by Joe Duncanson. All Rights Reserved.