For years, ohmmeters have been integrated into the electrician’s multimeter. This is convenient, because ohms and volt are the most-performed measurements. Also, only a single instrument must be purchased and maintained. Multimeters are further subdivided into ac, bench-type multimeters and hand-held, battery-powered multimeters.
The hand-held instruments are more convenient. Most often just grab your handheld meter, press the on button and take your measurement. The bench multimeter has many more features, but in recent years the hand-held has rapidly caught up. Now it can do capacitor checks, three-phase balance and motor quality measurements, not to mention interfacing with mobile devices, Windows, MAC OS and other operating Systems.
Most ac-powered bench-type multimeters are grounded at the building’s electrical service. This is a great safety features, but for certain multimeters, another hazard is introduced: When a ground-return lead is touched to a wire or terminal on a live device under test, that wire or terminal will undergo an intense fault current. It happens even if the measuring instrument is off. If the user is fortunate, the ground return lead will serve as a fuse, interrupting the circuit before much damage is done. Otherwise, the fault current could injure the user and damage the device under test, as well as the test instrument.
Some partially aware technicians have figured out that this problem can be quickly eliminated by cutting off the ground plug. This stratagem, of course, defeats the potentially life-saving purpose of the grounding conductor. A further hazard is in measuring the output of a switching power supply, even at the modest output of a ceiling fixture dimmer switch.
There are two legitimate solutions:
Use differential probes. For heavy wires, 4/0 and larger, the electrician’s Amprobe works quite well. Clamp it on an insulated, live conductor and it will provide a digital readout. Use on one conductor only. Opposite polarity conductors of the same circuit will cancel and display a zero reading. For very small wires,
oscilloscope manufacturers offer a small camp-on meter that plugs into a BNC probe port. For small readings, wrap two or more turns of wire inside the clamp, which is the primary winding of a transformer. Connect the ends into the primary circuit.
Connect an isolation transformer temporarily ahead of the test instrument. These instruments neither step-up nor step-down the through-voltage nor allow grounding to pass through. If they do, they are known as auto transformers.
There is one other approach: Bring out your hand-held, battery-powered multimeter. Besides being readily portable it has the advantage of being isolated from ground. Of course, the usual safety precautions apply. Check the specifications and operate it only within voltage and current limits. If working within a damp location or on a concrete floor (all concrete should be considered grounded), lay down a dry rubber mat or dry wooden pallets.
There are a wide range of hand-held, battery-powered multimers available, sometimes in big-box stores for less than $10. These models will quickly become non-functional, starting with the circuit tone continuity and progressing to the battery and thin plastic housing.
In contrast, the Fluke 87V True RMS meter is a good buy, selling through a network of dealers for around $450. It is an industrial digital full-featured meter with a highly accurate temperature probe. The True RMS means the meter calculates the energy present in all waveforms based on the energy they contain rather than some other averaging system. This guarantees accurate readings through a wide range of non-linear signals.
The 87V is by no means Fluke’s most expensive model. Some multimeters go for twice as much and look about the same. However, each of the more expensive models has additional features which you may never need.
Meters like those from Fluke are a far cry from the first resistance-measuring instruments. The first good ohmmeters–or ratiometers as they were called–used conducting ligaments to supply the restoring force. These primitive ratiometers consisted of two coils. One of them was connected via a resistor to the battery supply. The second coil was connected to the same battery and the resistor under investigation. The meter readout was proportional to the ratio of the currents through the two coils. This ratio was determined by the resistor under test. An advantage was that the voltage reading remained the same as long as the meter had a certain minimum charge. Accordingly, the user did not have to zero in the meter before each reading. And despite the non-linear scale, it remained correct throughout the entire deflection range.
By interchanging the two coils, a second range became available, the reverse of the first. Once the test leads were disconnected, the instrument would continue to exhibit its reading. Disconnecting the test leads also disconnected the battery from the instrument.
Ratiometers only measured ohms because they could not be incorporated into a multimeter. Insulation testers, which operated on a hand-cranked design, operated on the same principle. The readout was independent of the produced voltage, and these were the venerable first Meggers.
Later, improved ohmmeter designs relied on a small battery to apply voltage to a resistance. They measured current through the resistance by means of a galvanometer. In this new design, battery, galvanometer and resistance all connected in series. The galvanometer scale was marked in ohms because, due to the fixed voltage from the battery, as the resistance rose, the current through the motor and deflection at the readout would drop.
This design was the most common form of analog ohmmeter because it was far simpler than previous models. It was also easier to integrate these mechanisms into a multimeter. The design was cheaper to manufacture than its predecessors. Despite some calibration problems and the need to zero the meter with each new use, the meter reigned supreme for years.