Electricians use multimeters for many everyday tasks and would certainly not want to be without one. But there is another instrument that is finding increased application throughout the world, and it is called the loop impedance meter. David Herres explains.

Loop impedance meters are of great importance in European countries, where IEC standards mandate that all new electrical installations be tested with this instrument prior to utility connection, and in Australia and New Zealand where Standards Australia has published similar requirements.

To properly conduct the test, the loop impedance meter is connected to an upstream test point such as a receptacle, and it purposely introduces a simulated fault. The pulse is sufficiently brief so that the existing overcurrent device (fuse or breaker) will not trip out, and the impedance of this fault loop is measured.

To understand what is going on, we have to look at two definitions:
•    Impedance is similar to resistance. It is a measurement of the opposition to the flow of current, and like resistance it is measured in ohms. But unlike straight resistance it also includes capacitive and inductive reactance, which are frequency dependent. At the ordinary international power supply frequencies of 50 or 60 Hz, aside from long utility transmission lines, we can disregard the reactance contribution. For ordinary electrical work, resistance and impedance are the same. However, impedance is a little bit more sophisticated term because it shows you are touching all the bases.
•    Loop is similar to circuit. The difference is that a circuit is an intentional electrical path that is designed and constructed to serve a definite purpose, such as the branch circuit from an entrance panel, where electrons flow along the hot conductor, through a switch and light bulb filament and back to the neutral bar in the box. A loop is an electrical path where the fixture has become faulty, so that the current shunts to the normally non-current-carrying metal fixture housing. If it is properly grounded by the third equipment grounding conductor, the current will flow via that route to the neutral bar, and the flow will be sufficient to trip the overcurrent device assuming the ground loop is intact and of low impedance.
That’s what the loop impedance meter tests. Besides enabling the electrician to comply with requirements for utility connection, the loop impedance meter is an awesome troubleshooting tool. But the problem some manufacturers have had is that their loop impedance meters are not designed to work on other than individual power systems due to differing voltages and frequencies. Moreover, while many countries throughout the world have required pre-connection loop impedance tests, the US National Electrical Code has not jumped onboard, nor have individual jurisdictions. Consequently, some manufacturers have not offered a 120V, 60Hz model.

An excellent full-featured line that deals with this problem is available. The Megger LTW315, 25 and 35 two-wire non-tripping loop testers are truly universal since they operate at 50 to 440 volts, multiple frequencies. The LTW335 performs a broad range of measurements and displays a profile of branch-circuit parameters including harmful loop impedances. Additionally, it has a USB port and with included software it can download results to a computer so that a printed certification can be generated.

A typical procedure is to start the test at the farthest downstream receptacle. Take successive reading at each outlet working back toward the entrance panel until any bad reading disappears. The cause of the fault may be a loose equipment grounding conductor termination, or a bad spot in the wire caused by a chewing rodent or a tradesman’s drywall screw. Once you know where it is, you can fix it.

A less pricey but quite competent instrument is the Ideal Suretest 61 165 circuit analyzer. It sells for about $350 and performs these tests:
•    Arc-fault testing.
•    Tests for shared neutral.
•    Measures voltage drops with 12-amp, 15-amp and 20-amp loads.
•    Measures root mean square voltage.
•    Measures line voltage.
•    Measures peak voltage.
•    Measures frequency.
•    Measures ground to neutral voltage.
•    Measures ground impedance.
•    Measures hot and neutral conductor impedances.
•    Verifies proper wiring in three-wire receptacles.
•    Identifies false (bootleg) grounds.
•    Tests GFCI’s for proper operation.
•    Conducts testing while circuit is live without disturbing sensitive loads.
•    Verifies isolated grounds (with an adapter).
To summarise, the loop impedance meter is a valuable instrument in the electrician’s toolbox. You can check a finished installation to ensure that latent faults won’t surface the day after you leave the job, or far in the future with potentially catastrophic consequences.