Meters and measurement

An electrician's meter is a safety tool first and a measuring tool second. The same multimeter that tells you a circuit carries 118 V is the tool that proves a circuit is dead before you touch it, and a meter used wrongly, set to the wrong function, connected the wrong way or rated for the wrong environment, can explode in your hands. This chapter covers the instruments you will use every day, how to connect them, how to read their accuracy, how meters can fool you, and the measurement category ratings that decide where a meter can be used safely.

The digital multimeter

A digital multimeter (DMM) combines a voltmeter, an ammeter and an ohmmeter, and usually adds continuity, diode, capacitance and frequency functions. Most electricians' meters are "true-RMS", meaning they read the effective value of AC correctly even when the waveform is distorted, as it often is with electronic loads.

A DMM has a rotary function switch and usually three or four jacks: COM (common, the black lead), V/Ω (red lead for volts, ohms and continuity), and A or mA (red lead for current). The single most common way to destroy a meter, or start an arc flash, is to leave the red lead in the A jack and then touch the probes across a voltage.

Measuring voltage

A voltmeter is connected in parallel with (across) the component or between the two points whose potential difference you want. It has a very high internal resistance, typically 10 MΩ for a DMM, so it draws almost no current and barely disturbs the circuit.

On AC circuits, typical readings in North America are about 120 V line to neutral and 240 V line to line in a home, 120 V and 208 V in a 120/208 V building, and higher on commercial systems. Readings within roughly 5% of nominal are normal; much lower readings under load suggest loose connections, undersized conductors or an overloaded supply (Lesson 11).

Measuring current

An ammeter must be connected in series so that the current passes through it, which means opening the circuit. Its internal resistance is very low, a small fraction of an ohm, so it does not reduce the current it measures.

Clamp meters

Opening a circuit to measure current is slow and often unsafe, so electricians normally use a clamp meter (clamp-on ammeter). Its jaws surround one conductor and sense the magnetic field produced by the current, without any electrical contact. Clamp meters measure AC easily; models with a Hall-effect sensor also measure DC.

Measuring resistance and continuity

An ohmmeter passes a small current from its own battery through the component and measures the resulting voltage. Because it supplies its own current, it must only be used on de-energized, isolated components. Measuring resistance on a live circuit gives wrong readings and can damage the meter.

A continuity test is a quick resistance check that beeps when the resistance is low (a few ohms or less), used to check fuses, switches, conductors and connections. Always disconnect at least one end of the component, or the meter may measure a parallel path through other parts of the circuit.

Insulation resistance testing

An ordinary ohmmeter uses only a few volts, too little to reveal weak insulation. An insulation resistance tester (often called a megger, after a well-known brand) applies a DC test voltage such as 500 V or 1000 V between conductors, or between a conductor and ground, and reads resistance in megohms. Good insulation reads hundreds or thousands of megohms; a low reading means damaged, wet or contaminated insulation. Before testing, the circuit must be de-energized, and loads and electronic equipment that the test voltage could damage must be disconnected.

Accuracy and resolution

No meter is perfect. Manufacturers state accuracy as a percentage of the reading plus a number of counts (digits) in the last place shown.

Resolution is the smallest change the display can show (0.1 V on a 600.0 V range); accuracy is how close the reading is to the truth. A display showing many digits is not necessarily accurate.

How meters can fool you

Meter loading

A 10 MΩ voltmeter barely affects most power circuits, but in high-resistance electronic circuits it can change the very voltage it measures.

Ghost voltage

A high-impedance meter can show a voltage, sometimes tens of volts, on a conductor that is disconnected from the supply but runs alongside energized conductors. Capacitive coupling between the wires produces the reading, but it cannot deliver real current. Many electricians' meters have a low-impedance (LoZ) mode that loads the circuit enough to make ghost voltages collapse while a real supply still reads correctly. Never assume a reading is a ghost voltage; prove it.

Non-contact voltage testers

Pen-style non-contact voltage testers light up near energized AC conductors. They are convenient for a first check but can miss voltage (shielded cable, DC, a dead battery, a conductor deep in a box) and can light up on ghost voltages. They are never sufficient on their own to prove a circuit is de-energized.

Measurement categories (CAT ratings)

When a meter is connected to a circuit, a transient voltage spike, from lightning, switching or a fault, can reach the meter. The closer you are to the utility supply, the more energy such a transient can carry. The international meter safety standard IEC 61010-1 (adopted in Canada as CSA C22.2 No. 61010-1 and in the United States as UL 61010-1) defines measurement categories:

• CAT II: circuits connected to receptacles: appliances, portable tools.
• CAT III: building distribution: panels, feeders, branch circuit wiring, fixed equipment such as motors and lighting.
• CAT IV: the origin of the installation: the utility service, meter base, service entrance conductors and outdoor overhead or underground lines.

A meter is rated by category and voltage, for example "CAT III 1000 V, CAT IV 600 V". Use a meter only where both ratings meet or exceed the circuit. Check that the test leads carry matching ratings too, and look for the mark of an accredited testing laboratory; inexpensive meters with unverifiable ratings have failed violently in real arc flash incidents.

Proving a circuit dead: live–dead–live

Before working on any circuit, you must establish an electrically safe work condition: identify the sources, open the disconnects, lock and tag them out, and then test to prove there is no voltage. CSA Z462, Workplace Electrical Safety, and provincial occupational health and safety regulations set out the requirements. The test itself follows the live–dead–live method:

• Live: test your meter on a known energized source of similar voltage to prove it works.
• Dead: test the circuit you will work on, phase to phase and each phase to ground (and to neutral), to prove it is de-energized.
• Live: test the meter again on the known source to prove it did not fail during the test.

Use a meter rated for the circuit, wear the PPE your employer's procedures require until the absence of voltage is proven, and treat the circuit as live until all three steps pass. Course ELEC-420 and the safety stream course HSE-224 go further into energized work and arc flash.