How to use a multimeter safely and correctly

Level: Basic 12 min read 38 questions

The multimeter is the tool you use most in a control panel, and also the one that causes the most accidents when it is used wrongly. Measuring well is not just reading a number: it means choosing the right function, putting the probes in the right place, understanding what the reading tells you and, above all, not turning the meter into a short circuit. A technician who measures with a method finds a fault in minutes; one who measures blindly can spend the afternoon replacing parts that were fine.

What a multimeter measures

Function	Usual symbol	What it is used for in a panel
AC voltage	V~ or VAC	Mains supply (230/400 V, 120/240 V, 480 V), transformer outputs
DC voltage	V⎓ or VDC	24 V control circuits, PLC inputs and outputs, 0–10 V signals
Resistance	Ω	Coils, temperature sensors, motor windings
Continuity	Speaker symbol	Wires, fuses, push button and relay contacts
Current	A or mA	Small loads and 4–20 mA signals
Diode	Diode symbol	Diodes and bridge rectifiers

Many multimeters are auto-ranging: you choose the quantity and the meter finds the range. If the display shows OL (overload), the value is out of range: on resistance or continuity it means "open circuit".

For large currents you use a clamp meter, which measures without opening the circuit. Many clamp meters also include the functions of a multimeter.

Before you measure: safety

A control panel has more than enough energy to do serious harm. A short circuit through the meter can create an arc flash with severe burns, even if nobody touches a wire. So, first of all:

Measurement category (CAT). It tells you which transient overvoltages the meter withstands without failing or arcing (IEC 61010-1). CAT II is for plug-in equipment; CAT III for the building installation: distribution boards, fixed wiring and motors; CAT IV for the origin of the installation: the service entrance and the meter. For industrial panels use at least CAT III with enough voltage, for example CAT III 600 V or 1000 V. The test leads must have the same rating as the meter.
Test leads in good condition. No cuts in the insulation and with finger guards. In CAT III and CAT IV use probes with very little exposed metal or with tip covers (in the UK, HSE guidance GS38 recommends 4 mm of exposed tip or less).
Leads in the right jacks. For voltage, resistance and continuity: COM and VΩ. The amps jack is only for measuring current.
Prove the tester. Before and after verifying that something is dead, measure a source you know is live. That way you know the meter was working: if it failed in between, a "0 V" reading means nothing. This is the live-dead-live test that NFPA 70E and lockout/tagout procedures call for.

Typical mistakeMeasuring voltage with a lead left in the amps jack. That input has almost zero resistance: the meter becomes a short circuit between the two points. In the best case its fuse blows; in the worst, there is an arc flash in your hands. Always look at where the leads are before touching anything.

To work on a circuit, measuring is not enough: it must be isolated, locked out and tagged, proved dead with a suitable tester and, where needed, earthed. Our electrical safety topic explains the five safety rules step by step.

Measuring voltage: always in parallel

Voltage is a difference between two points, so the probes go in parallel with what you want to measure, without opening anything. The multimeter has a very high internal resistance (about 10 MΩ on digital meters), so the circuit hardly notices it.

Voltage is measured in parallel with what you are interested in. Current forces you to open the circuit and put the meter in series, unless you use a clamp meter.

Values you will find

Europe, UK and most of the world: about 400 V between phases and about 230 V between phase and neutral or phase and earth.
North America: 120/240 V split-phase in homes and small shops; 208Y/120 V or 480Y/277 V three-phase in commercial and industrial buildings.
Neutral to earth (ground): it should be close to zero; a few volts can be normal under load.
Control circuit: usually 24 V DC. Power supplies are often set slightly higher, for example 24.5 V, to make up for the voltage drop in the wires.
Active PLC input: about 24 V between the input terminal and the 0 V common.

When measuring DC, the red probe goes to the point you think is more positive and the black one to the reference (0 V). If you swap them, a digital meter shows the value with a minus sign: no harm done, but it is telling you the probes are the wrong way round.

Fault finding by measuring voltage step by step

Imagine a contactor will not pull in. Its coil gets 24 V through a chain of contacts in series: stop button, overload relay, limit switches… With the black probe fixed on 0 V, move the red one from the power supply towards the coil. Where the 24 V disappears, the open contact or the broken wire is between that point and the previous one.

The contact ruleMeasuring across the two terminals of the same contact, with the circuit powered: if the contact is closed, you read almost 0 V; if it is open, you read almost the whole supply voltage. It sounds backwards, but it makes sense: no voltage "drops" across a closed contact, and an open one takes the whole difference.

Ghost voltage

A wire disconnected at one end that runs in the same trunking as live wires can show tens of volts on the multimeter, sometimes more than 100 V AC. It is not a real voltage that can power anything: it is capacitive coupling with the neighbouring wires, which the meter picks up because its input resistance is so high.

To find out, some meters have a low impedance (LoZ) function: it places a low resistance between the probes that makes the ghost voltage disappear, while a real voltage stays. It is not used on sensitive electronic circuits, because that load can affect them.

True RMS and drives

With waveforms that are not sinusoidal, such as those produced by variable frequency drives, switch-mode power supplies or LED lighting, an ordinary multimeter can give wrong readings. True RMS meters measure those waveforms correctly. Even so, the output of a drive is hard to measure: some meters include a dedicated low-pass filter for it.

Measuring current: in series or with a clamp

To measure current with a multimeter you have to open the circuit and make all the current flow through the meter: the red lead moves to the A or mA jack. Those inputs are fuse-protected and have a limit, usually 10 A at most. That is why, in practice, the multimeter is used for small currents and 4–20 mA signals, and the clamp meter for everything else.

The clamp meter

A clamp meter measures the magnetic field created by the current flowing through a conductor. Advantages: you do not need to open the circuit or touch live parts. One condition: the clamp must go around a single conductor.

Cross-section view: the clamp must go around a single conductor. With line and neutral inside, the currents flow in opposite directions and the result is almost zero.

If you clamp the whole cable of a motor or a single-phase appliance, the meter will read practically zero, because the current that goes out on one conductor comes back on another. That effect has a use: if you clamp all the live conductors and the reading is not zero, some current is escaping by another path, such as a leakage to earth. That is how electricians track down why an RCD or a GFCI keeps tripping, with leakage clamps that are much more sensitive.

On a balanced three-phase motor, the three phases should draw about the same. If one draws clearly more than the others, something is wrong: a damaged winding, a loose connection or a phase with low voltage.

The 4–20 mA signal

Many industrial sensors, such as pressure, level or temperature transmitters, send their reading as a current between 4 and 20 mA. 4 mA is 0 % of the range and 20 mA is 100 %. Current is used instead of voltage because it is not affected by the length of the cable or the resistance of the connections, and starting at 4 mA makes a broken wire detectable: 0 mA is not "zero", it is a fault.

To convert current into a value:

value = (I − 4) / 16 × range

Example: a 0 to 10 bar pressure transmitter sends 8 mA. (8 − 4) / 16 × 10 = 2.5 bar. And 12 mA, half of the current scale, is exactly 50 %: 5 bar. The 4–20 mA converter does this for any range.

There are three ways to measure it: open the loop and put the multimeter in series on the mA range, use a milliamp clamp or, if the PLC input has a known resistor, measure the voltage across it. With 250 Ω, for example, 4–20 mA becomes 1–5 V.

On siteThe programmer calls you to say the PLC reads 0 mA from a transmitter. Before replacing the transmitter, check that the loop has power and look for a loose terminal or a broken wire. A faulty transmitter is much less common than a bad connection.

Resistance and continuity: always with the power off

To measure resistance, the meter injects a small current and measures the voltage across the part. If the circuit is live, the reading is meaningless and the meter can be damaged. That is why these measurements are always made with the power off, and ideally with the component disconnected: if it is still wired in, other parallel paths can give a false reading.

To know what a resistor should read from its colour bands, use the resistor colour code calculator.

Continuity: the meter beeps if the resistance is below a threshold, usually a few tens of ohms depending on the model. It is used to check wires, fuses and contacts. A normally open push button should beep only when pressed; a normally closed one, only when not pressed.
Contactor and relay coils: they should give a specific value that depends on the coil. OL means an open coil; almost 0 Ω, a shorted coil.
Pt100 sensors: 100 Ω at 0 °C, rising by about 0.385 Ω per degree. At 20 °C, about 107.8 Ω; at 100 °C, about 138.5 Ω. If you read OL, the sensor or its cable is open.
Three-phase motor windings: all three should give very similar values. A clear difference indicates a damaged winding.

A multimeter does not test insulationA motor that shows no continuity to earth on the multimeter does not have proven good insulation. Insulation is tested with an insulation resistance tester (megger), which applies test voltages of hundreds of volts. A multimeter only finds dead shorts.

Three-wire PNP and NPN sensors

Three-wire inductive, capacitive and photoelectric sensors usually follow this colour code: brown to +24 V, blue to 0 V and black is the output. What changes is what the output does when it detects:

PNP (sourcing): when it detects, the output gives +24 V. Measure between black and 0 V (blue): about 24 V when detecting and almost 0 V when not.
NPN (sinking): when it detects, the output connects to 0 V. Measure between +24 V (brown) and black: about 24 V when detecting and almost 0 V when not.

This splits the problem in two. If the sensor detects and there are 24 V on the black wire but the PLC does not see the input, check the terminal, the input card common and the configuration. If there are no 24 V, the problem is the sensor, its supply or its adjustment. And an obvious first check: the sensor must be the type the input card expects. An NPN sensor on a card wired for PNP will not work. The PNP and NPN sensor simulator lets you try both.

Typical mistakes

Measuring voltage with a lead in the amps jack.
Measuring resistance or continuity on a live circuit.
Accepting a dead reading without proving the tester before and after.
Using a meter or test leads without a high enough CAT rating for where you are measuring.
Mistaking a ghost voltage for a real one, or the other way round.
Clamping the whole cable and concluding the load draws nothing.
Measuring a component without disconnecting it and ignoring parallel paths.
Thinking that 0 mA in a 4–20 mA loop means "value zero".

Practise without riskIn the multimeter simulator you can measure on a three-phase supply, a 24 V control circuit and a 4–20 mA loop, make the mistakes in this topic with no consequences and find faults by measuring point by point.

Summary

Voltage in parallel; current in series or, better, with a clamp around a single conductor.
Resistance and continuity, always with the power off.
Before measuring: the right CAT rating, test leads in good condition and in the right jacks.
To verify a circuit is dead, prove the tester before and after (live-dead-live).
With the circuit powered, a closed contact gives almost 0 V across it and an open one the whole voltage.
In 4–20 mA, 4 mA is the bottom of the scale and 0 mA is a fault.

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