Short answer: A multimeter is best for voltage, resistance, continuity and small currents, measured through its test leads. A clamp meter measures larger AC (and sometimes DC) currents by closing its jaws around a single conductor, so you do not need to break the circuit.
How does a multimeter measure?
A digital multimeter connects to the circuit through two test leads. It measures:
- Voltage. The leads go across the two points in parallel.
- Resistance and continuity. These are measured on a dead circuit.
- Current. The meter must be wired in series. That means opening the circuit and passing all the current through the meter's internal fuse. Most handheld meters are limited to about 10 A.
Many models also measure capacitance, frequency, diode drop and temperature.
How does a clamp meter measure?
A clamp meter has hinged jaws that close around one conductor. For AC, a current transformer in the jaws picks up the magnetic field around the wire. Models that also read DC use a Hall-effect sensor. Because nothing is disconnected, you can check a motor's load current or a feeder cable in seconds. Ranges of hundreds of amps are common.
Most clamp meters also have test-lead sockets for voltage and resistance, so they overlap with a multimeter. Their resolution on low ranges is usually coarser.
Which tool suits which job?
| Task | Multimeter | Clamp meter |
|---|---|---|
| Checking socket or supply voltage | Yes | Yes (with leads) |
| Continuity and resistance | Yes, with better resolution | Basic |
| Small currents, such as milliamps and sensor loops | Yes, wired in series | Only with a special low-range clamp |
| Motor, heater or cable load current | Limited, and means breaking the circuit | Yes, without contact |
| Electronics and circuit boards | Yes | Rarely |
| HVAC compressors and fans | Partly | Yes |
Many electricians and HVAC engineers carry both tools: a multimeter for precision and a clamp meter for load checks.
Why does the clamp read zero around a plug lead?
A flexible cable carries live and neutral together. Their currents flow in opposite directions, so the magnetic fields cancel out. Clamp a single conductor only, or use a line-splitter adaptor.
What safety ratings should you look for?
Meters and leads used on mains systems are rated by measurement category under BS EN 61010. HSE guidance GS38 summarises the categories:
- CAT II: equipment plugged into sockets, such as appliances and portable tools.
- CAT III: the fixed installation, such as distribution boards, sockets and hard-wired equipment.
- CAT IV: the origin of the supply, such as meter tails and main protective devices.
A higher category means the meter is built to withstand larger voltage spikes. Choose a rating, together with a voltage, that matches the point where you will measure.
What does HSE GS38 say about test leads?
GS38 is HSE's guidance on electrical test equipment for low-voltage systems. Key points for multimeter users include:
- Probes should have finger barriers. The exposed metal tip should be no more than 4 mm, and ideally 2 mm or less, or the probes should have retractable shrouds.
- Leads should be well insulated, coloured so you can tell them apart, and of suitable length.
- Multimeter leads should normally have suitable high-breaking-capacity fuses, usually not exceeding 500 mA.
- To prove that a circuit is live or dead, a proprietary two-pole voltage detector or test lamp is preferred over a multimeter.
- Inspect the equipment before each use. Live testing should only be done when it is justified, and by competent people.
What goes wrong most often?
- Leaving the leads in the current socket and then measuring voltage. This can blow the fuse or cause an arc flash. Always check the dial and the sockets.
- Reading AC on a DC setting, or the other way round.
- Clamping two conductors, which gives a zero or misleading reading.
- A clamp jaw that won't close fully, often because of dirt. This reduces accuracy.
- A low battery. Many meters read unreliably before the low-battery warning shows.
FAQ
Can a clamp meter replace a multimeter? For general installation work, often yes. For electronics, low currents and precise resistance readings, a multimeter is better.
Is "True RMS" important? Yes, for modern loads such as variable-speed drives, LED drivers and switch-mode supplies. These distort the waveform, and meters without True RMS can misread it.
Do clamp meters need GS38-style leads? If you use the meter's own test leads, the same guidance on probes and leads applies.
Do clamp meters measure DC? Only models with a Hall-effect sensor. Check the specification.
Sources
- HSE, GS38 "Electrical test equipment for use on low voltage electrical systems": https://www.hse.gov.uk/pubns/priced/gs38.pdf
- HSE, GS38 publication page: https://www.hse.gov.uk/pubns/books/gs38.htm
- NICEIC, "When is it necessary to use fused test leads?": https://professional-electrician.com/technical/when-is-it-necessary-to-use-fused-test-leads-niceic/
Related products: Electrical Testers
