How to Test Capacitance with a Multimeter: A UK Step-by-Step Guide

Testing capacitance with a multimeter is straightforward once you know how to discharge the component safely and select the right range. This UK-focused guide walks through the procedure for motor run capacitors, PCB capacitors and everyday fault-finding.

When You Need a Capacitance Reading

Capacitance measurement is not an everyday task like voltage or continuity, but when a single-phase motor hums without starting or a control board behaves erratically, checking the capacitor is often the fastest diagnostic step. Maintenance engineers on UK commercial sites regularly test run capacitors on fan motors, compressors and pumps.

A standard digital multimeter with a capacitance (often labelled F or nF/µF) mode measures the stored charge capacity of a capacitor in farads — typically displayed in microfarads (µF) for motor applications.

What You Need

• A multimeter with a dedicated capacitance function (not all cheap meters include this)
• Insulated test leads with proper probe tips
• A discharge resistor (typically 10–20 kΩ, 5 W) for larger capacitors
• Insulated gloves and eye protection when working on powered equipment

The Fluke 107 pocket multimeter includes capacitance measurement alongside voltage, resistance and continuity — a practical combination for maintenance call-outs at £186.19 inc. VAT.

Safety First: Discharge Before You Measure

Capacitors store energy after power is removed. Motor run capacitors and power-supply capacitors can deliver a painful — occasionally dangerous — shock if handled carelessly.

1. Isolate the circuit at the appropriate breaker or isolator and lock off where required.
2. Prove dead using a two-pole voltage tester following the prove-test-prove sequence. Our voltage continuity tester guide covers safe isolation in detail.
3. Discharge the capacitor through a suitable resistor. Do not simply short terminals with a screwdriver — this damages the component and creates a spark hazard.
4. Verify zero voltage across the terminals before touching.

UK forum users often report confusing readings when skipping the discharge step — residual charge skews the multimeter result or triggers an overload warning.

Step-by-Step: Measuring Capacitance In-Circuit vs Out-of-Circuit

Out-of-circuit (most accurate)

1. Disconnect one or both capacitor leads so parallel circuit paths do not affect the reading.
2. Select capacitance mode on your multimeter.
3. Connect the red probe to the positive terminal and black to negative (polarity rarely matters for capacitance mode, but follow your meter's manual).
4. Wait for the reading to stabilise — larger capacitors take a few seconds.
5. Compare the reading to the value printed on the capacitor body (e.g. 25 µF ±5%).

In-circuit (quick check, less reliable)

In-circuit readings can be influenced by parallel resistors, inductors and other components. Use in-circuit checks as a rough indicator only. If the reading is suspicious, remove the capacitor and retest out-of-circuit.

Interpreting Results on Motor Run Capacitors

Motor run capacitors commonly fail high, low or open:

• Near nominal value: Capacitor is likely healthy (also check for physical bulging or leakage).
• Significantly below rating: Classic failure mode — motor may hum, draw high current or fail to start.
• Zero or OL (overload): Open circuit — capacitor has failed completely.
• Unstable reading: Internal degradation — replace even if the average looks acceptable.

Always replace with the correct voltage rating (typically 370–450 V AC for UK motor applications) and match the µF value printed on the nameplate.

Common Mistakes UK Engineers See

• Not discharging first: Residual charge damages the meter input or gives false readings.
• Wrong range: Nanofarad mode on a 25 µF motor capacitor will overload. Let auto-ranging handle it if available.
• Testing with power on: Never attempt capacitance measurement on energised circuits.
• Ignoring tolerance: A 25 µF ±5% capacitor reading 23.8 µF is fine; reading 12 µF is not.

Choosing a Multimeter for Capacitance Work

Look for these practical features:

• Dedicated capacitance mode with adequate range (nF to hundreds of µF)
• Auto-ranging to avoid manual range errors
• Overload protection on the input
• CAT III safety rating if you are also measuring live circuits on the same meter

The Fluke 107 combines capacitance measurement with auto-ranging voltage and continuity in a pocket-sized CAT III 600 V body — ideal for maintenance engineers who need capacitance checks without carrying a full bench meter.

FAQ

Can all multimeters measure capacitance?

No. Many entry-level meters omit capacitance mode entirely. Check the dial or function list before buying. The Fluke 107 includes capacitance measurement as standard.

What does OL mean on capacitance mode?

OL (overload) typically means the capacitance exceeds the selected range, the capacitor is open circuit, or residual charge is confusing the input. Discharge the component and retry, or switch to a higher range.

Is it safe to test capacitance on a live circuit?

No. Always isolate, prove dead and discharge before measuring capacitance. Capacitance mode is for de-energised components only.