Multimeter
The core electrical instrument, what each function measures, and how to use it safely.
Tool
Beginner
#tools#electrical#testingSafety
Verify the meter on a known live source. De-energize, lock out and discharge capacitors before resistance or continuity testing.
What it is
A multimeter measures voltage, resistance, continuity and often capacitance, temperature and microamps. In HVAC it is the instrument that answers most electrical questions, provided the technician understands which function applies.
| Function | Use it for | Rule |
|---|---|---|
| AC voltage | Line and control circuit power | Measure with the circuit energized, across the load or component |
| DC voltage | Electronic boards, sensors, ECM controls | Observe polarity |
| Resistance | Windings, coils, heaters | Only with the circuit de-energized and the component isolated |
| Continuity | Switches, fuses, wiring | De-energized only |
| Capacitance | Run and start capacitors | De-energized, and only after discharging the capacitor |
| Microamps DC | Flame rectification signal | Meter placed in series with the flame sensor wire |
Using it well
- 1Choose a meter with a category rating appropriate to the circuits you work on.
- 2Inspect the leads for damage before every use.
- 3Verify the meter on a known live source before and after testing a circuit you expect to be dead.
- 4Measure voltage across a component to find the drop, and across an open switch to see whether it is open.
- 5For resistance, de-energize, lock out, discharge capacitors and isolate at least one leg of the component.
Voltage drop tells you where the problem is
With the circuit energized and running, full voltage across a closed switch or a connection means that device is the open point. Voltage across the load means the load is receiving power.
Safety
Live testing
Energized testing is done only when there is no safe alternative, with appropriate personal protective equipment, one hand where practical, and awareness of arc flash risk. Discharge capacitors before touching terminals. Never use resistance or continuity functions on a live circuit.
Practice & Mastery
- Questions1
- Multimeter Troubleshooting Scenarios2
Practice is part of HVAC Pro — $5/month or $50/year.
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