Digital Multimeters (DMMs) User Guide and Working Principle.
A digital multimeter (DMM) is an electrical test instrument that can measure more than one electrical property. See Figure 1. Basic DMMs measure voltage, current, and resistance. Advanced DMMs include additional measurement capabilities, such as capacitance, continuity, and temperature. They may also include special functions, such as recording, logging, and comparing stored measurements.
Guidelines for DMM usage and safety
A DMM is used on new construction wiring installations, temporary wiring installations, maintenance tasks, and retrofit work with previously energized conductors and equipment. Guidelines for DMM usage and safety include the following:
- Verify that the DMM ratings exceed the system ratings.
- Verify the CAT rating of the DMM for the system to be measured.
- Inspect the DMM for physical damage, such as a cracked case or faulty switches or jacks.
- Never use a damaged DMM or test leads.
- Inspect the test leads for damage. Frayed test leads, damaged insulation, burnt tips, and damaged input plugs can all cause meter failure. Discard and replace any damaged leads.
- Do not allow fingers to protrude below the finger shrouds when taking a reading. The finger shroud is the raised portion of a test lead.
- Do not use a DMM (or any electrical test instrument) in an environment where hazardous vapors or dust particles are present. Small electrical arcs can be created when taking voltage measurements, which may be an explosion hazard.
- If unsure of the voltage level to be measured, begin on the highest scale possible and work down until the appropriate reading is obtained. (Many DMMs have only one setting for AC and one setting for DC and automatically adjust to the proper voltage level range as required.)
- Always wear PPE when measuring an energized circuit. Typically, with any circuit operating at 50 V or more, rubber insulating gloves and eye protection must be worn at a minimum.
Voltage Measurements using DMM.
Circuit voltages, energized status, or electrical test instrument accuracy must never be assumed. Best practices for measuring circuit volt- ages involve verifying electrical test instrument operation and voltage accuracy by first testing a known voltage source. See Figure 2.
Verify that a DMM is in proper working condition
To verify that a DMM is in proper working condition and that a circuit is de-energized, apply the following procedure:
- Connect the DMM to a known voltage source of the same voltage level as the circuit to be measured.
- Verify that the DMM reads the appropriate value.
- Connect the DMM to the circuit to be tested and note the measurement.
- Verify that the DMM reads a measurement of 0 V.
- Reconnect the DMM to the known voltage source.
- Verify that the DMM reads the appropriate voltage level and is in proper working condition.
Current measurements are used to determine the amount of circuit loading or the condition of an electrical load. Every time a load is switched on or a new load is added to a circuit, the power source must provide more current through the circuit for the new loads. Large amounts of current are measured in amperes (A). Small amounts of current are measured in milliamperes (mA) or microamperes (μA).
In-line current measurements are made by opening the circuit and connecting the DMM’s test leads so that the circuit is closed. See Figure 3. Current flows through the meter as it becomes part of the circuit. This method of current measurement is very accurate. However, this method must be limited to circuits that can be easily opened and are known to have currents less than 10 A, which greatly limits its applicability in industrial settings. In-line current measurements also present greater opportunity for electrical shock.
Resistance measurements may indicate the condition of a circuit or component. See Figure 4. Components designed to conduct electricity, such as conductors or component contacts, have very low resistance values. When conductors are damaged by burns or corrosion, resistance increases. Other components such as heating elements and resistors have a fixed resistance value. Any significant change in the fixed resistance value typically indicates a problem.
To measure resistance, a DMM uses an internal battery to supply voltage to the test leads. The battery sends a small amount of current into the circuit to be tested. The level of current indicates the circuit resistance.
Resistance measurements must be taken with the circuit de-energized. If the resistance of a particular component must be known, the component is removed from the circuit. When a circuit includes a capacitor, the capacitor must be discharged before taking any resistance readings of the circuit.
Measuring resistance is also a way to test continuity. Continuity is the presence of a complete path for current flow. Testing the presence of circuit continuity is often required for performing wiring installation to correctly identify and label all conductors prior to termination. A DMM emits an audible beep for little or no resistance, which indicates continuity. For example, a short length of copper wire should read about 1 Ω or less, and the meter should indicate continuity by beeping. A copper wire reading infinite resistance or overload (OL) indicates an open in the wire, which means there is too much resistance for the meter to measure.
Never exceed the current and voltage ratings of a DMM. Never connect a DMM set to measure resistance on an energized circuit. Always consult the DMM’s user manual before operating the equipment.