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Difficulty: MediumMultimeter: Resistance and Continuity

An industrial technician is troubleshooting an unpowered control cabinet and needs to measure the electrical resistance of a 24-volt relay coil to determine if the internal winding is defective. What is the correct chronological sequence of steps the technician should follow to perform this measurement safely and accurately?

  1. 1Verify that all electrical power to the control cabinet is completely shut off and de-energized.
  2. 2Disconnect at least one wire lead from the relay coil terminals to isolate the component from the circuit.
  3. 3Insert the black test lead into COM, the red lead into V/Ω\Omega, and set the meter selector dial to the resistance (Ω\Omega) function.
  4. 4Firmly contact the test probes across the isolated coil terminals and observe the steady value on the digital display.
  5. 5Compare the measured ohm reading against the manufacturer's nominal coil specifications to diagnose its condition.

Answer

The technician must first verify the circuit is de-energized, disconnect at least one coil terminal to isolate it from parallel paths, configure the multimeter into resistance mode with correct lead placement, apply the test probes across the coil terminals, and finally compare the resulting reading against manufacturer specifications.
The correct procedural order begins with personal and equipment safety by de-energizing the circuit, followed by component isolation to eliminate parallel resistance paths. Next, the multimeter is properly configured with leads in COM and V/Ω\Omega ports on the resistance setting. The probes are then placed across the terminals to obtain a reading, and the sequence finishes by comparing the measured value to the manufacturer's technical specifications.

Step-by-Step Solution

1
Ensure all power to the system is shut down and locked out.
The control circuit is fully de-energized and safe for ohmmeter connection.
Ohmmeter circuits inject their own small reference current; measuring on a live circuit will blow meter fuses or cause severe damage.
2
Disconnect at least one conductor from the relay coil terminals.
The relay coil is electrically isolated from any parallel components.
Parallel circuit paths provide alternative routes for test current, resulting in false, lower-than-actual resistance readings.
3
Connect the black lead to COM, red lead to V/Ω\Omega, and turn the dial to the Ω\Omega range.
The meter is correctly set up to inject reference current and compute resistance from voltage drop.
Placing leads in current (A/mA) jacks or selecting incorrect functions causes measurement failure or errors.
4
Apply probe tips firmly to the two coil terminals and wait for a stable reading.
The multimeter displays the true resistance of the coil winding.
Firm probe pressure minimizes contact resistance and ensures accurate measurement.
5
Evaluate the measured resistance relative to published technical data.
Accurate diagnostic determination of whether the coil is operational, shorted (near 0 Ω0\ \Omega), or open (OL).
A reading matching nominal specifications indicates a good coil, while significant deviations indicate faults.

Key Concept

Multimeter resistance testing requires a strictly ordered procedure: de-energizing power for safety, isolating the component to eliminate parallel paths, configuring proper meter settings, taking the measurement, and comparing results with specifications.
Estimated Time:1m 0s
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Topics

Multimeter: Resistance and Continuity

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