Electrician Red Seal · Question
A technician is troubleshooting a motor control circuit where a 24 V DC control relay (CR1) intermittently fails to energize. When CR1's coil is tested with an ohmmeter, it shows 500 ohms. However, when 24 V DC is applied directly to the coil, it draws 10 mA. What is the most likely cause for the intermittent failure?
According to Ohm's Law (R = V/I), if 24 V DC is applied and the coil draws 10 mA (0.010 A), the actual resistance is 24 V / 0.010 A = 2400 ohms. The ohmmeter re
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Question: A technician is troubleshooting a motor control circuit where a 24 V DC control relay (CR1) intermittently fails to energize. When CR1's coil is tested with an ohmmeter, it shows 500 ohms. However, when 24 V DC is applied directly to the coil, it draws 10 mA. What is the most likely cause for the intermittent failure?
Answer options:
- A short circuit in the coil.
- An open circuit in the coil. ✅ High resistance in the control circuit wiring.
- Incorrect control voltage supply.
Correct answer: High resistance in the control circuit wiring.
Explanation: According to Ohm's Law (R = V/I), if 24 V DC is applied and the coil draws 10 mA (0.010 A), the actual resistance is 24 V / 0.010 A = 2400 ohms. The ohmmeter reading of 500 ohms suggests high resistance elsewhere in the control circuit wiring that is not part of the coil's intrinsic resistance, causing an insufficient voltage drop across the coil during normal operation. A short circuit would show very low resistance, an open circuit infinite resistance, and incorrect supply voltage would be a constant issue, not intermittent.
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