Master Electrician Exam (NEC)General Electrical KnowledgeMedium
A master electrician is troubleshooting a control circuit that occasionally experiences nuisance tripping due to inductive kickback when a large contactor de-energizes. Which of the following components would be most effective in mitigating this issue?
- AA larger transformer for the control circuit.
- BA longer control circuit wiring run.
- CA snubber circuit (RC network or MOV) across the contactor coil.
- DA higher-rated circuit breaker for the control circuit.
Show answer & explanationAnswer & explanation
Correct answer: C. A snubber circuit (RC network or MOV) across the contactor coil.
Inductive kickback, or voltage spikes, occur when the magnetic field in an inductive load (like a contactor coil) collapses, inducing a high voltage. A snubber circuit (Resistor-Capacitor network) or a Metal Oxide Varistor (MOV) placed across the coil provides a path for this energy, suppressing the voltage spike and preventing nuisance tripping or damage to sensitive components.
Why the other options are wrong
- A. A larger control transformer would not mitigate voltage spikes from inductive kickback; it addresses power capacity.
- B. A longer wiring run could potentially worsen the issue by adding more inductance or capacitance, but would not mitigate inductive kickback.
- D. A higher-rated circuit breaker would only allow more current before tripping, not address the underlying voltage spike issue from inductive kickback.
Inductive Kickback Mitigation
Inductive kickback refers to the high voltage spike generated by an inductor when its current is rapidly interrupted. Mitigation techniques aim to safely dissipate this energy to protect components.
- Caused by the collapse of a magnetic field in an inductor (e.g., relay coil, motor).
- Can damage sensitive electronics, cause nuisance tripping, or generate electromagnetic interference.
- Common mitigation devices include snubber circuits (RC networks) and Metal Oxide Varistors (MOVs) across the inductive load.
Memory trick: Kickback Spikes Need Snubbing or MOV Shielding.