Nuisance Trip on a Refrigeration Circuit After Maintenance
A maintenance tech services a walk-in cooler. After replacing a contactor and re-tightening line-side lugs, the compressor starts but the circuit breaker trips after about 90 seconds. The breaker is a 30 A thermal-magnetic type; the compressor nameplate shows 24 A RLA and 120 A LRA. The tech measures 26 A per phase on a clamp meter during the run and 27.5 A just before the trip. Supply voltage is 208 V three-phase, within nameplate tolerance. The tech notices the new contactor coil is rated 240 V and the control circuit is 208 V. Ambient at the breaker is 40 C.
1. Evaluate whether the trip is an overload or a fault, using the current readings and breaker characteristics.
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Running current of 26 A against 24 A RLA is about 8 percent over, and 27.5 A just before trip is about 15 percent over. A thermal-magnetic breaker trips on overload according to its time-current curve; at 27.5 A on a 30 A breaker (about 92 percent of rating) a trip after 90 seconds is not consistent with a simple overload, since thermal elements typically hold well above rating for minutes. This points toward either a breaker thermal element degraded by the 40 C ambient (which derates the breaker, effectively lowering its trip point) or a high-resistance connection causing localized heating. The 240 V coil on a 208 V control circuit is a separate concern: it may chatter or fail to pull in reliably, causing repeated compressor starts and high inrush cycling that heats the breaker.
2. What is the significance of the 240 V coil on a 208 V control circuit?
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A 240 V coil operated at 208 V receives about 87 percent of rated voltage. Many contactors will pull in but with reduced holding force, leading to chatter, incomplete seating, and arcing at the contacts. Chatter produces repeated inrush events near the 120 A LRA, and each inrush deposits heat in the breaker's thermal element. Over 90 seconds of cycling, cumulative heating can trip a breaker that would hold a steady 26 A. The correct fix is a coil rated for the actual control voltage (208 V) or a control transformer, not adjusting the breaker.
3. Given the 40 C ambient, how should the breaker be evaluated, and what is the safe corrective sequence?
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Breakers are calibrated at a reference ambient (often 40 C for some, 25 C for others); at 40 C a standard thermal-magnetic breaker may be at or beyond its calibration point, so its effective rating is reduced. Check the breaker's ambient derating curve; if the corrected rating is below 27.5 A, the breaker is undersized for the actual load and ambient, and a higher-rated or better-ventilated enclosure is needed. Corrective sequence: de-energize, replace the coil with the correct voltage rating, verify lug torque to spec, confirm the breaker's ambient derating, and re-measure current and starts. Do not upsize the breaker without confirming conductor ampacity supports it.