A different job than a breaker
A breaker or fuse clears a short circuit fast. An overload relay protects the winding from sustained overcurrent a breaker will ignore for hours: a jammed pump, a seized bearing, a motor starting against a load it can't move. None of that trips a breaker. All of it cooks a winding.
Electronic relays model the heat, not just the amps
Thermal relays use a bimetallic strip as a rough analog for winding heat. Electronic relays calculate an I-squared-t curve from current transformer data, a closer match to real winding behavior, and can also catch phase loss, imbalance, and reversal that a thermal unit misses.
Trip class and the setting people miss
Trip class (10, 20, 30) sets how long the relay tolerates locked rotor current, matched to how the motor actually starts, not just its horsepower. And the relay should always be set to nameplate FLA, not breaker size. Set too high, it never trips while the motor runs in a mild overload indefinitely.
Nuisance trip or real fault?
● Trips on every start: check trip class against start time and load inertia
● Trips under normal load: check FLA setting and actual draw with a clamp meter
● Trips randomly: check for voltage imbalance or a loose lead connection
● Won't reset at all: don't force it, check for a genuine ground fault or locked rotor condition first
Why it's worth getting right
A relay set correctly doesn't just prevent a catastrophic failure, it protects insulation life on every single overload event that falls short of catastrophic. Insulation degrades a little every time a winding runs hot, whether or not it ever trips. A well-tuned overload is one of the cheapest reliability upgrades available on an existing motor, since it usually just means checking a setting rather than buying a part.
BOTTOM LINE
A motor that keeps tripping and nobody's sure why is usually a five-minute phone diagnosis before it becomes a bigger repair.
