A compressor that keeps tripping its breaker is not a breaker problem. The breaker is doing exactly what it's supposed to do, protecting the motor and wiring from overcurrent. Resetting it and hoping the problem goes away is how you turn a £400 repair into a £4,000 motor replacement.
The question is why the current is too high. That's usually one of about six things, and they're not difficult to work through systematically.
Start with the electrical supply, not the machine
Before you pull the compressor apart, check the basics:
Breaker rating. Compressor motors draw two to three times normal current on startup. If someone has fitted an undersized breaker at some point, a 20A where a 32A should be, it will trip every start. Check the manufacturer's specification for minimum circuit ampacity and required breaker rating. These are different numbers and both matter.
Supply voltage under load. Low supply voltage forces the motor to draw higher current to maintain power output. A 10% voltage drop can increase current draw by 15-20%. Measure voltage at the motor terminals while the machine is running, not at the panel while it's idle. The difference can surprise you, particularly on older industrial estates with long cable runs.
Star-delta starting sequence. This catches people out more often than it should. On compressors with a star-delta starter, the transition from star to delta needs to happen after the motor has accelerated. Set the timer too short and you get a current spike on every transition. Check the transition time against the manufacturer's recommendation, typically 4-8 seconds depending on machine size.
Mechanical causes: what loads the motor at start-up
Back pressure on restart is one of the most common causes on machines without a functional unloader. When the compressor tries to start against full system pressure, motor load is dramatically higher than a no-load start. The receiver should be at atmospheric or very low pressure before the motor attempts to turn. If it isn't, check the unloader valve, a failed or stuck unloader is a straightforward repair once diagnosed.
Seized or stiff airend. A tight airend draws high current from the first revolution and often won't fully start. You'll usually hear the motor struggling before the breaker trips. This doesn't self-resolve, a stiff airend needs investigation, either an oil-related seizure or bearing damage.
Worn bearings. Bearing wear increases mechanical friction and raises running current progressively. The machine may start fine and run for 20-30 minutes before tripping as bearings heat up and clearances tighten. If the symptom has developed gradually over several weeks, bearing wear is the likely cause.
Electrical causes: when the motor itself is the problem
Winding insulation breakdown is indicated by a trip that occurs after a period of running, resets once things cool down, and happens again after a similar interval. An insulation resistance (IR) test on the motor windings will confirm or rule this out, a reading below 1 megohm at 500V indicates compromised insulation. This is worth catching before the winding fails completely.
Cooling fan failure. A motor running without its cooling fan will eventually draw higher current as it overheats and insulation degrades. Check that the fan is turning and that cooling airflow through the motor casing is unobstructed.
Capacitor failure on single-phase machines. A failed or degraded run capacitor causes high running current and reduced motor efficiency. Capacitor testing is quick and a new one costs £20-40. Worth checking before anything more involved.
What to have ready when you call for help
When you contact your service engineer, have the following available: the breaker rating and type, the measured voltage at motor terminals under load, the current draw at start and during running if you can measure it safely, whether the trip occurs immediately on start-up or after a period of running, and whether this is a new problem or has developed over time.
That information cuts the diagnostic time considerably. An engineer arriving cold to a "it keeps tripping" fault can spend an hour narrowing down what you can establish in ten minutes with a multimeter.