Why Does My Extractor Fan Keep Tripping?

Reading what the trip is telling you

The first thing worth knowing is that not every trip is the same trip. Walk to the distribution board and look at what has actually operated. A device that protects against electric shock - a residual current device, or RCD - is doing a different job from the overcurrent protection on the circuit, and the two failures behind them have almost nothing in common. Getting this wrong is how kitchens end up replacing a perfectly good motor to cure a fault that was really damp insulation, or endlessly resetting a breaker that is warning them the fan is working itself to death.

Earth leakage - the slow creep

An RCD watches the current flowing out to the fan and the current coming back. When those two no longer match, it means some current is escaping to earth - through a person, or through a fault - and the device cuts the supply. Under BS 7671, the 30mA RCD protecting a kitchen circuit must operate within 300 milliseconds at its rated current, and it will do so the instant leakage climbs past its threshold. In a commercial extraction motor, that leakage rarely arrives all at once. It creeps.

A motor might sit happily at a couple of milliamps of leakage for months, then drift upward as moisture, grease and heat quietly degrade the winding insulation. When the running total across the circuit nudges past the trip point, the RCD lets go - often at the worst moment, when the motor is hot and working hardest. That is why a fan can trip at eight o'clock every night and behave itself by morning. The usual culprits behind rising earth leakage are worth committing to memory:

• Moisture ingress - condensation forming inside poorly insulated ductwork or the motor housing, tracking across terminals and bridging live parts to earth.
• Grease-laden windings - a motor mounted in the extract airstream slowly coats internally, and warm grease is a far better conductor than clean, dry insulation.
• Aged winding varnish - once a motor has run hot enough, long enough, the enamel darkens and stops insulating; BS 7671 wants at least 1 megohm at 500V DC, and a healthy circuit should read a hundred times that or more.
• A neutral fault or miswire - a neutral returned outside the RCD, or a nicked cable in a spur or isolator, will trip the device the moment the fan is switched on.

The tell for earth leakage is that the RCD, not the breaker, is the thing that operates - and that it often trips on damp mornings, on start-up, or when the system has been running long enough to warm through.

Overload - when the motor asks for too much

The other family of trips has nothing to do with leakage. Here the overcurrent protection - the breaker, or a separate thermal overload in the fan's control gear - operates because the motor is simply drawing more current than it should. A motor that is straining draws harder, heats up, and eventually pulls enough to trip. The question is always the same one: why is it straining?

Very often the answer is airflow, or the lack of it. A fan choked by a grease-blocked filter, a collapsed flexible or a canopy that has not seen a deep clean in a year is fighting the system every second it runs. Worn bearings that have lost their grease drag on the shaft. A failing capacitor leaves a single-phase motor struggling to get up to speed. And a fan that was oversized for its duct in the first place can spend its whole life running against resistance it was never meant to meet - one of the quiet reasons an oversized extraction fan carries hidden costs long after the install invoice is paid. An overload trip usually arrives after a run of a few minutes rather than on start-up, and the motor is hot - sometimes too hot to hold a hand against - by the time it goes.

From nuisance trip to lasting fix

Resetting the board is not a repair - it is borrowing against a fault that is still there. Every time you flick the switch back without knowing why it went, you are betting that the leakage has not crept higher or the motor has not cooked itself a little further. The way out of the cycle is a proper diagnosis, followed by the sort of routine that stops the fault ever building again.

What a proper diagnosis looks like

A competent engineer does not guess. The starting point is to separate the two failure modes described above, and that means dead testing before anything is switched back on. An insulation resistance test - a megger sending a safe 500V between the conductors and earth - reveals in seconds whether the motor and its wiring are leaking, and puts a number on it you can track over time. From there the diagnosis narrows quickly:

• Insulation resistance - readings near or below 1 megohm point at damp or degraded windings; a motor reading in the low kilohms is on its way out.
• Earth leakage under load - measuring the actual leakage the RCD sees, ideally kept well under a third of its 30mA rating, shows whether the circuit is living on borrowed time.
• Running current against the motor plate - a fan drawing above its rated amps is overloaded, and the reason will be airflow, bearings or a tired capacitor.
• The control gear itself - contactors, thermal overloads and isolators that have baked in a hot ceiling void fail in ways that mimic a motor fault.

Only once the fault has a name does a fix make sense - a strip and dry, a rewind or replacement, a new capacitor, a bearing change, or a rewire of a botched spur. Guessing skips the one step that tells you which of those you actually need.

Keeping the trip from coming back

Most repeat trips are preventable, because most of them are grease and moisture doing their slow work. The motor that runs in a clean airstream, breathes freely and stays dry does not drift toward its leakage threshold, and it does not strain against a blocked system until the overload lets go. That is the real argument for planned extraction cleaning and a periodic electrical check on the fan - not a box ticked for the insurer, but a fan that simply keeps running through service. There are genuine, measurable savings in a system kept in good order, from lower running current to a motor that lasts years longer - the quiet savings of a well-maintained extraction system add up well beyond the cost of the visit. To keep the board quiet, build in the basics:

• Deep clean the canopy, filters, ductwork and fan on a schedule matched to how hard the kitchen cooks, so grease never reaches the motor.
• Have the fan's insulation resistance and running current logged during cleaning, so a slow decline is caught long before it trips.
• Insulate and drain ductwork properly so condensation cannot pool near live parts.
• Replace tired capacitors, bearings and worn control gear as they show up, rather than after they fail mid-shift.

A fan that keeps tripping is rarely broken beyond help. It is a system asking for attention in the one way it can - and once you know whether it is leaking or labouring, the fix is usually straightforward, lasting, and a great deal cheaper than a kitchen that keeps going dark at the worst possible moment.