Why Does My Kitchen Trip the Electrics Under Load?

How to trace a kitchen that trips under load

Work through these in order. The sequence moves from the cheapest, most likely causes to the ones that need a qualified engineer, and it separates an overcurrent trip from an earth-leakage trip early - which is the single most useful thing you can establish.

1. Read the device that tripped. An MCB or the overcurrent side of an RCBO trips on too much current - an overload or a short circuit. An RCD, or the residual side of an RCBO, trips on current leaking to earth, typically at 30mA. Which one moved tells you whether you are chasing load or leakage, so note it before you reset anything.
2. Time the trip against the extract fan. If it goes the instant the canopy fan starts, you are likely seeing inrush - a motor can pull six to eight times its running current for a fraction of a second on start-up. A tired motor or a curve that is too tight turns that surge into a trip, especially when several loads start together.
3. Check whether it only trips at full service. A circuit that holds all morning but drops when the last fryer, combi and dishwasher come on is being asked for more current than it is rated to carry. That is an overload, not a fault - the answer is usually redistributing loads across circuits, not forcing more through one.
4. Feel and listen to the fan motor. A dry bearing, a grease-choked impeller or a motor starved of airflow runs hot and drags. As friction climbs, so does current draw - a motor rated at 3.6A can creep to 4.1A or beyond - until the thermal overload or the breaker calls time. A rumbling, ticking or scorching-hot motor is a strong tell.
5. Split the circuit to find the culprit. With the power isolated, disconnect appliances one at a time, reset, and bring them back individually. When the trip returns, you have found it. Leakage often builds slowly - starting at a couple of milliamps and climbing as a faulty appliance warms - so give each one time under load.
6. Look for water, steam and grease near the fault. Moisture bridging a terminal, condensation running back down ductwork, or grease that has crept into a connection box will all leak current to earth once the kitchen heats up. This is why so many under-load trips are damp-and-warm problems rather than pure electrical ones.
7. Get insulation resistance and RCD type checked. A qualified engineer can measure insulation resistance - BS 7671 wants at least 1.0MΩ at 500V DC - and confirm the RCD is the right type. Modern EC fan motors, variable-speed drives and electronic controls produce non-sinusoidal leakage that an older Type AC device cannot read correctly, and Regulation 531.3.3 now calls for Type A or better.

Why extraction sits at the heart of it

Your extract system is the hardest-working electrical load in the kitchen, and it lives in the worst possible conditions for cables, motors and connections: continuous heat, airborne grease and, at the canopy, a steady bath of steam. Every one of those attacks the two things that keep a circuit healthy - the motor's ability to draw a sensible current, and the installation's ability to keep that current out of the earth path.

Take the load side first. A fan motor is a fixed job of work, but only when it is clean and free-running. Let grease build on the impeller and let the bearings dry out, and friction rises; as the bearing runs hotter the lubricant degrades faster, which raises friction again - a loop that accelerates quickly once it starts. The motor answers by pulling more current, and at some point that extra draw meets the breaker or the motor's own thermal cut-out. From the floor it looks like a mystery trip at service; on a clamp meter it is simply a motor working far harder than it should. A clean canopy, clean ductwork and a fan that spins freely is genuinely part of your electrical maintenance, not just your hygiene regime. If the fan itself is the thing dropping the breaker, it is worth reading why an extractor fan keeps tripping alongside this.

Now the leakage side, which is where the “under load” pattern really lives. Every modern appliance with an electronic power supply leaks a tiny, deliberate current to earth through its filter capacitors - that is by design and within safety limits. The trouble is that BS 7671 Regulation 531.3.2 caps the accumulated leakage downstream of an RCD at 30% of its rating: for a 30mA device that is just 9mA before it is entitled to trip. Stack a combi oven, a dishwasher, a couple of EC fan motors and a rack of controls on one RCD and the standing leakage can sit close to that ceiling before you have a fault at all. Add a genuine fault - a damp terminal, tired motor insulation, a grease-bridged joint - and the total tips over exactly when the kitchen is hottest and busiest. That is why Regulation 314.1 asks for the load to be divided across enough circuits to keep nuisance tripping down in the first place.

Moisture is the quiet accelerant here. Steam that condenses under the canopy and inside the first run of duct does not stay put - it works into fittings, drips onto connections and lowers insulation resistance until a warm, working circuit starts leaking past the RCD's threshold. If your extract runs cold, sweats heavily or drips, the electrical symptom and the comfort symptom share a root cause; why there is condensation under your canopy covers that side of it. Deal with the grease, the airflow and the damp, and a surprising share of “the electrics keep tripping” calls simply stop.