What is this?
When an immersion heater trips the electrics, it is causing either an earth leakage (tripping the RCD) or an overcurrent (tripping the MCB). This is a very common fault, especially in hard water areas where limescale attacks the element over time, degrading its insulation.
Common causes
- Heating element insulation breakdown — the most common cause by far
- Water ingress into the element terminal housing
- Corroded wiring connections at the immersion heater
- Faulty thermostat causing overcurrent
- Damaged cable between the spur and the immersion heater
- Leaking immersion heater boss allowing water onto connections
Is it dangerous?
An immersion heater that trips the electrics should not be used until repaired. The tripping indicates current is flowing where it should not — typically through the water or the earth conductor. This creates a genuine electric shock risk, particularly near the hot water cylinder.
Can I fix it myself?
You cannot safely fix this yourself. Turn off the immersion heater at the spur switch and leave it off until an electrician has attended. Do not keep resetting the trip switch.
When to call an electrician
Call an electrician as soon as you can. If the immersion heater is tripping the RCD and affecting other circuits in your home, treat it as more urgent. If you rely solely on the immersion for hot water, explain this when booking so the electrician can prioritise accordingly.
What will an electrician do?
Isolate the immersion heater and test the circuit
Perform insulation resistance testing on the element
Inspect the terminal housing for water ingress and corrosion
Drain the cylinder and replace the element and thermostat
Check the wiring and connections
Test the repaired installation to confirm safe operation
Typical cost
£120 – £300
Element and thermostat replacement including draining the cylinder is the standard repair. Parts are relatively inexpensive but the labour for draining and refilling adds to the cost.
