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Immersion Heater Tripping The Electrics
Your immersion heater keeps tripping the RCD or MCB every time it turns on.
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What's happening?
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.
What usually causes it?
- 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 should you 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 the electrician actually 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
What will it cost to fix?
£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.
Find an electricianFrequently asked questions
We’re happy to answer all your questions.
Why does my immersion heater trip the electrics?
In nearly all cases, the heating element's insulation has broken down, allowing current to leak to earth. This is especially common in hard water areas where limescale damages the element over time.
Can I still use other electrics in the house?
If the immersion trips only its own MCB, your other circuits will work fine. If it trips the RCD, it may affect other circuits on the same RCD. Turn off the immersion at the spur to prevent it tripping the RCD.
How often do immersion elements need replacing?
In hard water areas, every 3-5 years. In soft water areas, they can last 10-15 years. Fitting a water softener or scale reducer can extend the element's life.
Should I switch to a different hot water system?
If you are replacing elements frequently, consider whether a combi boiler, heat pump, or point-of-use water heater might be more cost-effective in the long run.
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