If your RCD (Residual Current Device) keeps tripping, it's easy to assume it's just being "sensitive" and reset it without a second thought. In reality, an RCD trips because it's detected a genuine imbalance in the electrical current — most of the time, that means something is genuinely wrong, not that the device is faulty.
What Causes an RCD to Trip But Not the MCB?
This is one of the most common questions we're asked. An RCD and an MCB (Miniature Circuit Breaker) protect against different things. The MCB protects against overload and short circuits — too much current flowing through the wire. The RCD protects against earth leakage — current escaping to earth through a fault, often through you, if you're touching something faulty. Both live inside your consumer unit, so it's entirely possible for the RCD to trip while the MCB stays on, because the fault is an earth leakage issue, not an overload.
Common Causes of RCD Tripping
- A faulty appliance — often the most common cause. Unplug everything and reconnect one item at a time to isolate it.
- Damaged or ageing wiring — insulation breaking down over time can cause current to leak to earth. Older properties are especially prone to this; our electrician Nottingham team regularly finds this in Victorian and inter-war housing.
- Water ingress — an outdoor socket, light fitting, or security lighting circuit can develop a moisture leakage path over time.
- An overloaded circuit with multiple appliances sharing a single RCD-protected circuit.
- A genuinely faulty RCD or outdated consumer unit — more common in older units without individual RCBOs per circuit.
Why Does My RCD Keep Tripping Specifically at Certain Times?
If it trips when you turn on a specific appliance, that appliance (or its lead) is the likely culprit. If it trips randomly, especially in wet weather, water ingress into an outdoor circuit — such as an EV charger or garden lighting — is a strong possibility.
When Should You Call an Electrician?
If resetting the RCD only holds for a short time, or it trips again as soon as you turn something back on, don't keep resetting it repeatedly — this is your cue to get it properly diagnosed. Our electrical fault finding service uses proper testing equipment to trace the exact circuit and cause, rather than guessing. If the fault turns out to be your consumer unit itself, our fuse box and consumer unit replacement page has current fixed pricing. For landlords, a repeatedly tripping RCD is also worth flagging at your next EICR inspection.
FAQs
How fast should a 30mA RCD trip?
A 30mA RCD should trip within 300 milliseconds under normal fault conditions, and even faster (within 40ms) at higher fault currents.
Is it dangerous to keep resetting a tripping RCD?
Yes, if it keeps tripping and you keep resetting it without investigating, you're likely leaving a genuine fault unaddressed.
Can a faulty RCD itself cause tripping?
Yes, though it's less common than an actual circuit fault. If testing finds no fault, the RCD itself may need replacing.
