How to Test a Consumer Unit RCD
Last updated: 7 September 2026
Press the test button on the RCD, it should trip instantly. If it does not, call an electrician immediately.
Overview
RCDs are critical safety devices that protect against electric shock. This guide covers testing your RCD using the test button.
Step-by-Step Fix
Before beginning any repair, ensure you have isolated power or water supplies. Clear your workspace, gather standard hand tools, and wear appropriate personal protective equipment (PPE).
- Before you begin, ensure you are standing on a dry surface and have dry hands. This is essential for your safety when working with electrical equipment. Make sure the area around the consumer unit is well-lit so you can clearly see the test button and the status of the RCD switches.
- Locate your consumer unit, usually found in a hallway, under the stairs, or in a cupboard. Identify the RCDs within the unit - they are typically larger than MCBs and often have a Test button marked with a T or the word Test printed on them. Some consumer units have multiple RCDs, so identify each one individually.
- Press the Test button firmly and listen for a click. The RCD should trip immediately, which means it switches to the OFF position, cutting power to all circuits it protects. You should also notice that the lights or appliances on that circuit go off. This confirms the RCD is working correctly.
- After the test, reset the RCD by pushing the switch firmly back to the ON position. The switch should click into place and stay on. If it trips again immediately or refuses to stay on, this indicates a fault on the circuit, and you should contact a qualified electrician.
- If the RCD does not trip when you press the Test button, it is faulty and may not provide protection in the event of an electric shock. Do not attempt to test the RCD multiple times if it is not tripping, as this could indicate a serious fault. Contact a qualified electrician immediately to have the RCD replaced.
- If the RCD trips when you press the test button but does not reset, there may be an earth fault on the circuit. Turn off all MCBs on that RCD, reset the RCD, and then turn the MCBs back on one by one. When the RCD trips, you have identified the circuit with the fault. This may be due to a faulty appliance or a wiring issue.
- If you have identified a faulty appliance, unplug it and try resetting the RCD again. If it holds, the appliance is the cause of the fault and should be repaired or replaced. If the RCD still trips with all appliances unplugged, there is a wiring fault that requires professional investigation.
Common Symptoms & Diagnostic Resolutions
Review the common error codes and operating symptoms below to quickly identify the root cause of the fault before ordering replacement parts.
| Symptom / Code | Likely Root Cause | Recommended Resolution |
|---|---|---|
| RCD does not trip when test button pressed | Faulty RCD mechanism or internal wiring issue | This is a serious safety issue. Do not assume the RCD is working. Contact a qualified electrician immediately to replace the RCD. |
| RCD trips but will not reset | Earth fault or leakage on one of the circuits protected by the RCD | Turn off all MCBs on the RCD, reset the RCD, and turn MCBs back on one by one to identify the faulty circuit. |
| RCD tripping at the same time each day | A timed appliance such as a water heater or freezer is causing the fault | Identify which appliance runs at that time and isolate it for testing. Unplug the appliance and see if the RCD trips at the usual time. |
| RCD tripping during or after rain | Water ingress into an external circuit, socket, or light fitting | Check all external sockets, lights, and junction boxes for signs of water ingress. If you find any, turn off that circuit at the consumer unit and call an electrician. |
Frequently Asked Questions
How often should I test my RCDs?
You should test your RCDs quarterly, which means at least four times a year. This is a quick and simple test that takes only a few seconds but provides essential verification that your safety devices are working correctly.
What is the difference between an RCD and an MCB?
An RCD protects against electric shock by detecting earth leakage. An MCB protects against overcurrent and short circuits. Modern consumer units often use RCBOs, which combine both functions.
Why does my RCD trip when I turn on a specific appliance?
This indicates that the appliance is causing an earth fault, which means there is a leakage of current to earth. Stop using the appliance immediately and have it inspected by a qualified electrical engineer.
Is it safe to run an RCD with no earth connection?
No, an RCD with no earth connection will not provide protection against electric shock. If you suspect your earthing system is faulty, contact a qualified electrician immediately.