An RCD trip happens when the device senses a difference between the current flowing in the live and neutral wires. This means some electricity finds a path to earth, which may signal a fault. You use an RCD to protect yourself from electric shock, while an MCB guards against overloads and short circuits. To see how they work differently, check this table:
| Device | Trip Current | Response Time |
|---|---|---|
| RCD | 10-30 mA | ~30 ms |
| MCB | 3-20x rated | 0.04-13 sec |
If you ever feel unsure or see repeated trips, you should always contact a qualified electrician for safety.
Key Takeaways
- RCDs keep people safe by turning off power fast when they find small electrical leaks. MCBs keep wires safe by stopping too much current from flowing.
- RCDs can trip because of broken appliances, water, bad wires, or neutral to earth faults. MCBs cannot find these problems.
- To fix RCD trips, unplug one appliance at a time. You can also turn off circuits to find the problem.
- Test your RCD with its test button every three months. This helps make sure it works and keeps your home safe.
- Call a trained electrician if you cannot find the problem. Also call if you see sparks, smell burning, or the RCD trips again and again.
How They Work
RCD Function
You use a Residual Current Device (RCD) to help keep you safe from electric shock. An RCD always checks if the current in the live wire matches the current in the neutral wire. If it finds even a small difference, like 30 milliamps, it shuts off the circuit very fast—usually in less than 40 milliseconds. This quick action helps stop bad injuries or fires. You can test your RCD by pressing the test button. This button makes a small leak on purpose to see if the RCD works. Electricians use special tools to check how fast and how sensitive the RCD is. You should test your RCD about every three months to make sure it keeps working.
Tip: Press the test button on your RCD to check it, but this does not take the place of a professional test.
MCB Function
A Miniature Circuit Breaker (MCB) keeps your wires and devices safe from too much current. It works in two ways. First, it has a strip that bends when it gets hot from too much current. Second, it has a coil that acts fast if there is a short circuit. When either part works, the MCB turns off the circuit. You have to turn it back on by hand after it trips. MCBs do not find earth leakage, so they cannot protect you from electric shock like an RCD.
Here is a table that shows how RCDs and MCBs work:
| Device Type | Operating Principle | Rated Current | Breaking Capacity | Trip Settings | Sensitivity | Application |
|---|---|---|---|---|---|---|
| MCB | Bimetallic strip (overload), solenoid (short circuit) | Up to 125A | Up to 10kA | Fixed (B, C, D curves) | N/A | Lighting, fans, small appliances |
| RCD | Detects imbalance between live and neutral | N/A | N/A | Trips on leakage | 30mA typical | Electric shock and fire protection |
Key Differences
RCDs and MCBs do not do the same job. An RCD trips when there is a current imbalance, which means some electricity is leaking to earth. This keeps people safe from electric shock. An MCB trips only when there is too much current, like during an overload or short circuit. This keeps your wires and devices safe.
- RCDs act very fast (in milliseconds) and help keep people safe.
- MCBs work when there is too much current (in seconds or minutes) and keep equipment safe.
- RCDs have a test button, but MCBs do not.
- You often see both devices used together for better safety.
Note: RCDs do not take the place of MCBs. You need both to keep your home safe from electrical problems.
Common Causes of RCD Trip
Faulty Appliances
Sometimes, an RCD trip happens when you use certain appliances. Faulty appliances can let small amounts of electricity leak to earth. This leak might happen slowly or all at once. For example, a kettle or toaster with bad insulation can let electricity escape. A Whirlpool bath with a rusty wire or a new microwave can also cause trouble. In one case, a person unplugged the bath and had fewer trips. Electricians say to unplug things one at a time to find the problem. Even small things like computers or medical gear can make the RCD trip. If your circuit trips a lot, check for these devices. MCBs do not find these problems because they only work with high current, not small leaks.
Tip: If you think an appliance is the problem, unplug it and see if the RCD stops tripping. Always call a professional if you cannot find the cause.
Moisture and Dampness
Water is a common reason for RCD trips. Water can get into sockets, outdoor lights, or appliances and let electricity leak to earth. This happens a lot in kitchens, bathrooms, or outside. Heating parts, like in water heaters, can also let in water over time. When water mixes with dust or dirt, the risk of leaks goes up. You might see this after heavy rain or if you spill water near outlets. MCBs do not trip for this because the current is not high enough.
Here is a table that shows how experts measure the effect of water and dirt on electrical safety:
| Metric | Description | Unit/Measure |
|---|---|---|
| Equivalent Salt Deposit Density (ESDD) | Measures salt on insulator surfaces, showing contamination level | mg/cm² |
| Leakage Current | Shows how much current leaks due to moisture and dirt | Amperes (A) |
| Root Mean Square Error (RMSE) | Checks how well experts can predict leakage current | Unit of leakage current error |
| Mean Absolute Percentage Error (MAPE) | Shows the accuracy of leakage current predictions | Percentage (%) |
| Mean Absolute Error (MAE) | Measures the average error in leakage current predictions | Unit of leakage current error |
| Coefficient of Determination (R²) | Shows how well the prediction matches real leakage data | Dimensionless (0 to 1) |
Damaged Wiring
Broken wires are another big reason for RCD trips. Wires can get damaged by mice, nails, or just get old. When the cover on the wire breaks, electricity can leak to earth. This leak may not be enough to trip an MCB, but it is enough for an RCD. Every year, bad wiring causes many fires and injuries. Experts say to check your wiring if your RCD trips a lot. In one case, a broken wire in a light circuit made the RCD trip over and over. When the wire was fixed, the problem stopped.
Note: If your RCD trips often, have your wiring checked by a qualified electrician.
Neutral to Earth Faults and RCD Trip
Neutral to earth faults can also make an RCD trip. This happens when the neutral wire touches the earth wire or a metal part. Bad insulation and water make these faults more likely. When this fault happens, the RCD sees a difference in the live and neutral currents and trips to keep you safe. You may not see any problem with your appliances, but the RCD still trips. MCBs do not find these faults because the total current does not go up.
You should know that neutral to earth faults are hard to find. They often need special tests and tools. If you think you have this kind of fault, always call a professional.
Troubleshooting
When you face an RCD trip, you can follow a step-by-step process to find the cause. This helps you stay safe and may solve the problem before you need to call an electrician.
Unplugging Appliances
Start by unplugging all appliances from their sockets. For built-in devices like fridges, use the dedicated switch to disconnect them. Here is a simple way to check if an appliance is causing the issue:
- Unplug every appliance in the affected area.
- Try to reset the RCD to restore power.
- If the power comes back, plug in each appliance one at a time.
- Watch for another RCD trip as you reconnect each device.
- If the RCD trips again after plugging in a certain appliance, you have likely found the faulty item.
- If the RCD does not reset, move on to isolating circuits.
Tip: Always make sure the power is off before you touch any plugs or wires.
Isolating Circuits
If unplugging appliances does not fix the problem, you may need to isolate circuits. This means turning off different circuit breakers to see which part of your home is causing the RCD trip. Electricians often use a method called selectivity. They set up RCDs with different sensitivities and time delays. This setup helps only the RCD closest to the fault to trip, making it easier to find the problem area. You can isolate circuits by switching off one breaker at a time and checking if the RCD resets. This process helps you narrow down the source of the fault.
Insulation Resistance Testing
Sometimes, the problem comes from damaged wiring or poor insulation. Electricians use insulation resistance testing to check the health of wires. They measure how well the insulation keeps electricity from leaking. The test uses a special meter and follows strict rules, like adjusting for temperature and making sure the wires are dry. If the resistance drops below a safe level, the test shows a fault. This method helps find hidden problems that you cannot see.
When to Call a Professional
You should call a qualified electrician if you cannot find the cause of the RCD trip, or if you notice signs like sparking, burning smells, or repeated trips. Professionals use special tools and wear protective gear to stay safe. They follow strict safety rules, such as turning off the power before working. In some cases, like hearing sparks inside a panel, you must shut off the power and get help right away. Electricians can test, repair, and make sure your system meets safety standards.
Note: Never try to fix wiring or open electrical panels yourself. Always put safety first and let a professional handle complex problems.
Now you know why an RCD might trip but the MCB does not. Things like broken appliances, water, bad wires, or neutral to earth faults can cause this. You can try unplugging things and turning off circuits to find the problem. Testing and taking care of your system often can stop these issues from happening again. If you cannot fix it or feel worried, call a licensed electrician for help. These steps are safe and work well in many homes.
FAQ
What should you do first if your RCD keeps tripping?
You should unplug all appliances in the affected area. Try to reset the RCD. If it stays on, plug in each device one at a time. Watch for the RCD to trip again. This helps you find the faulty item.
What makes an RCD trip but not the MCB?
An RCD trips when it detects a small current leaking to earth. This can happen with faulty appliances, water, or damaged wires. The MCB does not trip because the total current stays low.
What is the difference between an RCD and an MCB?
| RCD | MCB |
|---|---|
| Protects people from electric shock | Protects wires from overload |
| Trips on earth leakage | Trips on high current |
| Has a test button | No test button |
What signs show you need an electrician?
If you see sparks, smell burning, or the RCD trips often, call a licensed electrician. Do not try to fix wiring yourself. Safety comes first.
What can help prevent RCD trips in the future?
You can check appliances for damage, keep sockets dry, and test your RCD every three months. Regular checks help you catch problems early and keep your home safe.
The following is information that you may be interested in
How to Identify and Fix RCD Tripping Issues
What Causes RCD Intermittent Tripping and How to Identify It
The difference between type A and AC RCD
What Makes Type B RCD Essential for EV Chargers?
Why is Type A AC RCD not suitable for electric vehicle charging equipment?
