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Residual-current devices (RCD) are utilised to prevent electrocution to people coming into contact with conductive electrical material. The device will disconnect the power when it detects a specific quantity of electrical current leakage, for instance if 30mA was detected then the device would immediately disconnect the circuit. Any amount of current above 30mA can be enough to cause a cardiac arrest leading to death. In addition, usually the maximum time it will take to trip and disconnect the circuit is less than ≤0.1s (100 Milliseconds).

RCD protection is available in several types to the consumer, for example, fixed RCD, combination of RCD with circuit breaker and portable RCD. Each type will be canvassed first to ensure it’s appropriate for the particular safety requirement of the installation, as well as, meet the local electricity authority regulations. Hence domestic installations, or where electrical equipment is being continuously moved and workplaces with high-risk of physical damage to electrical equipment may utilise varying types of safety protection.

The residual-current devices can be tested with the built-in ‘Test Button’ and should be tested before use then regularly according to manufacturers’ recommendations or workplace guidelines. Furthermore most devices will have a LED indicating power is on and a reset button. You can connect a small electrical appliance such as a desk lamp to check for correct function. It’s a simple procedure to perform and when the test button is pressed power is disconnected. If the check was successful, press the reset button to operate as intended. However if the device failed, do not use and have it checked by an electrician.

The product monitors the electrical current between the active conductor and neutral return conductor for detection of imbalanced or difference in current i.e. small leakage current. This is commonly referred to as a short-circuit hence the product disconnects the circuit turning off the power. It’s worth noting these products operate differently than a circuit breaker. A circuit breaker is designed to protect against overcurrent that may lead to fire and damage to electrical equipment.

Several pivotal aspects should be understood when relying on these devices. These products will switch-off the circuit power when electricity has leaked to earth and has reached the fatal level. Therefore to summarise the device will operate within the scenarios listed below:

  • RCD Trips… Positive to Earth detected.
  • RCD Trips… Positive to Negative with a good earth detected.

While the RCD won’t trip in situations below:

  • Positive to Negative.
  • Positive to a metal alloy type structure that hasn’t been earthed.

The aforementioned positive conductor to negative conductor scenario; care must be taken because if direct contact by a person touching both conductors simultaneously, won’t trip, hence they’ll be electrocuted.

RCD protection is recognised and utilised in countries around the globe however it may be titled differently, for example RCD can be called safety switches. Finally each product should be approved for your location of use. Therefore check the product specifications and compliance with relevant local electricity authority approvals.


Source by Keith G Painter

This blog is made available by BC High Light Electric for educational purposes only to give the reader general information and a general understanding on the specific subject above. The blog should not be used as a substitute for a licensed electrical professional in your city or region. Check with city laws before performing any household project.