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Important Factors Should Be Considered for RCD Testing

When choosing a testing method for residual current devices, there are several important factors that you need to consider. Some of these factors include the delay angle of the residual current, reliability, and Earth loop impedance figures. These factors are important when choosing an RCD, and they should be consulted with the manufacturer for advice. Ultimately, your safety and protection are of utmost importance.

Current delay angles of the residual current

A laboratory test was conducted to measure the current delay angles of the residual-current-delay curve during RCD testing. The results are presented in Figure 20. Hence, the current delay angles of the residual-current-delay curve are the same in both tests. This implies that the effectiveness of RCD protection in the presence of harmonics is not affected. However, high-delay-angle curves signify a potential nuisance tripping of RCDs.


Reliability of RCD testing can be achieved by using a laboratory stand. This stand is designed to test the sensitivity of A-type RCDs and has a structure similar to Figure 7. The study was performed using eight representatives RCDs. Figure 20 shows the current delay angle of an A-type RCD. The F200 type B delivered the best trade-off between the resistance to high temporary leakage currents and the safety level required. It ensures the accuracy and reliability of the device and the entire installation.

Earth loop impedance figures

Knowing the correct earth loop impedance figures for your RCD tests is important. There are two main methods of measuring earth loop impedance: calculation and using a loop test instrument. Earth loop impedance should be low enough to allow the circuit breaker to work properly. In the example below, we’ve used a 15-mA test current. The lower the figure, the better.

Protection against electric shock

The purpose of an RCD is to protect you from electric shock, but there are some caveats. This protective device only works if the current flows through it. It cannot protect you if the current flows from phase to neutral or vice versa. It cannot differentiate between the intended load and a person. Consequently, it is important to consider the type of protection you need when installing an RCD.


An RCD is a safety device designed to interrupt electrical currents in the event of contact between a human body and a live conductor. Its purpose is to protect people from an electric shock, but it should only operate if they detect a leakage current of more than 30 milliamps. Even a very small leakage current can cause a fatal electric shock, and a conventional circuit breaker will only interrupt a circuit when the total current is too great. Consequently, a high sensitivity RCD is essential to prevent death from electric shock.

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