RCD Tests

Residual Current Device (RCDs) are a key safety component in electric installations. In practice, however, there are frequent reports of an RCD allegedly being faulty because it failed to trip during a test. It often transpires, however, that the test was carried out with a residual current that was too low, or that the tripping behaviour was misinterpreted.

The requirements for the tripping current and tripping time of RCDs are defined in the standards EN 61008, EN 62423 and the national implementation VDE 0664. The rated residual current IΔn – for example, 30 mA or 300 mA – does not represent a fixed tripping value, but rather a design value with defined tolerances.

A tripping range between 0.35×IΔn and 1.4×IΔn is permitted by the standards. A type A RCD must not trip below 0.35×IΔn and must trigger a shut-down at the latest when 1.4×IΔn is exceeded. In the case of a 30 mA type A RCD, this means that failure to trip at a test current of around 12 mA is entirely in accordance with the standard and does not indicate a fault.

Similarly, the trip times specified are critical values rather than target times. For non-time-delayed type A Residual Current Device, EN 61008 and EN 62423 specify maximum trip times of 300 ms at 1×IΔn, 150 ms at 2×IΔn and 40 ms at 5×IΔn.

False alarms are often caused by unsuitable test procedures or incorrectly selected settings on the measuring instrument. It is therefore crucial to select the correct type of RCD, to be familiar with the applicable critical values and to interpret the measurement results correctly. This helps to avoid misdiagnoses and unnecessary replacements.