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Selecting an RCD comes down to three questions: how many milliamps, which type, and how it will discriminate with the upstream device. Without answering these, the device either fails to protect or keeps tripping and stops production.
What the threshold means
- 30 mA — personal protection (additional protection). Socket circuits, hand-held equipment, wet areas.
- 100 mA — intermediate; used on some distribution feeders.
- 300 mA — fire protection. Common on main incomers and large load groups.
- 500 mA and above — installation protection only; not sufficient for personal protection.
Type letter: AC, A, F, B
In industrial plants, electronic loads such as drives, power supplies and LED drivers produce residual currents with DC components. A classic type AC device cannot see them. Lines with drives need at least type A, and the drive output side needs type B. Skip this and the RCD exists on paper but will not trip in a real fault.
Discrimination: the downstream device should trip first
If the incomer has 300 mA and the sub-distribution has 30 mA, there must be both a current and a time margin between them. A time-delayed (type S) device is used at the incomer. Otherwise a small leakage blacks out the whole plant and the fault cannot be located.
Frequently asked questions
Why does the RCD keep tripping on a line with a drive?
The EMC filters in drives inherently leak current to earth, and this rises with cable length. The fix is usually the correct device type (A or B), re-evaluating the threshold and, if needed, feeding the drive from a separate circuit.
Can I use 30 mA at the incomer?
Technically possible but not advisable: the plant's total natural leakage easily exceeds 30 mA and the device trips constantly. A 300 mA time-delayed device at the incomer with 30 mA downstream is the usual solution.
How often should an RCD be tested?
The test button on the device should be pressed monthly. That only proves the mechanism works; tripping time and threshold must be measured with an instrument during periodic inspection.
When is a combined device (RCBO) preferred?
RCBOs are preferred where rail space is limited and each feeder needs its own protection. Leakage on one circuit then does not affect the others, giving per-feeder rather than group protection.