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RCM or RCD — the life-support questionWould you put a life support circuit on an RCD?
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A good question: if you're monitoring a life-support system with an RCM, would you gang that with an RCD on the same circuit? Not one to answer from memory — it needs checking against the standards. There are three separate things going on in it.
- Do you want a critical system interrupted at all? If a fault elsewhere trips a shared RCD, the life-support system goes off with it. So no — you don't put life-support systems, or anything similar, on an RCD in the first place.
- Medical locations have their own rules. They are covered to an extent by BS 7671, but they also sit under separate standards and guidance that you'd have to follow. Don't assume BS 7671 is the whole answer on a medical installation.
- So would you fit an RCM? Only where the book provides for it, and for a specific monitoring purpose — see 532.3 below.
Why domestic is RCDs and nothing else
- 411.3.4: in domestic (household) premises, AC final circuits supplying luminaires need additional protection by a 30 mA RCD.
- 411.3.3: socket-outlets up to 32 A, and mobile equipment up to 32 A used outdoors, need it too, in any premises, not only homes.
- Socket outlets don't have to be protected from the consumer unit — you can use individual RCD socket outlets. Almost nobody does; the protection normally sits in the board.
| Device | Rough cost |
|---|---|
| RCD | £10–£15, sometimes less |
| RCBO | more than an RCD |
| RCM | around £500 |
That price gap is the real answer to "why wouldn't I just fit an RCM?" — nobody is choosing to spend £500 protecting a domestic circuit. RCMs are specialist devices for specialist reasons.
Ways in the board — RCD vs RCBO
Worth knowing because it comes up constantly in design questions. One RCD serving several circuits still occupies board space, and every circuit behind it shares its trip. Three RCBOs in the same footprint give you three independently protected circuits. You get more circuits per board with RCBOs, and only the faulty one goes off.
532.3 · p.168 · RCMs (BS EN 62020) · may be used instead of RCDs in IT systems Part 7 · medical locations (plus separate standards)
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