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RCMs, AFDDs and fuse standardsWhat's the difference between an RCD and an RCM?
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The question at the end: What's the difference between an RCD and an RCM? The answer is in the notes below.
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532 — devices for protection against fire
Residual current monitors
An RCM monitors residual current and warns, rather than disconnecting. That matters where cutting the supply would be worse than the fault itself.
Testing: BS 7671 requires an RCD to be tested at one times its rated residual current, and Guidance Note 3 recommends a half times test as well, where it should not trip — so 15 mA and 30 mA for a standard 30 mA device (Guidance Note 3 · Table 2.18(a) · p.96).
AFDDs, again
Arc fault detection devices appear here as fire-protection devices, having already appeared in 421.1.7 earlier. They shall be installed at the origin of the final circuit to be protected, on single-phase AC final circuits supplying socket-outlets not exceeding 32 A, in higher-risk residential buildings, HMOs, purpose-built student accommodation and care homes.
Fuse standards worth recognising
| Standard | What it is |
|---|---|
| BS 88 | Cartridge fuse — including the supplier's cut-out fuse |
| BS 1362 | The plug-top fuse |
| BS 3036 | Semi-enclosed rewireable fuse — the 0.725 factor |
| BS 646 | Small ceramic cartridge fuse, similar in appearance to a plug-top fuse |
532 · devices for protection against fire · RCD ≤ 300 mA 532.3 · p.168 · residual current monitors 421.1.7 · AFDDs (Part 4)
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