Step 00a.10 — The fuse in the inlet, and what it does not protect¶
Step 10 of 12 · Chapter 00a — Mains safety: read before Chapter 09
Parts: none.
Do: Learn the inlet's three properties, because 10.5 asks you to prove all three with the meter. Keep the fuse at the rating supplied and never fit a larger one.
Check
You can find the fuse drawer with the machine upright, and its rating is written in the build log (verify on bench).
What you're looking at
LDO's diagram of the combined AC inlet: one moulded body carrying the C14 socket where the cord plugs in, the illuminated double-pole rocker switch, and a pull-out fuse drawer. C14 is the male inlet on the machine, C13 the socket on the cord.
The kit's AC inlet is a single module — C14 socket, illuminated rocker switch and fuse drawer in one body — not the official manual's separate filtered inlet plus rocker (09.11, 10.4). The three properties:
- The fuse is in series with Live only. Pull it and the L path opens even with the rocker on.
- The rocker switches both L and N. RepRap's requirement — "You always shall be capable to unpower totally the printer, so the printer power switch shall be on the mains" — is met by this module, and only because both poles are switched.
- The Earth spade is neither switched nor fused. It goes straight through. That is correct and it is what makes PE a protective conductor rather than a circuit.
What the fuse is for: Omron's datasheet for your SSR says "The G3NB may rupture if a short-circuit is applied to it. To protect against short-circuit accidents, install a protective device, such as a quick-burning fuse, on the power supply line." The inlet fuse is that device. What it is not for: it does not protect a person. A fault current small enough to kill will not blow a mains fuse — that is the RCD's job (00a.5) and the PE chain's (00a.6). Never fit a larger fuse to stop nuisance blowing; a fuse that keeps blowing is reporting a fault. LDO does not publish the fitted value, so read it off the drawer.
Source: Omron — G3NB datasheet (J164-E1) · RepRap — Safety · LDO wiring guide § Preparing the inlet · image: LDO inlet_layout.png
