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Safety analysis

This machine combines four independently dangerous elements: mains voltage, sustained heat, steam, and a powered blade. Each mitigation below is a requirement, not a recommendation.

Read this before building or modifying anything.

Hazard register

# Hazard Consequence Mitigation Independent of firmware?
H1 Mains contact Electrocution Heater is a sealed commercial appliance; mains section in its own earthed enclosure; line fused ✅
H2 Earth fault through wet mass Electrocution RCD / GFCI on supply ✅
H3 Thermal runaway Fire 120 °C non-resettable thermal fuse in series with heater ✅
H4 Controller failure with heater on Fire H3 above; plus firmware over-temperature abort as a second layer Partly
H5 Blade contact Severe hand injury Hardware lid interlock switch cutting motor power ✅
H6 Steam burn Scald Vent routed away from the operator; no opening above 60 °C core ❌
H7 Vent blockage Pressure build-up Vessel unsealed by design; filter checked for condensate loading ❌
H8 Motor jam under load Motor burnout, fire Current sensing with auto-reverse; motor thermal fuse Partly

The right-hand column matters most. Any mitigation that depends on software is not a safety system. Software fails, and this is a first firmware written by a first-time team. H1, H2, H3, and H5 are all handled in hardware and would still function with the ESP32 removed entirely.

The heater decision

The single most hazardous fabrication step in a machine like this is wiring a mains heating element by hand. We do not do it.

A used rice cooker is purchased whole and switched on its mains cord by a solid-state relay. This preserves the appliance's factory thermal cut-out, insulation, and earthing, protection engineered and type-tested by a manufacturer, which we cannot reproduce.

The SSR switches the appliance. It does not replace anything inside it.

Wiring rules

  1. Physical separation. Mains section (SSR, appliance cord, fusing) lives in its own earthed enclosure, separated from the low-voltage electronics. No mains conductor shares a cable route or terminal block with signal wiring.

  2. Bond metalwork to earth. Vessel and frame both bonded. Verify continuity with a multimeter before first power-on.

  3. Fuse the line at the supply, ahead of everything.

  4. Thermal fuse is series-wired to the heater, not to the controller. It must open the heater circuit regardless of what any code is doing.

  5. Lid interlock is a hardware switch in the motor supply, not a GPIO read. Firmware also checks it, as a second layer, but the switch is the safety system.

Operating rules

  • Never run unattended until twenty logged, uneventful cycles are recorded in the build log. Not "it seems fine", twenty logged cycles.
  • Never open the lid above 60 °C core temperature. Steam.
  • Never bypass the interlock to observe the blade running. There is no observation worth a hand.
  • First power-on: one person present at the disconnect switch, eye protection on, extinguisher within reach.
  • Confirm the vent path is clear before every run. The carbon filter loads with condensate over time and must be inspected.

Team responsibility

If any team member is under 18 or working unsupervised, an adult with mains wiring experience must inspect the electrical work before first power-on. This is not optional and it is not a formality.

Every safety-relevant modification is recorded in build-log.md with the date and who made it.