Mitigation of emission of flames, molten metal, poisonous or ignitable substance


Bart de Visser

We are developing a 1kW power supply (PSU) for medical equipment. The PSU needs to be 60601-1 2012 certified.

Subclause 13.1.2 states that emission of flames, molten metal, poisonous or ignitable substance in hazardous quantities shall not occur in single fault conditions.

To prevent this from happening, we considered the following options:

  1. We cannot use a fire enclosure. 60601-1 requires that the bottom of a fire enclosure cannot have holes. Our PSU can be used in any direction, so any side of the PSU can be the bottom. We need holes for several connectors (which don't cover the hole completely).
  2. Interpretation sheet 2 of the 60601-1 explaines that "Fire ENCLOSURES are intended to be used only where there is a significant likelihood of fire due to the presence of a source of ignition (as described in the subclause) and a significant source of fuel." We have several big 300uF 400V capacitors in our design. I think those are a great source of ignition and a great source of fuel.
  3. The PSU is placed in equipment. Using the equipment enclosure as fire enclosure is not allowed by the equipment manufacturer.
  4. We cannot test all single points of failure, because the number of SPFs is infinite.

Are we missing an option? Is our reasoning behind options 1 and 2 correct?
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Peter Selvey

Super Moderator
The problem here is your client, not your design.

Most medical devices use the fire enclosure option as the only practical solution.

Solutions which do not make use of a fire enclosure are likely to be very simple, allowing the fault testing to be performed with confidence. Typically it would be applied only to parts of a device which contains only limited components and where a fire enclosure is impractical.

Fault testing as a means to prove no fire hazard is onerous because (a) the number of parts, (b) the number of failure modes, (c) number of operating conditions such as supply voltage and loading that can influence the result, and (d) the confidence that a test result is representative for regular production.

Bart de Visser

Peter, thank you for helping out.

Are UL94 V0 connectors allowed to fill holes in the bottom of a fire enclosure?

Small round holes are allowed in the bottom, but is small spacing around a connector allowed?
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