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Door & safety interlocks switches Door & safety interlocks switches

Summary

  • Failure mode Breakage of parts can occur if they are poorly attached or distorted.
  • Durability feature Use of simple door pull mechanisms and robust plastics can help prevent failure.
  • Cost

Good practices

  • Good practicesMeet safety standard for doors and switches
  • Good practices Use metal hinges and robust crack resistant plastics

Bad practices

  • Bad practicesFlimsy plastic parts
  • Bad practices Weak door hinges
  • Bad practices Complex door openers

Key facts

  • 15% of microwaves are replaced within 3 years because they fail or are unreliable. 

Icon key

  • Failure mode Failure mode
  • Durability features Durability features
  • Cost saving Cost saving
  • Low cost Low cost
  • Cost neutral Cost neutral
  • High cost High cost

Key failure modes

  • Microwave radiation is generated only when the door is shut so microwave ovens have several safety interlock switches, one of which senses if the door is closed.
  • Doors must be locked shut during cooking by a mechanism that prevents the door from opening.
  • Some ovens have doors that open when the door handle is pulled, others have complex mechanisms that open the door through a push button. If the door hinge distorts, the safety interlock will not function.

Early life

  • Flimsy and poorly attached parts can be distorted or broken from impacts such as door slamming.
  • Over-complex door latch mechanisms can fail if a part becomes dislodged or broken.

Later in lifetime

  • Some types of plastic may fracture if there is a notch (e.g. v-shaped groove) when under cyclic stress.
  • Repeated opening and closing can cause wear to the hinge, so that the door becomes misaligned and the safety switch does not actuate when the door is closed.
  • Switch contacts will wear and eventually fail.

Specification for improved performance – door Interlocks

The specification below provides the recommended improvements needed to improve product durability. They are ranked from bronze to gold based on the effectiveness of the change in improving durability, with bronze being the minimum and gold being the maximum.

Compliance with safety standards

  • See testing section below for minimum standards.

Use of stress crack resistant plastic; simple door mechanism

  • Ensure that all door latch and opening parts are robust and firmly attached.
  • If plastic parts are used, avoid notches. If notches in plastics are necessary use a stress crack resistant type of plastic such as ABS.
  • Avoid over-complicated door opening mechanisms; doors with handles can be pulled open and pushed shut, which avoids complex door openers completely.

Testing to demonstrate performance

  • The domestic microwave oven safety standard  EN60335-2-25 specifies that the door  opens / closes for 100,000 times with the power switch on for half of these actuations which will also test the function of the safety switch. 
  • Household appliance safety standard EN60335-1 requires switches to operate >10,000 cycles.
  • The door should open and close with the safety switch operating correctly after these tests.
  • Suppliers should provide evidence e.g. a test certificate/report from an independent test house.

Cost implications– door interlock

Improved reliability of components can reduce early returns and improve brand reputation which, in turn, can lower business costs and increase long-term sales.

To calculate overall cost savings for your business, use the WRAP cost/benefit analysis tool (currently in draft form and available on request).

Saving

Simple door pull 
Door opening design to avoid complex door openers will reduce material and manufacturing costs.

Neutral

Robust polymers 

Slightly larger material cost for using crack resistant polymers and thicker more robust mouldings.

Low cost

Switches

Switches that pass the safety standard’s switch durability test should be used, so there should be no additional cost. However, low cost inferior switches may be used by some suppliers if durable switches are not specified.

High cost