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Week #46

Understanding the Push Force Tests

The “Understanding the Product Safety Tests” Series

Push Force Testing, more appropriately titled “Enclosure” Push Force Testing, involves applyinga steady pushing force to the product’s enclosure using a specified device and force. Force
Testing requirements take into account a number of variables that affect the likelihood, location,
and level of force that a product may be subjected to in normal use. These variables include the
weight of the product and the enclosure material. In all cases, the pushing force itself is only the
pre-cursor to an engineering review of the results.

The Purpose of the Test:

  • To verify that the product’s enclosure has adequate strength to withstand the typical pushing forces associated with normal use over the life of the product

Test Method: These tests involve pushing on the product at a defined force using a defined device,
and holding the force steady for a period of time (i.e. 5 seconds). Additional test considerations

1) Force Gauge Attachment: A calibrated force gauge is used for this test, with an attachment
device for the end of the force gauge that is used to push against the enclosure while
applying the force. The standard provides specifications for the force gauge attachment
device. The size and shape of the device used to push against the enclosure can have a
significant effect on how the enclosure responds to the applied force. The devices include:

  • 30 mm diameter disc
  • Rigid version of the test finger probe
  • 12 mm diameter rod with a hemispherical tip

2) Enclosure Flex: In some standards, you must also monitor the inward displacement of a
metal enclosure while the force is applied, measuring the spacing between hazardous
electrical circuits and the enclosure. Metal enclosures should not flex to the point that they
bridge low voltage hazardous energy circuits. Some standards require a minimum clearance
to hazardous voltage circuits during the test, even if the enclosure is grounded (earthed).

3) Test Force Level: Areas of the enclosure protected and less likely to be subjected to
mechanical abuse are tested with a low force level such as 30N, while unprotected areas
are tested at a much higher force, typically 250N.

Test Locatons:

a) All Sides Tested: All operator accessible sides of the product are to be tested. Specific test

points on the enclosure should be selected with consideration to areas capable of failing the test (see next page).

b) Enclosure Bottom: Most standards also specify testing the bottom of the enclosure on products weighing up to 18 KG.

c) Enclosure Material: Plastic enclosures are more susceptible to the type of failures that would allow access. Enclosure flex and spacing to hazardous circuits is only a concern with metal enclosures.

d) Seams in the enclosure are important to test, especially for enclosure panels with wide gaps between securement screws.

e) Enclosure Vents in areas involving shock and injury hazards must be tested – with the force applied in a manner to try and push the force gauge attachment through vent openings that can bend.

Test Specifications: Several different push force tests may be required depending on the product

1) External Enclosures: Most standards specify that a push force of 250N be applied to all
enclosure external surfaces. The typical application device is a 30 mm diameter disc.

2) Internal Enclosure Walls: When a product has an internal area that is considered “operator
accessible” (i.e. behind an access door or panel), the inner walls around this area are also
subjected to a steady force test. The force is usually 30N and, depending on the standard,
the application device could be a rigid version of the test finger or the hemispherical end of
a 12 mm diameter rod.

3) Vent Openings: Vent openings are usually tested with a 30N force using a rigid version of
the test finger probe.

Test Objectives: The objectives are to find weakness in the product enclosure which includes:

  • ‍Identifying enclosure materials that are too thin.
  • Identifying plastic enclosure materials that are too brittle.
  • Identifying metal enclosure panels that are too flexible.
  • Identifying enclosure vent patterns which have inadequate strength for the material used.
  • Identifying seams and enclosure securement methods which are inadequate.

Pass/Fail Criteria:

  1. ‍The enclosure should not be damaged to the point that it allows access to a “hazard” = if a push force causes an opening in the enclosure, the opening is checked for “access” to hazards using the accessibility probes.
  2. Metal enclosures should not bend inward to the point that they contact hazardous electrical circuits. Some standards prohibit bridging high energy circuits. Some standards also require a minimum clearance distance to hazardous voltage circuits during the test.
  3. No damage to the strain relief system for cord connected products – when in doubt, the product must continue to comply with the strain relief test after the push tests.
  4. No sustained reduction to creepage & clearance distances below the distances specified in the standard – the product must continue to comply with the creepage & clearance distance requirements after the push tests.
  5. No damage to the protective earthing system – the product must continue to comply with the P.E. requirements after the push tests.

Conclusion: As you can see, we don’t simply perform the tests because they are in the standard.
Each test in the standard has a set of objectives that relate to the 6 Hazards of Product Safety.
The Force Tests are performed as part of the accessibility review for Shock, Energy, & Injury
hazards. Verifying the physical integrity of the product’s enclosure is crucial to insuring the product
continues to provide protection from a Risk of Shock, a potentially serious hazard that could lead
to death by electrocution. It is also critical to insuring that no opening develops in the enclosure
that could allow access to hazardous moving parts. It is therefore an extremely important test –
another test that directly saves lives.

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