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Understanding Accessibility Testing

The “Understanding the Product Safety Tests” Series

Accessibility Testing, is an integral part of the product construction review for Risk of Shock and Risk of Injury hazards. As such, it is usually done earlier in the product safety certification process, separate from the full product safety test program. Accessibility testing identifies circuits which are considered “accessible”. In most cases, it involves identifying circuits that are inadequately protected against access. The test appears to be simple to perform = checking to see if various probes will fit into openings in a product’s enclosure. However, there are many considerations in determining what probe to use, where to use it, and under what use conditions.

Risk of Shock – Accessibility: In order for the user to get “shocked”, the user must touch an electrical circuit. Ability to touch an electrical circuit is typically referred to as “accessibility”. The product safety standards specify “accessibility probes” that are used to determine what circuits are considered to be user accessible. There are different accessibility probes in different standards. Typically these same probes are also used to determine access to parts involving the potential for injury.

Accessibility Probes: The shape and size of the probes specified in the standard depend on the type of product and the age/size of the typical user (adult, children). IEC, EN, and “IEC harmonized” UL/CSA standards typically specify one set of probes while UL & CSA standards that are not harmonized to IEC standards tend to have a different set of probes. Be sure to check your product safety standards for the accessibility probes that apply to your product. Some of the more common probes include:

  • International Finger – based on an adult male hand size • UL Finger – smaller than the International Finger as it is based on the average hand size of women
  • Child Finger Probes (2) – representing two different age groups of children, with each probe including an extended, detachable length that represents a child’s arm
  • Test Pin – short straight metal pin aimed at contacting parts too close to vent openings
  • Test Chain – simulates a user’s necklace drooping into a product top vent opening
  • Fingernail Probe – used to try and pry open snap-together enclosures

Purpose of the Test: To verify that there are no openings that would allow a typical user’s finger or handheld tool to access parts involving a shock or injury hazard. For example, a user’s finger contacting a rotating motor or high voltage part. Or, a screw driver inserted into an opening.

Test Objectives:

  1. Identify any openings that provide inadequate user protection from shock and injury hazards.
  2. Identify all user accessible electrical circuits for SELV circuit review = there are ways to design an electrical circuit so that it is permissible to be accessed by the user. In many standards, these circuits are called “SELV” (Safety Extra Low Voltage) or Class II.

Test Preparation: Most safety standards require opening and removing all parts that can be removed or opened without tools before applying the accessibility probes. This can include using a Fingernail probe to try and pry open snapped-together parts.

Test Method:

  1. Opening Location: Identify all openings and their location on the product – the location of the opening may dictate which probe is used. For example, there are probes that are used only on top openings.
  2. Probe Identification: Identify the accessibility probes specified by your standard. Also identify any use restrictions or limitations such as probes only used on top openings or probes that should not be used on NEMA configured mains power receptacles.
  3. Probe Application: Apply the probes using minimal force unless indicated otherwise by your standard.
  4. Results: Identify all electrical circuits that can be contacted by an accessibility probe. Also identify any parts involving a risk of injury that can be contacted by a probe. These results must be interpreted during the product construction review.

Other Considerations:

  1. User Instructions: Circuits which the operator is instructed to reach per the user instructions are considered accessible.
  2. Connectors: For products with electrical connectors for communication or accessories (USB, headphones, etc.), these connectors are assumed to be accessible regardless of the type of connector. This is very important – all circuits connected to external connectors are generally considered user accessible (other than certified power receptacles).

Pass/Fail Criteria: Verify the exact criteria in your standard. Most standards indicate:

  1. The accessibility probes may not contact any part of the mains circuit, regardless of voltage.
  2. The accessibility probes may not contact secondary circuits operating at hazardous voltage.
  3. The accessibility probes many not contact low voltage secondary circuits that are not suitably insulated from hazardous voltages by two levels of protection (determined during the product construction review).
  4. The accessibility probes many not contact parts involving an injury hazard (high temperature, sharp edge, moving part, etc.)
  5. The accessibility probes may contact grounded parts (earthed) as well as electrical circuits verified to meet the SELV or Class II criteria. Some standards may have other secondary circuit designations that are allowed to be user accessible (i.e. impedance protected or limited current circuits).

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. Accessibility Testing is performed as part of the Risk of Shock & Risk of Injury compliance review. User access to hazardous voltages or moving parts can cause serious injury or death. It is therefore a very important test – another test that directly saves lives.

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