IEC 61496: A Safety Standard for Electrosensitive Protective Equipment

Electrosensitive protective equipment (ESPE) can be used for a variety of purposes, from keeping fingers, hands, or arms away from a particularly hazardous part of a machine to scanning the path of an automated vehicle to encircling and safeguarding a buffer area around an industrial robot. In each of these applications, the ESPE will produce an output signal when a person or an object comes within the detection zone; the dangerous movement of the machine can then be stopped or reversed. Such equipment has long been subject to national regulations and standards, but over the past several years it has become clear that an international framework of safety requirements and a recognized terminology are also badly needed.

Taking up the challenge, standards makers within the European Committee for Electrotechnical Standardization(CENELEC) began work on an ESPE standard, provisionally designated prEN 50100. The effort became a global one with the involvement of the International Electrotechnical Commission (IEC), whose new numbering system changed the standard's designation to IEC 61496 (or EN 61496). The requirements remain essentially the same from one standard to the other, though the differing numbers have caused a measure of confusion.

Although the original designation has now been superseded, some products on the market still carry claims of having been "developed according to the draft prEN 50100." All new products should, however, refer to IEC (or EN) 61496.

The first part of the standard, IEC 61496-1, sets out the general requirements and necessary tests for ESPE, including testing for functionality (e.g., response time and number of outputs), design (e.g., electrical supply and software), and environmental stress (e.g., stress and mechanical vibration).

Four different types of ESPE are defined by IEC 61496. Fault-detection capability is the most important parameter to be considered in determining whether an apparatus should be categorized as Type 1, 2, 3, or 4. The four types are related to the categories of EN 954-1, but because parameters other than fault detection are also taken into account, they do not have precisely the same meaning here.

Originally, only what are now Types 2 and 4 were to be defined. Later work, however, pointed to a need to define four different types. The fault-detection capability of Types 1 and 3 is still under consideration in the first issue of the standard.

It does not fall within the scope of the standard to prescribe appropriate applications for the various types of equipment; rather, the standard restricts itself to the actual functioning of the ESPE and how it interfaces with the machine. When it comes to projected use, it is thus up to the manufacturer to conduct a risk analysis and decide which type is required. Merely stating that a piece of equipment complies with IEC 61496 will not be sufficient; the particular type must also be specified to prevent equipment with limited fault-detection capability from being installed in high-risk applications.

As indicated in Table I, the minimum number of outputs (output signal switching devices) required suggests that single-fault tolerance is the anticipated norm for Types 3 and 4, but not for Types 1 and 2.

The fault-detection capability of a device must be validated, and a list of faults must be assigned to electronic components such as resistors, transistors, and integrated circuits. A very useful catalog of single faults to be employed in the failure mode effects analysis (FMEA) may be found in IEC 61496-1, Annex B. (No such list exists in any other standard relating to the safety of machinery.)

Tests to evaluate the effects of single faults are to be carried out on all relevant components of the ESPE. For Type 4, fault-accumulation testing must be performed where a single fault is not detected. Testing of more than three accumulated faults is unnecessary, provided that the probability of a greater number of faults is low.

A quality system compliant with the requirements of ISO 9001 is mandated for those designing with software or complex integrated circuits. The IEC standard does not require the manufacturer to actually hold a certificate, but a quality system must be in place. It is interesting that this requirement covers not only the functionality and features of the product but also its development. In fact, most of the standard's requirements concerning software and complex electronics address the documentation and development process; measures for fault avoidance during development are strongly emphasized.

IEC 61496 specifies which kinds of environmental disturbances must be tested. Certain requirements are common to all types of ESPE, but in some cases a higher severity level will be needed for Type 4 products (see Table II). Several aspects of EMC are covered, but emissions requirements are not included.

Optional Functions

The accuracy of the sensing function of ESPE will to a large extent depend on the optical design of the particular device, which will be required to detect objects of a certain size throughout the detection zone. This parameter is tested by placing a test piece in the detection zone, often a rod of a specific diameter.

Objects with reflective surfaces positioned close to the detection zone may cause the light beam to be transmitted even if a target is present (see Figure 1). This possibility is covered by the requirement for a maximum effective aperture angle (EAA) in the ESPE. Note, here, that misalignment may also be the source of undesirable hazards.

Figure 1. Risks with reflective surfaces.