Glossary

This parameter provides information on the effectiveness of a system’s ability to self-detect any possible failures within the system. Using the percentage of the detectable dangerous failures, one obtains a diagnostic coverage of better or worse quality. The numerical DC parameter is a percentage value which is calculated using values taken from a table (EN ISO 13849-1 Annex E). Depending on the measures for failure detection taken by the manufacturer, example values are provided there.

Because multiple measures are normally taken to rectify different anomalies in the same circuit, an average value or a DC_avg is calculated and can be assigned four levels:

• High DC_avg ≥99%
• Medium 90% ≤DC_avg <99%
• Low 60% ≤DC_avg <90%
• None DC_avg <60%

A diagnostic coverage of none is only permissible for systems of category B or 1.

Contact element opening or closing the circuit’s conducting path at two points in series.

Control switch whose actuator is actuated by exerting force with a foot on the pedal.

International Electrotechnical Commission

The IMQ (Italian Institute of the Quality Mark) is an association in Italy (independent third body) whose task is to check and certify the compliance of materials and equipment with safety standards (CEI standards in the electric and electronic sector). This voluntary conformity certification is a guarantee of quality, safety and technical value.

Contact element closing a circuit’s conducting path when the control switch is actuated.

Number of no-load operating cycles (i.e. without current on the main contacts), under the conditions defined by the corresponding product standard, which can be carried out without repair or replacement of mechanical parts.

Minimum force to be applied to the actuator that will cause all contacts to reach their switched position.

This parameter is used to determine the functional system quality over the mean lifetime in years before a dangerous failure occurs (other failures are not considered). The calculation of the MTTF_D is based on numerical values supplied by the manufacturers of the individual components of the system. In the absence of this data, the values can be taken from the tables with guide values included in the standard (EN ISO 13849-1 Annex C).

The evaluation results in a numerical value, divided into three categories: High, Medium or Low.

For components that are susceptible to high wear (typical for mechanical and hydraulic devices), the manufacturer supplies the value B10D for the component, i.e., the number of component operations within which 10% of the samples failed dangerously, instead of the MTTF_D of the component.
The B10D value of the component must be converted to MTTF_D by the machine manufacturer using the following formula:

Where n_op= means number of annual operations for the component.

By assuming the daily operating frequency and the daily operating hours for the machine, n_op can be calculated as follows:

Where
d_op= work days per year
h_op= operating hours per day
t_cycle= cycle time (s)

For components that are susceptible to wear, note that parameter MTTF_D is dependent not only on the component itself but also on the application.
An electromechanical device with low frequency of use, e.g. a remote switch that is only used for emergency stops, has a high MTTF_D; if the same device is used for normal processes in the operating cycle, the MTTF_D of the same remote switch could drop dramatically.

All elements of the circuit contribute to the calculation of the MTTF_D depending on their structure.
In control systems with single-channel architecture (as is the case in categories B, 1 and 2), the contribution of each component is linear and the MTTF_D of the channel is calculated as follows:

To avoid overly optimistic designs, the maximum value of the MTTF_D of each channel is limited to 100 years (for categories B, 1, 2 and 3) or 2500 years (category 4).
Channels with an MTTF_D of less than 3 years are not allowed.
For two-channel systems (categories 3 and 4), the MTTF_D of the circuit is calculated by averaging the MTTF_D of the two channels using the following formula:

Sequence of two operations, one for opening and one for closing.

Given these data, the EN ISO 13849-1 standard provides the PL of the system through a correlation table (Annex K, EN ISO 13849-1) or, alternatively, using a simplified graphic (section 4.5 of EN ISO 13849-1), through the following figure:

Maximum current for heating tests on equipment without enclosure, in free air.
Its value shall be least to equal to the maximum value of the rated operational current Ie of the equipment without enclosure, in eight-hour duty.

TÜV SÜD is an international authority claiming long-standing experience in the certification of operating safety for electrical, electromechanical and electronic products. In the course of type approval, TÜV SÜD closely inspects the quality throughout all the stages concerning product development, from software design and completion, to production and to the tests conducted according to ISO/IEC standards. The operating safety certification is obtained voluntarily and has a high technical value, since it not only certifies the electrical safety of the product, but also its specific operating suitability for use in safety applications according to the IEC 61508 standard.

Verband Deutscher Elektrotechniker (DE)

Directive on low-voltage switchgear and controlgear