1. The respective C standards will determine the manner in which locks in safety circuits of a machine control system are to be used. If the designer satisfies this norm he simultaneously also satisfies the requirements of the Machine Directive.

Deviations from the C standard are nevertheless possible at the responsibility of the designer but in these cases the designer must be able to prove that the deviating solution guarantees a safety level which is at least comparable.

If there is no C standard for the case of use, additional information is provided on the use of locks in the B standards.

2. Additional monitoring switches:

An additional monitoring switch serves the purposes of error control and error detection in the event of the safety critical failure of the lock e.g. in the case of an important damage to the internal mechanics.

It is possible to forego the use of an additional monitoring switch

o if an optimal fit is guaranteed, i.e. if a robust door guide with a stable end stop is provided for the entire serviceable life of the door,
o if the admissible locking force has been observed,
o if the actuator is permanently linked with the protective devices, preferably in a concealed position,
o if the actuator consists of one part is has not made of plastic or cast metal,
o if locks have a failsafe mechanism.

Irrespective of this we recommend the use of a second monitoring switch:

o in applications in control category 4 to EN 954-1,
o in non-visible hazard sources, e.g. in the case of radiation, electrical shock and similar.

1. In the case of interlocking devices without locking (in the case of safety switches) a double arrangement is advisable for increased risks if nothing else is proposed in the respective C standard.