4.3.2 Moving guards
If the access to the danger zone is required during the normal operation or for maintenance work, electro-sensitive protective equipment, such as safety light curtains or moveable guards such as protective doors or flaps, must be used. These kinds of moveable protective devices must be position-monitored via safety switches or safety locking devices, and electrically connected with the control unit (for further requirements see EN ISO 12100).
EN ISO 14119:2014-03 essentially differentiates two types of safety switches (referred to as "interlocking devices" in the standard). "Interlocking devices without guard interlocking" and "Interlocking devices with guard interlocking". These safety switches must be set up so that they cannot be easily manipulated.
Moveable guards with safety switches (without guard interlocking)
Safety switches (without guard interlocking) are used for position monitoring of protective doors or flaps, for example. The guard can be opened at any time. As soon as the hard guard is no longer closed a stop command is generated. An appropriate safety distance from the protective device to the point of operation must be observed so that the dangerous movement is stopped in good time before the point of operation can be reached.
If a C standard or other machine-specific specifications are not available, the required safety distance S can be determined with the calculation formula provided in EN ISO 13855, for example:
|S = (K * T) + C|
|S||Minimum distance in millimeters measured from the danger zone to the safety switch|
|K||1600 mm/ms approach speed of the body or body parts in millimeters per second|
|T||Run-on of the entire system in seconds|
|C||additional distance |
(taken from table 4 of ISO 13855 if it is possible to insert fingers or hands through the opening in the direction of the danger zone before a stop signal is produced.)
Moveable guards with safety locking devices
Safety locking devices keep the guard in a closed position. They are always used when the dangerous machine function has not ended after the protective device has been opened, before a person can reach the point of operation (e.g. with long machine stopping times). With the guard interlocking the guard stays closed until the dangerous state has ended.
Machine protection is a further application area. Safety locking devices are frequently also used when undefined interruptions of the production process are to be prevented for process safety reasons (see also, EN IEC 60204‑1, Item 9.4.1).
EN ISO 14119:2014-03 differentiates with the technical configuration of power-actuated interlocking devices between two variants:
- Spring force-actuated and electrically unlocked (e.g. electrical signal)
- Power-actuated (e.g. electro-magnet) and spring-force unlocked
Safety locking devices with spring force-actuated interlocking also remain interlocked with a power failure on the entire machine and therefore keep a protective door blocked, including during the machine's stopping time. Because of this property they are preferred over the power-actuated (magnetic-force actuation) Safety Locking Devices for people protection applications. Magnetic-force actuated guard interlockings are frequently used for machine guarding.