Chapter 3.6
Doors, lids and covers must be monitored to ensure the machine control system can stop movements before a person enters a hazard zone. The safety switches used for monitoring are often also referred to as guard, door safety or position switches. Throughout this chapter we will guide you choosing the right safety switch.
1 - Machine running | 2 - Main switch off | 3 - Trapped key removed | 4 - Trapped key transferred to machine cover | 5 - Machine cover open, key trapped
Key transfer system
Interlocking of operating modes and doors
In many applications, operator access to a hazard area must be prevented as long as a machine is powered. This can be achieved by movable guards in connection with mechanic/electromechanic interlocking. A long-standing technology in this field is the so-called key transfer system also referred to as a "trapped key system" in the USA and the UK.
Mandatory
Emergency stop switches/push-buttons
The emergency stop function must be available and operational at all times. It must not be considered an alternative to other required safeguards. The international standard for it is (EN) ISO 13850.
Safety expert Matthias Schulz explains how to determine the optimal safety switch for your specific needs in this webinar. Gain practical insights and expert tips to make informed choices for enhanced safety and efficiency. The webinar starts at the 2:00 minute mark.
The switches fitted to guards are called interlocking devices in standards and come in four types. These switches are critical for safety, especially on high-risk guards. They must be installed securely to prevent unauthorised access and ensure proper operation.
1. Switches that are mechanically actuated by a rail, plate, or cam on the guard or even by the door itself. Popular types are the roller lever, roller plunger and hinge switches. These switches are all uncoded and require significant design and assembly effort, which is why they are rarely used today.
If you use them, ensure the following:
2. Switches that are actuated by a mechanic mating piece or key, known as the actuator, are still the most used safety switches today. They are easy to install and come in single or double channel technology. Many switches can also be set up or supplemented for guard locking, which keeps the guard securely closed using a magnetically powered pin or lever.
Unfortunately, these safety switches with separate actuator are vulnerable to manipulation. Simply removing the actuator from the door and inserting it into the switch allows the machine to operate with the door open.
To prevent this manipulation:
3. Proximity switch, uncoded. These switches are similar to mechanical ones with separate actuators. However, instead of being actuated mechanically, they are triggered electronically when the actuator is near the switch. For guard locking, an additional mechanical lock is required and many switch manufacturers offer both functions in a single unit. Like the uncoded mechanical switches, uncoded proximity switches are easily manipulated, and the additional requirements mentioned under switch type 2 therefore apply to them.
4. Proximity switch, coded. In the (EN) ISO 14119 standard, which is referenced in ANSI B11.19 for the USA and CSA Z432 for Canada, three different coding levels for these switches are identified, as listed in the chart.
Such key transfer sets come with high level coding and therefore reliably prevent manipulation of the guard.
There is a wide variety of options for key transfer systems:
An emergency stop is often misunderstood as a selectable safeguarding measure, but it is not.
An emergency stop is a function required in addition to other safeguards.
The Machinery Directive and Machinery Regulation say:
“The emergency stop function must be available and operational at all times, regardless of the operating mode. Emergency stop devices must be a back-up to other safeguarding measures and not a substitute for them.” MD Annex I/MR Annex 3 section 1.2.4.3
The only exceptions are:
The emergency stop comes in two categories:
EN/IEC 60204-1 also describes a stop category 2, but it must not be used for emergency stop.
Until the late 1980s all safety-related control functions were achieved with electromechanical switchgears. However today we are using all kinds of electronic and microprocessor-controlled safety devices, so a clarification concerning how an emergency stop should function became necessary.
Emergency switching off is different from emergency stop in two ways:
Normally, the only emergency switching off device on machinery today is the main switch. If it is meant for use in case of emergency, then it must be coloured red and yellow. If the main switch is not red and yellow, then it must not be switched off in case of emergency, as this could prevent the braking of hazardous movements for example.
