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.
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.
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 is (EN) ISO 13850
Axelent Safety Webinar: What performance level do my door safety switches need?
The switches fitted to guards are called “interlocking devices” in standards and come in four types:
1. Switches that are mechanically actuated by a rail, plate or cam on the guard or even by the guard itself. These switches are all uncoded. Popular types are the roller lever, roller plunge and hinge switches. But they are rarely used today, because their installation requires quite some design and assembly effort.
If you use them, ensure the following:
2. Switches that are actuated by a mechanic mating piece or key that is mounted to the guard. It is referred to as the “actuator”. To this day, this is the “bread and butter” safety switch. It is easy to install and comes in single or double channel technology. Many switches can also be set up (or supplemented) for guard locking, holding the guard closed with a magnetically powered pin or lever.
Unfortunately, safety switches with separate actuators are very easy to defeat: Just remove the actuator from the door and plug it into the switch; then the machine will run with open door.
To prevent this dangerous manipulation:
3. Proximity switch, uncoded. These are very similar to the mechanical switches with separate actuator. They are not actuated mechanically, but electronically when the actuator is near the switch. For guard locking you need an extra mechanical locker (many switch manufacturers offer both functions in one unit). Like the uncoded mechanical switches, uncoded proximity switches are easily defeated and the additional requirements shown for type 2 also apply to them
4. Proximity switch, coded. There are three different coding levels in (EN) ISO 14119 standard. It is also referenced in ANSI B11.19 for USA and CSA Z432 for Canada:
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.
Basic function of a key transfer system:
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:
Emergency stop is generally considered a selectable safeguarding measure, but it is not.
Emergency stop is a required additional function.
The Machinery Directive and Machinery Regulation both 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:
Emergency stop comes in two so-called “stop categories”:
EN/IEC 60204-1 also describes a stop category 2, but it must not be used for emergency stop.
This term is not used very often. It refers to an emergency stop circuit with electromechanical components only.
Until the late 1980s all safety-related control functions were achieved by electromechanical switchgear. Today however, we are using all kinds of electronic and even microprocessor-controlled safety devices, often even controlled by software. Today however, we are using all kinds of electronic and even microprocessor controlled safety devices, often controlled by software.
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 red and yellow. A black or gray main switch must not be switched off in case of emergency as this could prevent braking of hazardous movements.
The international standard for emergency stop devices is (EN) ISO 13850.
