How to work safely during maintenance
Prevent a machine restart
Emergency stops are placed around machines and equipment, but the use of an emergency stop alone will not prevent unintended machine restarts during maintenance work.
A more unheard-of international standard describes the requirements for preventing machine restarts and keeping maintenance workers safe.
Safety device manufacturers and safeguarding designers continue to develop products and solutions to protect operators who have whole body access to machines. Detecting and tracking operators within safeguarded areas has been the goal of protecting employees for a long time, this is reflected in today’s presence sensing systems. However, presence detection is only one way to provide safety for operators while they are working within the safeguarded area.
ISO 14118 (2018) – Machine Safety – Prevention of unexpected start-ups was revised in 2018. This document describes the requirements to safely perform work in safeguarded areas. It guides the reader through identifying energy sources, stored energy and external influences.
Performance Levels (PL) and Safety Integrity Levels (SIL) are not part of ISO 14118, instead, the emphasis is placed on locking energy sources in the isolated mode. This is important when safety components or fences are temporarily removed or out of use during maintenance work.
By using mechanical interlocks or Trapped Key Interlock systems there can be made sure that the safety procedures are followed correctly. Trapped Key Interlocks can be applied to all types of energy sources such as electrical, pneumatic, hydraulic and stored. When removing the key from the trapped key interlock system, the energy source gets locked in an isolated position. This key can then be used to gain access to a safeguarded area, where this key remains locked as long as the safety device is open. This prevents the key from being removed and the energy source from being switched on again.
In more complex applications, the flexibility of interlock systems ensures that multiple energy sources can be locked and/or access to the safeguarded area is possible via multiple access points. This can also be carried out sequentially so that the switching off and locking of the energy sources takes place in the correct sequence. The use of interlocks for locking energy sources is described in ISO 14118 (2018) section 5.3.
Although the ISO 14118 (2018) does not mention Categories, Safety Levels and Performance levels, this does not mean that components that are part of the safety-related part of the control cannot be used to command a machine stop (see section 6.3 for more clarification). As mentioned earlier, an emergency stop cannot be used as a safeguard against unintentional machine start-ups (ISO 14118 section 6.3.2).
When “full body access” is chosen in the design, mechanical interlock systems with a personal safety key can be considered. This key is taken into the guarded area by the operator. As long as the operator has this key in his or her possession, a machine restart is excluded. With the Fortress trapped key system, it is even possible to make the removal of this safety key mandatory. This ensures that the operator can only open the safety device (door) after the safety key has been removed.
Multiple personnel keys or lockouts can be used when more than one operator enters the safeguarded area. The recent publication of ISO / TS 19837 (2018) – Safety of machinery – Trapped key interlocking devices (interlocks) – Principles for selection and design, provides more clarity on the use of personal safety keys.