Basic understanding of safety circuits
emergence and resolution of one open switch or fault obscures the presence of another open switch or fault. Fault masking is most likely to occur where an installation includes volt-free contacts such as relays having no other power connections beyond that for the switch connection. Where such risk is unacceptable, more sophisticated wiring systems and methodologies may be necessary. Trapped-key interlocks are often used to ensure that all guards are locked shut before operating a machine. In these systems, locks on each safety guard have keys which can only be removed when the guard is barred shut. The keys can then be taken to the control or power unit and used to activate the machinery. Similarly, the keys are held captive while the machine is activated and can only be removed from the power unit after the machine has been shut down. The keys can then be used to open the guards again. Risk assessments and the requirements of governing standards ISO 14119 covers the safety of machinery with interlocking devices associated with guards and outlines design and selection principles to ensure machinery safety. It refers to other standards for general principles of risk assessment and risk reduction in the design of machinery.
The basic function of an interlocking guard is to prevent the execution of hazardous operations it covers until that guard closes. So, if something or someone forces the guard open during operation, the guarded operation should stop. In some cases, a guard-locking device may be fitted to prevent opening of the guard during machine operation. It should be noted that although machines can operate when the guards are closed, the closure of a guard shouldn’t trigger the beginning of a hazardous operation. Instead, such operations should require a separate start command. One exception is something called a control guard
— a special type of interlocking guard with a start function capable of starting a hazardous operation when the guard is closed, without a separate start command. Also covered in ISO 14119 is the concept of a safety-system defeat. This is an action that bypasses a machine’s interlocks. For example, an operator may accidentally or deliberately rest a heavy object on a position switch while the guard is open, which in turn may grant access to workspaces that become dangerous when the machine is in operation. Properly designed safety systems make it impossible to defeat interlocks in any reasonably foreseeable manner — either manually or
with readily available objects nearby. This includes the removal of switches or actuators using tools that are used to operate the machine or are readily available such as screw drivers, hex tools, adhesive tape or wire. This also means that spare keys should not be accessible for trapped key systems. ISO 14119 puts interlocking devices into four categories: n Type 1 interlocking devices have mechanically actuated position switches with uncoded actuators such as a rotary cam, linear cam, or hinge. These are relatively easy to defeat by resting an object on the switch or holding it in position in some other way.
Figure 1: Shown here is a Banner Engineering SC10 Series safety controller designed to deliver the functionality of three safety relay modules. (Image source: Banner Engineering)
systems. Electronics give far greater flexibility in the
series so that if any section of the guard is not closed properly, the whole circuit will be open, and the machine will not run. In fact, controls in a safety circuit also require series wiring to ensure safe conditions in the event of any loosening of connections or sudden breaks in (such as severing of) the safety-component wiring. One caveat related to the series wiring of safety circuits: when a circuit contains more than four safety switches or includes frequently used switches or gates, there’s a decrease in the design’s performance level (PL r — which is detailed in the next article section) as well as an increased risk of fault masking . The latter is when the
arrangement of guards and the complexity of safety procedures than mechanical solutions. Typical electronic safety circuits only allow the machine to operate if the circuit is closed — a structure called normally closed (NC) operation. They also wire safety components in series to maximize effectiveness and minimize complexity and cost.
Consider a typical safety installation with a number of
position switches that are NC when the corresponding section of guard is closed. These position switches are wired into the installation in
we get technical
18
19
Powered by FlippingBook