The market is overwhelmed with various safety devices, making the design of machine safety a daunting task. Selecting an optimum safety device often depends upon understanding its proper use for a specific design.
There are different types of industrial automated equipment. They can be as simple as a pneumatic cylinder, or various automation components working simultaneously. Irrespective of the complexity of the system, it is imperative to consider the safety of the maintenance staff, integrator, or operator. Depending on the system they are installed in, safety systems can be very simple or complex. Moreover, the complexity of the safety system depends on the automated system, increasing with the latter’s complexity. The availability of different types of safety systems on the market makes the choice rather difficult.
As automated equipment have been around for so long, documentation on standard design principles, best practices, and guidelines for safety systems are available in plenty. When designing safety systems, these documents are a great resource, and it is necessary to consult them to ensure that the equipment is safe.
SIL or Safety Integrity Level is a measure of the failure rate of equipment expressed in terms of the probability of its failure. Typically, a safety-rated equipment will most likely have a published SIL number. This number is not a rating, but rather a guideline to the type of system with which it can be used. For instance, if the system has a SIL rating of 3, then all devices within the safety system must also have a SIL number of 3. There are four SIL levels and level 4 is the highest, meaning it has the lowest probability of failure.
The E-Stop or Emergency-Stop button is the most common safety device. It is the first safety device typically added to a system. Typically, the push button comes with one normally open contact for monitoring and two normally closed contacts for de-energizing. The button is colored bright red and has a yellow label. Its basic purpose is to stop all sources of motion or hazards by de-energizing the power within the system. For pneumatic equipment, engaging the E-Stop results in venting the stored pressure to the atmosphere, and turning off any STO signals for motion devices.
While there are many ways of using an E-Stop button, the most common is to couple it with a safety relay. Typically, two monitor circuits of the safety relay pass through the dual contacts on the E-Stop button. Pressing the button opens the contacts, causing a break in the redundant safety circuit, thereby triggering the safety relay, which then opens its contacts.
A safety controller or safety PLC using special safety inputs can also be useful for monitoring the state of the E-Stop button. The PLC program should incorporate opening the output contacts when the emergency stop button has been pressed. For simple systems, passing the STO signals or the control voltage for contactors through the E-Stop contacts should suffice.
The inexpensive E-Stop button is a simple way for easily stopping and de-energizing hazards within a system. It is possible to integrate them easily into either simple or complex safety systems.
