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Q&A: Safety Modules

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ANSI B11.19

Description: A safety module interfaces safety-related devices directly into the machine control, providing safety functionality with self-checking logic with either discrete analog components or redundant micro-processors. In addition to detecting faults within a safety-related device or circuit, a safety module can monitor its own circuitry and outputs.

Q: Why do I need a safety module?
Safety modulesA:
Safety modules can be thought of as “pre-packaged” machine logic functions that simplify the task of machine design, while increasing the level of safety. With the increased global requirements for worker safety, it has become more difficult to design control circuits that make a machine function AND that also ensures failures or faults within those circuits don’t expose workers to an increased risk of harm.

Q: For what kinds of machine logic functions are safety modules used?
Safety modules are used with two-hand control devices for the start function. Emergency Stop safety modules reliably monitor E-Stop and rope-pull devices. In addition, safety modules provide functionality like interlocked guard and safety mats monitoring, muting safeguards and providing delay outputs for controlled stops.

Q: If I add a safety module, will my safety application automatically become control reliable?
It really depends on the application, the safety module, and how the module is implemented. More than likely, you’ll improve the level of safety integrity and the fault tolerance of the circuit by adding a safety module. A word of caution however, in order to ensure that the expected level of safety is achieved, you must follow all manufacturer's instructions and all relevant standards.

Q: Are safety modules the only solution?
No, there are other solutions such as safety PLCs, safety BUS systems, configurable safety modules, interfacing modules, and extension modules. These solutions may be more appropriate, depending on your application. In comparison, safety modules are usually responsible for one or two safety functions and are an inexpensive solution.

Q: When would I use a solution other than a safety module?
Once again, what you use would depend on your application. However, in general:

  • Safety PLCs are responsible for all the safety functions of complicated machines and even at times some of the machine process.
  • Safety BUS systems are used in manufacturing cells to tie a large number of safety and safeguarding devices together, typically over longer distances.
  • Configurable safety modules solve applications in between safety PLCs/safety BUS and safety modules by providing a cost-effective, flexible platform for several safety functions.
  • Interfacing modules provide the hard contacts (relay contacts) needed for the safe switching of power for those safeguarding devices or safety modules with solid-state safety outputs. These modules have no logic and must be monitored by the safeguarding device for unsafe failures.
  • Extension modules provide additional safe switching outputs when required by a safety module. Similar to interface modules, extension modules have no logic and must be monitored by the safeguarding device.
  • Q: Are there certain applications in which a safety module is recommended?
    Yes. Two-hand control functions can be greatly simplified by using a safety module. Others include monitoring an E-Stop device string or an interlocked guard.

    Q: But aren’t safety bus systems better?
    In some situations, yes, but generally safety bus systems are expensive, complex and are considered excessive for most single machine applications. Some of the same things can also be said about safety PLCs.

    Q: What sorts of faults can a safety module detect?
    This will depend on the type of safety module, but generally safety modules detect a short circuit between input channels or secondary sources of power. They also detect improper operation, such as loss of concurrency (i.e. redundant input channels not operating together), the lack of response from a device under control (i.e. a control relay stuck ON), or an internal failure of the safety module itself.


    Q: Why can’t I just wire my safety devices to relays instead of a module?
    There are methods to do this and maintain the level of “safety” (i.e. fault tolerance) that is required. One such method is the “three-relay network,” a scheme using special relays with mechanical linkage to detect a failure of one of the three relays. Unfortunately, these methods are usually inconvenient, expensive and difficult to troubleshoot.

    Q: When do I use a muting module?
    Muting moduleA:
    Use a muting module when the safeguarding function must be suspended for machine operation, such as material-feeding or removal. However, be advised that muting can only take place during the non-hazardous portion of the machine cycle and cannot expose a worker to any hazard.

    Q: Why can’t I just use a photoelectric sensor for muting?
    Muting must occur with the same level of safety integrity as the safeguard being muted. A single photoelectric sensor can fail and cause an unintended mute cycle, thus possibly exposing an individual to a hazardous situation.

    Q: When would I use a time-delay module?
    Safety modules can provide a “safe” time-delayed output for a controlled stop function (as defined by NFPA79 Category 1 Stop). A controlled stop is useful in situations that might use regenerative braking, or with servo-controlled systems, by allowing the braking action to occur before removing power.

    Q: What’s the difference between single and dual channel?
    Typically the terms “single channel” and “dual channel” when referring to safety modules, describe the input wiring configuration. Single channel is a single control line or switching point to signal a stop. While simpler than dual channel, a single failure can cause the loss of the switching action, which would result in a failure to danger. In dual channel, redundant input channels back each other up and prevent the possibility of an unsafe situation due to a single failure.

    Q: Do all safety devices require the use of a safety module?
    Not always. Sometimes a risk assessment may determine that the safety device or the safeguard can be interfaced directly into the machine control. But, generally the use of a safety module is a very cost efficient means to greatly improve the level of safety of a machine control circuit.

    Q: Can I use one safety module for my light screen, my E-stop and my safety mat?
    Safety module with safety matA:
    It depends on the design of the safety module. The devices you’ve listed perform very differently from each other. You might require a safety module that is designed for each particular device in order to maximize functional reliability. However, there are safety modules that provide multiple functions. Always refer to the documentation provided by the manufacturer before installing your safety system.

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