Tough Interlock Switches Make Machinery Safer

Protection from the hazards of automated machinery and equipment with physical guarding can be improved by using safety interlock switches.

Designers and manufacturers of automated machinery and equipment must ensure the safety of their systems. Just installing signage or writing procedures is not enough; each of these methods of protection can be easily ignored. Therefore, guards and fencing are often required to keep operators safe from the hazards. Some guards are mechanically fixed into place, and others can be opened and closed by operators.

IDEC Corp. Safety, safety switches, standards

The IDEC HS1T interlock switch with solenoid is compact and adaptable. It’s designed to accommodate a variety of safety applications. (Image source: IDEC)

Yet even these protective measures can be removed or disrupted during operation. In many cases, the best practice is to use electrical controls to confirm the guard is closed, and to stop the equipment from operating if it is not. An even better tactic is to actively interlock the guard so it can’t be opened when the equipment is in operation. Interlock switches with solenoids are the ideal devices for providing this type of protection.

Guarding Requirements

Many types of OEM machinery or engineered systems require guarding. Some common examples are machines with hazardous mechanical motions like rotating, reciprocating, cutting, punching, shearing, and bending. This includes saws, presses, metalworking, and other machines. Even normally unattended equipment may present hazards. Packaged skids may have motion elements or heated sections. Automated manufacturing lines use industrial robotics with powerful high-speed articulated arms to move payloads and tools, presenting serious hazards.

Machines and packaged skids are fitted with guards protecting operators from the hazardous areas, but these may be removeable or have doors for maintenance activities, or to place or remove materials and tools at the appropriate safe times. Robotics systems are often enclosed in a “process cell,” essentially a locked cage to keep personnel out.

Moveable guards must be interlocked to prevent a machine from starting unless the guard is closed. Numerous standards apply to various operational equipment, industries and associated safety interlocking devices, such as:

  • 29 CFR 1910.212
  • ANSI B11.19
  • ANSI/RIA R15.06
  • ISO 14119
  • IEC 60204-1
  • ISO 12100
  • ISO14120

Best practices for applying these standards includes monitoring and locking guards.

Guarding the Guards

Even when comprehensive guarding has been applied, monitoring switches are required in some applications and recommended in others to ensure guards are closed and to de-energize or electrically interlock the equipment if they are not. Locking devices hold the guards closed while equipment is running. Combining these functions, de-energization and locking, provides an all-in-one solution.

The door monitoring portion of an interlock switch senses the presence of the interlocking tongue, which proves the guard is closed and closes an electrical contact. The lock monitoring portion assures the lock is energized. Contacts for both functions must be wired to a safety circuit or safety relay, which in turn stops the hazardous equipment, usually by de-energizing it.

Locking functionality is possible when the guard is positively closed and before the hazardous equipment starts. A safety circuit is used to command the locking function at the appropriate time. Two versions of interlock switch are available:

  • Spring lock types lock with a mechanical spring and must be energized to unlock
  • Solenoid lock types lock only while energized and unlock otherwise

Typically, a spring lock version is employed where the guarded machinery has hazardous elements that do not stop immediately, so the interlock switch must be energized to unlock only after the hazard is gone. This configuration is also fail-safe and will keep the guard locked during a power loss, which is required for some applications. For machinery where delayed motion is not a concern, it may be more appropriate to use a “solenoid lock” version where the safety circuit energizes the interlock switch to lock it as needed.

Interlock Switch Challenges and Features

Even though interlock switches seem to be common-sense technology, there are challenges with implementation. If the switch is too big or not adaptable enough, then it is hard for designers to implement. Another concern is if operators or the equipment itself can apply enough force to break the switch, rendering it unsafe. Equally worrisome is if personnel can bypass and defeat the interlock switch.

IDEC Corp. Safety, safety switches, standards

Automated car welding lines may be guarded with fencing and use operator entry gates outfitted with safety interlock switches. (Image source: IDEC)

For these reasons, interlock switches must be strong and are best installed in unreachable, covered or hidden locations. Here are key features to look for in an interlock switch to provide these functions:

  • High locking force:
    Metal fittings with minimal points of failure
  • Durability and integrity:
    Multiple rotary cams provide redundant strength for locking and an independent monitoring function, arranged so the lock monitoring contact can never indicate closed unless the guard is positively closed
  • Small footprint, adjustable/adaptable:
    Minimized form factor with easily rotated housing and multiple mounting options
  • Easy wiring and conduit/cable entry:
    Spring clamp wiring for vibration resistance and installation flexibility
  • Low energy consumption:
    Saves energy since solenoid versions of the device may be energized for long periods
  • Manual override capability:
    Mechanical button guarantees that local operators have ultimate control to release the switch if needed and therefore stop equipment operation

Designers should also look for interlock switches meeting the requirements of ISO 14119:203 for Lock Monitoring and marked as such. This means both the position status and the locking function of a protected guard can be monitored through the lock-monitoring contact, resulting in fewer on-board contacts and wiring to achieve redundant safety circuits and additional monitoring circuits. This integrated functionality is a significant advantage for machine designers as it simplifies their efforts, compared with the traditional method of handling these functions separately.

Application Example

Robotics is a common application where guarding and interlocking safety switches are applied. Consider an automotive manufacturing line using robots to place and weld parts (Figure 2). Operating robots create a significant hazard, so personnel must be kept away from them, in this case with fencing around the production line to create an enclosed work cell. However, access gates are still needed for personnel to service the line when stopped.

The best solution is outfitting each gate with one or more safety interlock switches wired to safety relays integrated with the robots. If any gate is open, the robots are not allowed to start. Before the robots begin operating, the controls interlock all the gates in the closed position. Once running, even if a robot improperly moves a part with enough force to overcome the interlock switch, operation will immediately stop. One concern with caged safety areas is that personnel could be locked inside the work cell. To protect against this, the interlock switch is available with an unlocking button, which is readily oriented so any personnel within the robot area can press it to release the gate and stop the robots.

Better Safety Through Toughness and Adaptability

The newest interlocking switches are tougher and more adaptable, ready to make machinery safer than ever. These switches are also easier to install and wire, providing a comprehensive safety solution.

Luiz Shida, a marketing specialist at IDEC Corporation, is responsible for safety product marketing. Since joining IDEC Japan HQ more than seven years ago, Shida has become certified as a safety sub-assessor in Japan and is a specialist in safety products.

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