Mechanical safety interlock switches couple a moveable guard door with the power source of the hazard. When the guard door is opened, the power is isolated, ensuring that the machine does not pose a hazard while an operator requires access.

These switches are suitable for areas with the following requirements:

• Delayed stopping or uncontrolled process interruption could cause injury or death
• Attempts to override the switch and defeat the system must be prevented
• Machinery or equipment that require frequent access or have a possibility of causing injury or death 

Selection Criteria

There are a number of elements to consider when selecting a mechanical safety interlock switch. These include interlocking mechanisms, materials, and environment.

Video Credit: GalcoTV / CC BY 3.0

In an environment where frequent access is required or the probability of damage to the interlock is high, a heavy-duty mechanism should be used. Switches may have mechanical failure modes such as cam overrides that keep the contacts closed even if the actuator is removed or broken. It is possible to install multiple switches onto a door to increase security.

Mechanical interlock safety switches are the most popular of the safety switches; however, they can become misaligned after time when doors or fences sag. 

Interlock Mechanisms 

Keyed (tongue-style, guard) products are mechanical interlocks in which the switching element and actuator do not form a design unit, but are combined or separated during actuation. They fit to the leading edge of sliding, hinged, or lift-off machine guards and provide interlock detection of movement. These mechanical safety interlock switches often use a solenoid that applies a controlled voltage.

• Ideal for applications with sliding, hinged, and lift-off guards.
• Designed to prevent easy cheating of the safety switch.
• Needs to remain aligned with entry hole in switch body.
• Can be difficult to clean thoroughly.

Tongue operated above, hinge operated below.

Image Credit: Safetyswitch.net

Hinged interlocks are mounted over the pin of a hinged guard that opens with the positive mode of operation. Hinged interlocks with a pre-bored actuator shaft allow the existing hinge pin to be used as the mounting point. For certain types of guards, solid actuator shafts can also be used as hinge pins.

• Ideal for applications where there is access to the hinge center line.
• May not be the best choice for wide guard doors due to weight and small margin of opening creating a gap.

Manually-operated bolt interlocks are retracted by hand and provide a time delay for machine run-down. The bolt locks the machine guard in place and operates the contacts. The first few turns of the bolt open the contacts; however, the bolt is not fully retracted until the knob is turned repeatedly. 

• Spring force locking - the guard is held closed by spring force.
• Magnetic force locking - the guard is held closed by activation of the magnetic coil.

Video Credit: Electrical Engineering Resource / CC BY 3.0

Contact Configuration

Positive-break contacts are normally closed (NC) electrical contacts which, upon actuation, are forced open by a non-resilient, mechanical drive mechanism. In the event of a mechanical failure such as the breakage of a spring or weld, the contact point remains in an activated position.

True spring-actuated safety interlock switches are not considered to be positive-break devices.

Mechanical safety interlock switches with normally open (NO) contact pairs and pairs of changeover contacts are also available. Typically, changeover contacts are used in single-pole double-throw (SPDT) devices. 

Actuators 

An integrated actuator monitors the position of movable components. Movement of the actuator opens positively driven safety contacts.

An actuator that is separate from the switch body closes the contacts when the actuator is inserted into the switch head. 

Materials

Metals used include aluminum and stainless steel. Some applications may require chemically-resistant steel.

Plastic is lightweight. Glass-reinforced thermoplastic housing provides a tougher material. 

Other Considerations

Electrical specifications include maximum current, maximum AC voltage, and maximum DC voltage.  Security and safety features include:

• Emergency overrides
• Visual indicators
• Hermetically-sealed
• Explosion-proof
• Tamper-resistant

Devices with integral control units can be used to monitor multiple switches.  Products with manual time delays continue to protect equipment even after machine controls are turned off.  Some switches come with a rotating head for multiple entry points.  Mounting sizes vary.

Standards

ANSI B11.19 - Machines – Performance Criteria for Safeguarding

BS EN ISO 14119 - Safety of machinery — Interlocking devices associated with guards — Principles for design and selection

BS EN ISO 13849-1 - Safety of machinery - Safety-related parts of control systems Part 1: General principles for design

BS EN ISO 13849-2 - SAFETY OF MACHINERY - SAFETY-RELATED PARTS OF CONTROL SYSTEMS PART 2: VALIDATION

ISA 84.00.01 P1 - Functional Safety: Safety Instrumented Systems for the Process Industry Sector - Part 1: Framework, Definitions, System, Hardware and Software Requirements

RIA R15.06 - Industrial Robots and Robot Systems - Safety Requirements

Agency approval markings and certifications include:

• BG
• CE
• CSA
• EN
• IEC
• UL

References 

AutomationDirect  - Key Interlock & Cable-Pull Safety Swtiches

Banner Engineering - Safety Interlock Switches

Euchner - Safety Interlock Switches

Leviton - Mechanical Interlocks

SICK - Safety Locking Devices

Image credit:

Euchner-U.S.A., Inc.