Interlocked Guarding

in Machine Safety Blog by

Estimated reading time: 11 minutes

Interlocked guarding on a gang drill press

We always tell people to get interlocked guarding over their non-interlocked versions.  In fact, most of the safety guards and shields that Ferndale Safety offers come with an electrical interlock switch. And when wired correctly, it prevents a machine from starting and running while open.

We believe that interlock guards prevent workers from doing unsafe operations and ensure that all guards and shields are in place.  For example, The model FB/FBB milling machine shields have interlock switches on all movement axes. These interlocks can remind an operator to return the shield to its safe position before running the machine.

In one case, a worker turning on a mill almost lost their life when an improperly tightened fly cutter catapulted into his chest. Unfortunately, nobody performed a proactive machine safety assessment, and interlocked guarding was never installed.

What is Interlocked Guarding?

According to ISO 14119 (Section 3.1), the definition of an interlocking device is: “Mechanical, electrical or other types of device, the purpose of which is to prevent the operation of hazardous machine functions under specified conditions (generally as long as a guard is not closed).”

When an operator opens or removes an interlocked guard, it will automatically stop and disable restarting a machine’s movement.  We can accomplish this by using an electrical interlocking device connected to the machine’s electrical control system.

Interlocked machine guarding is useful when an operator needs to frequently access guarded parts of machinery.  Typically, we recommend interlocking a safety guard or shield if an operator requires access more than once during a work shift.

Guard-Locking Device

“Guard-locking devices” or “interlocking guard with guard-locking” operates in such a way that a safety device remains in position and locked (cannot be moved.) The guard-locking device stays locked until the machine has completed all its cycles and has come to a full stop.  Always pair guard locking devices with an interlocking device to detect the position of the guard (closed or open.)

Due to angular momentum, machine components such as flywheels store much kinetic energy and may take a long time to stop.  In this case, a guard-locking device is great because we want to keep all guarding locked and in place until everything has come to a complete stop. 

A brilliant way to achieve guard locking is using electromagnets paired with a transponder-coded switch like the Euchner Safety switch CEM-AY-C40.

The Interlock Switch

Many of Ferndale’s safety products have a built-in electrical safety interlock switch or a “safety switch,” such as their flip-up drill press shield.  An interlock switch is a type of interlocking device that detects the position of components mechanically or through non-contact means.

You have Four types of interlock switches in interlocked guarding applications.  Carefully choose which one to use for your safety application, depending on your application.

Switch Types


  • Mechanically actuated (Through the movement of an actuator)
  • Many various designs and electrical contact combinations
  • Actuators can be plunger-type, roller-type, or lever
  • Not coded
Type 2 switches which are most commonly used


  • Mechanically actuated
  • Separate and removable actuator
  • Available with built-in guard locking
  • Coded


  • Non-contact actuation
  • Magnetic, capacitive, inductive, ultrasonic, or optical actuation
  • Not recommended for safety applications
  • Not coded


  • Non-contact actuation
  • Uses coded magnets, RFID, or coded optical actuation
  • Coded

Positive-Opening Contacts and Positive-Mode Actuation

By design, type 1 and type 2 switches with positive-opening contacts have a direct mechanical linkage with the contacts. The mechanical linkage forces the normally closed contacts open upon switch actuation.  For example, opening a guard will force open a welded contact.

Always look for this symbol (arrow with circle) on a switch to be sure it has positively driven contacts.

Positive opening contacts right arrow with circle


In positive mode, the OPENING of the shield actuates (pushes in) the switch.  As shown in figure 6, When an operator flips the screen open, a mechanical cam actuates the switch closed.  Pushing in the switch actuator also forces the contacts open.  If the contacts are welded together, forcing them open will allow the machine to stop safely.

Positive mode actuation on an interlocked guard.

Fig 6, positive mode operation

Never install a switch in the negative mode for safety applications.  In this mode, someone can bypass the guarding easily by forcing the actuator closed with tape or other means.

Examples of Interlocked Guarding Products

lathe chuck interlocked guarding
Ferndale milling safety shield model fb/fbb
  1. Ferndale Safety lathe chuck shield model TF is a type of interlocked guarding that prevents entanglement hazards and helps prevent leaving keys in the chuck.
  2. The Ferndale Safety model EXT-SG is a surface grinder enclosure with an interlock switch that prevents the table hydraulics from operating while the door is open.


Dr.-lng.  Peter Kocher, Frank Schmidt.  “Interlocking devices associated with guards – principles for design and selection according to the new standard”

Images (credits)

Type 1 switch, Type 2 switch, Type 4 switch – Source: Euchner-USA Inc.
Type 3 switch: Source: