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Machine Guarding Definitions:
Presence Sensing & Interlocks


Presence Sensing Device.

A device is a mechanism or control designed for safeguarding at the point-of-operation. Devices include presence-sensing devices, movable-barrier devices…two-hand-trip devices, and two-hand-control devices.2

The most common and versatile presence sensing devices used in machine guarding are light curtains; also referred to as photo-electric devices. Such devices use an optical mechanism to create a continuous beam of light around hazardous machine components. Light curtains are designed to automatically stop a machine if the sensing field (the light beam) is interrupted by an operator’s body or another object. Presence sensing devices may also sound an alarm or initiate a different machine action other than stoppage.

Presence sensing devices may monitor large areas such as an entire shop, a door or gate on a restricted machine area, the edges of a machine (called perimeter guarding), or the point-of-operation during normal machine operation.

Presence sensing devices also include, among others, motion detectors, presence detectors, and safety beams that use microwave and infrared technology (typical of commercial automatic doors), and pressure sensitive mats.


Interlocked-Barrier Guards.

The word guard often refers exclusively to barriers designed for safeguarding at the point-of-operation. Guards include…enclosure guards, fixed-barrier guards, interlocked-barrier guards, and adjustable-barrier guards.2

An interlocked-barrier guard shuts off or disengages machine power whenever it is opened or pushed out of position.


Other Machine Guarding Definitions


  1. National Safety Council, Accident Prevention Manual for
    Industrial Operations
    , 7th Edition, 1974.
  2. National Safety Council, Accident Prevention Manual for Business &
    Industrial Operations
    , 12th Edition, 2001.
  3. 29 CFR 1928, 1928.57 – Guarding of Farm Field Equipment,
    Farmstead Equipment, and Cotton Gins.

See the following Nelson & Associates Fact Sheets:

Also search "machine guarding" at http://www.osha.gov/ to include http://www.osha.gov/SLTC/machineguarding/standards.html.


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