Share
Nelson & Associates :: Machine Guarding :: Presence Sensing & Interlocks

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

  • Guarding / Safeguarding.

    Any means of effectively preventing personnel from coming in contact with the moving parts of machinery or equipment that could cause physical harm to the personnel.1,2

  • Enclosures.

    Guarding by fixed physical barriers that are mounted on a machine to prevent access to the moving parts.1,2 Enclosures are most effective when designed as part of the machine, but they can be bolted or welded to the frame or the floor.2

  • Fencing.

    Guarding by means of a fence or rail enclosure that restricts access to a machine except by authorized personnel.1 Such fence or rail should be locked to positively restrict access.2

  • Point of Operation.

    That area on a machine where material is positioned for processing or change by the machine,1 where work is actually being performed on the material.

  • Power Transmission.

    All mechanical components including gears, cams, shafts, pulleys, belts, and rods that transmit energy and motion from the source of power to the point of operation;1 to equipment of a machine.2

  • Functional Components.

    Functional components in agricultural equipment, such snapping and husking rolls, straw spreaders and choppers, cutter-bars, rotary beaters, mixing augers, feed rolls, conveying augers, rotary tillers, and similar units [in agriculture and general industry], which must be exposed for proper function.3 Such components shall be guarded to the fullest extent that will not substantially interfere with functioning of the component.3

  • Ingoing Nip Points.

    A hazard area created by two or more mechanical components rotating in opposite directions in the same plane and in close conjunction or interaction.1

  • Pinch Point.

    A pinch point is any place where a body part can be caught between two or more moving mechanical parts.2

  • Shear Points.

    A hazard area created by a reciprocal (sliding) movement of a mechanical component past a stationary point on a machine.1,2

  • Guarding by Location.

    Guarding that is the result of the physical inaccessibility of a particular hazard under normal operating conditions or use.1 Both fencing and location are very limited as safeguarding techniques and are permitted only if caution restrictions can be met.2


  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.


© Nelson & Associates, 2010