Muting safety light curtains

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March 2008 Sensors & Transducers

A muting safety light curtain permits the safety protection function to be temporarily disabled for reasons related to the equipment or machine work cycle.

While there are a number of situations in which the muting function may become necessary, there are two basic applications that can be considered typical. The first is to permit the access of personnel into the hazardous area during the non-hazardous portion of an equipment cycle, for example, removal or repositioning of a work piece. The second is to permit the access of production material and to prevent access by personnel, for example, pallet entrance and exit from the hazard area.

Muting must only be activated at a point in the machine cycle when there are no risks to personnel. Suitable measures must be taken to prevent any attempt to override the protection. Where practical, the possibility of personnel remaining undetected inside the hazardous area on termination of the muting cycle must be avoided. The material, not the pallet, must be sensed, to avoid the possibility of a person passing through the access point riding on a pallet.

The configuration chosen for the muting sensors and their positioning must allow a safe discrimination between personnel and material. The four most common muting safety light curtain configurations are:

* Four muting sensors with timing control.

* Four muting sensors with sequence control.

* Two cross-beam muting sensors with timing control in a T-configuration.

* Two muting sensors with timing control in a L-configuration.

The following are among the most frequent application problems encountered when using muting safety light curtains:

* No control of the activation and de-activation time or excessive muting time sensors making man-pallet discrimination impossible.

* The muting function is achieved by connection of the safety outputs of the ESPE (electro-sensitive protective equipment) via the contacts of a selector that is activated manually or by the transit of the pallet.

* Bypassing the safety outputs of the ESPE via a conveyor activation command.

* Activating the muting function using a single muting sensor.

* Incorrect positioning of the muting sensor or the protection zone of the ESPE, thus facilitating safety system failure.

ReeR provides a range of safety light curtains with a number of selectable features. These include height and angle adjustment of the muting sensor for reliable detection of the material in transit, as well as selectable maximum muting time-out from 30 seconds to 90 minutes. ATEX rated units are available for use in explosive or dust filled areas.

An integrated override function with time-out and two selectable modes of operation allows manual latched action and impulse with automatic latching. Models are available for one-way pallet exit or two-way pallet entry and exit transit.

For more information contact Chris Huxham, Shorrock Automation, +27 (0)12 345 4449, sales@shorrock.co.za, www.shorrock.co.za

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