Pepperl+Fuchs Blog

Safety Considerations for Thru-beam Photoelectric Sensors, Light Curtains, and Light Grids

Posted by Sean Miller on Tue, Oct 08, 2013

Pepperl+Fuchs offers a wide variety of functional safety products that include photoelectric safety sensors. We offer single-beam safety sensors, light grids, and light curtains. It is important to know the difference as the cost varies and safety standards must be met.

Photoelectric safety sensors
For many applications, a simple thru-beam safety sensor is all that's required. This would be something similar to the type found on a garage door. These are intended to cut power to a machine when the beam is broken. For many applications, a single beam will meet the protection requirements (functional safety). Others might require more. Applications that require more coverage will likely look to a safety light grid or a safety light curtain for a solution. The terms light grid and light curtain have been used interchangeably in the past, but they do have differences.

A light grid is basically a series of single beam light sensors stacked together, usually 2, 3, or 4 to a unit, depending on the height, and can range from 300 mm to 1800 mm. These grids are often used when trying to keep a person out of an area and there is little or no risk of injury if someone were to stick part of themselves through the beam gap. Light grids are used to create electronic fences to keep people out of potentially dangerous areas where there might be moving parts, pinch points, sharp edges, or other dangerous areas that could cause physical harm.

A light curtain is basically a set of thru-beam sensors in one emitter housing and one receiver housing. This set of sensors casts a curtain of light across the protected area. The beams are closer on a light curtain than they are on a light grid. We offer beam spacing, or resolutions, down to 14 mm. Light curtains are used when you want to safeguard users from placing fingers inside an area where they could be injured. Resolutions for light curtains are offered in 14 mm, 30 mm, 60 mm, and 90 mm. The 30 mm resolution models are used for hand protection, while the 60 mm and 90 mm models are used to safeguard against bypassing or getting inside the area, but where tighter beam spacing is needed. The same options commonly found for light grids are also available for light curtains such as self monitoring, automatic restart, and optional relay outputs.

While light grids do have safety ratings, light curtains provide a higher degree of safety and come with a higher price tag than a light grid. For example, the SLC-2-R/129/151 light grid receiver has a list price of $759 US while an SLC30-600-R/129/151 light curtain receiver has a list price of $1282 US, so the difference in price is significant. Light grids are still offered to meet the same safety category as light curtains, the beam spacing is just not as tight when compared to a curtain. This is important to understand as some applications might require beams that are closer together despite having a high safety rating.

Before selecting safety products for safeguarding machines, it is important to perform a risk assessment. This is typically performed by a facility manager or whoever is in charge of safety at the installation site. This involves the likelihood of injury that could occur based on severity. The results of the assessment then determine which equipment is required. For example, a piece of machinery that could cause serious injury or even death would likely require a light grid or light curtain with a Category 4 safety rating and the appropriate beam resolution.


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Topics: Light Grids, Photoelectric Sensors

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