There are important differences between Type 2 and Type 4 safety light curtains. For personnel and plant safety it is important that designers and purchasers select the appropriate type.
Introduction
The international standard IEC 61496 specifies requirements for the design, construction and testing for two levels or ‘types’ of light curtains or screens. Type 2 and Type 4 describe different requirements to ensure that the appropriate safety-related performance is achieved. Type 2 light screens are generally lower cost, with fewer features compared to the more robust and control reliable Type 4, but they definitely have their place in the industrial setting.
Q: What are the differences between Type 2 and Type 4?
A: Two significant functional differences determine the safety rating and the pricing between Type 2 and Type 4 light screens. The first difference is that, since it only has one microprocessor, the Type 2 uses fault exclusion as its main method to ensure the integrity of the safety function. The Type 4 achieves high levels of fault tolerance through redundancy and monitoring.
The other difference has to do with the effective aperture angle (EAA) or the field-of-view of both the emitter and the receiver. Type 2 light screens have a larger field-of-view or EAA and are therefore more susceptible to optical short circuits.
Q: Tell me more about optical short circuits.
A: This is the possible effect a highly reflective surface (eg, stainless steel, glossy paint, etc,) located next to a sensing field may have on a sensor’s performance. The reflective surface may deflect one or more beams around the target object in the sensing field. This affects the detection capability of the light screen. In the worst case, such a situation may allow an object to pass undetected through the sensing field.
Q: What is the difference between a ‘type’ and a ‘category’?
A: The term ‘category’ generally refers to the machine, including the control system, and how it interfaces with protective equipment. When talking about light screens, ‘type’ refers to the specific requirements for the design, construction, and testing as defined by the international standard IEC 61496-1/-2. These standards describe what is required to ensure that appropriate safety-related performance is achieved. In other words, a Type 4 device is typically used in a Category 4 application.
Q: What safety category can I achieve with a Type 2?
A: Typically you can achieve Category 2 with a Type 2 light screen when it is used as the primary guarding. In some cases, a Type 2 light screen may be used as a supplemental safety system, in situations where more reliability is required. This would depend on the installation or application. Contact a Banner application engineer to discuss your application.
Continued on the web
For the complete article visit www.motioncontrol.co.za/+091Q08.
For links to more safety related resources and tutorials visit www.bannerengineering.com/training.
Published by kind permission of Banner Engineering.
For more information contact Rodney Topham, RET Automation, +27 (0)11 453 2468, [email protected], www.retautomation.com
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