Safety Light Curtains

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GuardShield Type 2

Overview

The Allen-Bradley Guardmaster GuardShield Type 2 safety light curtain is a new addition to the ABGM portfolio of Opto-Electronic safety devices. The “Type 2” designation means that this particular safety light curtain meets a lesser requirement for performance under fault conditions than the GuardShield Type 4. As a result, the GuardShield Type 2 safety light curtain can only be applied in applications where a formal risk assessment has determined that a Type 2 device is sufficient to meet the level of risk posed by the hazard.

There are a number of regulations that discuss risk assessment. The US based methods of conducting risk assessments which provide guidance for the determination of the level of risk to personnel can be found as follows:

• ANSI TR3—for machinery
  
• ANSI/RIA 15.05—for the Robotics industry
  
• ANSI/ISO 12100-1 Safety of Machinery
  

Typically in North America the use of Type 2 light curtains is not allowed for machinery or equipment covered by ANSI/OSHA regulations and standards whose requirement for performance under fault conditions is "control reliability." Type 2 light curtains do not meet the requirements of "control reliability."

The European Union requires the Risk Assessment be performed per EN ISO 14121 "Principles for Risk Assessment."

Safeguarding of the machine must be appropriate to the foreseeable risks and identified hazards. The level(s) of risk determined from the completion of a formal risk assessment indicate the level of the safety device required. In many instances it is necessary to use a combination of safety devices for adequate protection of personnel from hazards based on the risk assessment.

What Is "Type 2" and "Type 4"

Three types of safety ESPEs (Electro-Sensitive Protective Equipment) are defined within International and European standard IEC/EN 61496 Parts 1 and 2. This standard specifies the requirements for the design, construction, testing and performance of ESPEs—designed specifically to detect persons as part of a safety related system. The three types differ in their performance in the presence of faults and under influences from environmental conditions.

A Type 2 safety light curtain requires:

• A single fault resulting in the loss of detection capability,
  
• An increase in the response time,
  
• Failure of the OSSDs to go to the OFF state,
  
• Shall cause the light curtain to lock out as a result of the cycling of power, the next periodic test, or the interruption of the sensing function.
  

A Type 2 device requires the need for a periodic test of the system prior to a demand on the system, i.e., interruption of the light curtain's sensing field. This periodic test may be initiated by a simple manually operated switch (i.e., pushbutton) or by a more complex automatically controlled PLC.

Ideally the periodic test will be twice the operator interface frequency of exposure to the hazard.

The Type 4 device is required to detect a fault in its safety circuitry and immediately switch to a lock-out condition when the fault occurs. The fault detection is assured through diverse redundancy and cross monitoring of the internal circuit. A Type 4 device will fail to a safe state in the presence of an accumulation of undetected faults.

	IMPORTANT: Type 2 safety light curtains do not meet OSHA regulations nor ANSI published standards which define the requirements of “Control Reliability,” therefore it is not allowable to use a Type 2 safety light curtain device on equipment or machinery that requires “Control Reliability.”