Specifications

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Publication S117-CA001A-EN-P
Presence Sensing Safety Devices
POC Type 2 Safety Light Curtains
2-42
GuardShield Type 2
General Principles
2-Opto-electronics
Safety Switches Operator
Interface
Logic Power
R
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,
 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.
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.”
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 (e.g., 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.