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| Type 2 vs. Type 4 Light Curtains |
| As machinery safety standards and safety light curtains evolve to meet new application demands,
users are faced with more choices and responsibilities than ever before. |
| A new breed of light curtain is
gaining recognition in the
United States. Developed in
Europe and classified as "Type 2," it is a
lower-cost, reduced-capability alternative
to the more robust "Type 4" high safety
performance level light curtains typically
used to safeguard machinery in the United
States. The terms and definitions of the
product "Type" are derived from the
international standard for light curtains,
IEC 61496. Understanding the capabilities
and differences between these two types
of machine safeguarding devices will help
users determine which is right for their
application. |
In most instances, Type 2 and Type 4
safety light curtains look much the same.
However, these photoelectric safeguards
are designed to satisfy vastly different
safety requirements. Essentially, Type 2
products are designed to a lower level of
safety integrity and must not be used in
applications where a Type 4 control is the
appropriate choice. Although the differences
are technical and based on various
industry standards, these devices differ in
three significant areas: |
| 1. Fault Detection Circuits |
| Type 2 light curtains lack the redundant
automatic self-checking circuits employed
in Type 4 light curtains. As a result, the
Type 2 light curtain does not meet the
OSHA or ANSI standard for the highest
safety performance level. Type 4 safety
light curtains are designed to immediately
detect the failure of a single component
within a defined response time. This is not
true of Type 2 light curtains. |
Traditional Type 4 safety light curtains
have an effective optical angle of ±2.5
degrees, while Type 2 devices have an effective optical angle of ±5 degrees. The
wider optical angle increases the possibility
of reflective surface interference,
where a reflective object near the sensing
field of the light curtain causes an optical
"short circuit." As a result, an object in the
sensing field may not be detected, as the
light "bends" or reflects around the object.
This possibility demands users take great
care during installation and alignment to
ensure proper operation of the Type 2
device. Fortunately, there are simple tests
to detect this potential hazard. The tests
must be performed during installation and
periodically afterwards for any light curtain. |
| The third difference is price. Type 2
devices are typically 15% to 30% less expensive
when compared to an equivalent
Type 4 device. The cost difference stems
from the less precise optical angle and
fewer fault detection circuits. In addition,
Type 2 light curtains typically have fewer
available features, such as exact channel
select, floating blanking, MPCE (Machine
Primary Control Element) monitoring and
MTS (Machine Test Signal). |
|
Application examples - type 2 or type 4?
|
Determining when to use a Type 2 or Type 4 safety light
curtain may best be demonstrated by reviewing a couple of
examples. While it is essential to perform a complete risk assessment
on all machines, the severity of the potential injury is
the overriding factor when deciding between a Type 2 and Type
4 safety light curtain.
First, a pharmaceutical company's packaging department
uses index tables, conveyors, filling and labeling equipment
and a multitude of moving parts.
In the assessment process, the user determined that the size
and force of the motors used on the index table and conveyor
was insufficient to cause serious injury. The worst-case injury
was defined as a potential bruise requiring simple first aid.
In this application, a Type 2 light curtain is the light curtain of
choice. It serves as an appropriate safeguarding device, while
doubling as a process control device.
In our second example, the assembly department of a
gas and pneumatic regulator manufacturer requires that an
operator continuously interface with the assembly equipment.
The workstation consists of a small pneumatic press and an
automatic self-feeding screwdriver. The operator must insert a
multitude of parts that need to be compressed while the driver
inserts screws.
The worst-case injury would require off-site medical attention.
In this application a Type 2 device is not an appropriate
safeguarding choice, but rather a Type 4 light curtain is
recommended. |
The Occupational Health & Safety
Act (OSHA) and the American National
Standards Institute (ANSI) both require
the highest level of safety performance for
safety-related machine control systems
when serious injuries can occur.
Examples of machines that require the
highest safety performance level include
machine tools, such as power presses,
shears, press brakes, robots, etc.
A Type 4 safety light curtain employs
self-checking circuitry to monitor itself for
internal faults. If it detects an internal fault,
the Type 4 safety light curtain immediately
sends a stop signal to the guarded machine
and the light curtain enters a lockout
condition. Only after replacement of the
failed component, and an appropriate
system reset, will the Type 4 light curtain
and the guarded machine be restored to
operating condition.
Because Type 2 light curtains lack the
redundancy of internal fault detection
circuits, they cannot achieve a suf-
ficiently high safety performance level and
therefore are not suitable as a safeguarding
option on machinery where OSHA or
ANSI requirements or risk assessments
require control reliability. Remember
also that a Type 2 light curtain is not
protected against dangerous failures when
exposed to extreme levels of electrical
interference sometimes found in industrial
environments. |
|
| Don't roll the Dice - Perform a risk Assessment |
Conducting a thorough risk assessment requires the user follow a formal procedure
that considers many factors when looking at machinery hazards. A risk assessment
must be applied in a consistent manner across all plant machinery. This will enable the
user to logically evaluate safety hazards and hazard-guarding solutions. The process
considers all hazards and each type of safety hazard on a given machine.
The risk assessment analyzes each hazard and estimates the risk level by breaking
it down into three components: Frequency of exposure, Probability of injury, and
Severity of the potential injury.
An operator, for instance, typically has a high level of exposure, while someone performing
maintenance does not. Probability considers machinery speed, and compares
it to a person's typical reaction time - so a fast-cycling machine will have a higher
probability of injury than one that is a relatively slow. The user must also estimate the
type of potential injury in terms of severity, ranging from a simple pinch on the low
end, to loss of a digit or even life at the other extreme.
Severity of injury should always dictate the assessed risk level. If the severity of
potential injury is high, but exposure and probability are low, a Type 2 device is not an
appropriate safeguarding option. Type 2 devices are not intended for use where ANSI
B11.19, OSHA 1910.212 or 217 apply, and should never be used on a mechanical power
press. Type 2 devices are not and cannot be made Control Reliable. |
|
A Question Of Interpretation
|
Because of these differences, Type
2 light curtains are intended for use in
machine-guarding applications where the
worst-case injury resulting from an accident
may be remedied by simple first aid.
The social, legal and political cultures of
Europe and the United States are distinct,
resulting in different interpretations of
"simple first aid." These differing interpretations
impact a user's decision as to
whether to apply a Type 2 or Type 4 device
in a given application.
In Europe, first aid is measured, in
part, on the amount of time an employee
misses from work. For example,
if a worker is injured and must go to the
hospital for stitches or other medical
procedures, yet returns to work the same
day or the next day, it would be considered
simple first aid.
In the U.S. these injuries would be
considered much more serious due to the
nature of the injury itself, lost machine and
worker productivity (the "gawk" factor,
lower employee morale, investigating the
cause of the injury, etc.), required injury
reports, preparing insurance claims and so
on.
In the U.S., first aid is defined in OSHA
1904.12. as any one-time treatment,
and any follow-up visit for the purpose
of observation of minor scratches, cuts,
burns, splinters, and so forth, which do
not ordinarily require medical care. Such
one-time treatments and follow-up visits
are considered first aid, even though they
are provided by a physician or registered
professional personnel. |
|
Based on the risk assessment results and the type of hazard, the
user can work with a safety expert to determine the most appropriate
machinery safeguards for each application.
Remember that a light curtain, whether Type 2 or Type 4, may
not be right for every machine safety application. Other safety
equipment, such as safety mats, safety switches, hard guards or a
combination of equipment may offer the optimum solution.
Machine users should reference ANSI B11.TR3, ANSI/RIA
R15.06-1999, and/or ISO 14121-1 prior to beginning their formal risk
assessment. A comprehensive discussion of safety strategy and risk
assessment, including formal procedure documentation, is available
at www.sti.com/safety/index.htm. |
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