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I. Man-Machine Safeguarding Principles & Practices 1
I. Man-Machine Safeguarding
Principles & Practices
Safety professionals recognize that, in many factories,
workers often override or bypass safeguards intended to
protect them from injury. Reported motivation includes
real or perceived inconvenience, production incentives,
familiarity with the equipment, or simply the challenge
presented by the presence of the safeguard to be defeat-
ed.
Consequently, manufacturers are increasingly recogniz-
ing the need for, and their obligation to provide, safety
interlocks which are not easily defeated/bypassed by the
operator or maintenance personnel.
Additionally, safety standards-making groups encourage
use of interlocks which are not easily defeated using
simple, readily-available means (such as a paper clip,
tape, rubber band, piece of rope, screwdriver, etc.).
For example, the American National Standards
Institutes (ANSI) B11.19 1990, Reference Standard for
Safeguarding Machine Tools specifically requires:
.
Barrier guards that protect against unauthorized
adjustment or circumvention.
.
Interlock devices that are not easily bypassed.
.
Reduced liability
With the growing number of product liability cases, com-
panies are recognizing the benefits of designing safety
circuits with interlock devices that are difficult to defeat.
To further reduce their liability exposure, firms are select-
ing only those devices that have been tested and certi-
fied for use in safety applications by a recognized, inde-
pendent third-party agency.
Manufacturers are encouraged to surpass safety design
expectations. As cited at a recent DESIGN NEWS sem-
inar entitled Product Liability A Survival Kit for the
90s, jurors expect companies to go beyond mere com-
pliance. They give greater benefit to firms who have
designed their products with the latest state-of-the-art
machine guarding safety devices.
Why should machine guard interlocks be
tamper resistant?
Difficult to defeat is a subjective term related to work-
ers propensity to override or bypass safety devices
intended to protect them from injury. Colloquially it
means that the relevant devices or systems cannot be
defeated or bypassed using readily available means
(such as a piece of wire, tape, simple hand tool, etc.). It
implies the basic safety interlock switch design serves as
a deterrent to easily overriding or bypassing its intended
function.
What is meant by the term difficult to defeat
when related to safety interlock switches in safety
standards such as ANSI B11.19, ANSI B11.20,
ANSI-RIA 15.02, OSHA 1910.212, et al?
1
2 2
Safety interlock switch manufacturers are addressing
this requirement by:
.
Designing two-piece keyed interlocks which feature a
geometrically-unique actuating key and associated
operating mechanism which function together to
deter bypassing.
.
Designing coded-magnet sensors whose multiple
reed contacts can only be actuated in the presence
of a matched magnetic field array.
.
Encouraging positive-mode mounting of single-
piece interlock switches.
How is this requirement (difficult to defeat)
being addressed by safety interlock switch
manufacturers?
3
Positive-break safety interlocks are electromechanical
switches designed with normally-closed (NC) electrical
contacts which, upon switch actuation, are forced to
open by a non-resilient mechanical drive mechanism.
(Spring actuators are not considered positive-break
mechanisms.)
One example of a positive-break safety interlock is
shown in the photo below. This third-party certified and
widely used safety switch features a two-piece construc-
tion: an electromechanical switching mechanism and a
geometrically-unique actuator key.
A simple illustration of this design concept is shown in
Figure 2.
The actuator key is typically mounted to a movable
guard such as an access door, protective grating,
equipment hood, or plexiglass safety cover. When the
guard is closed, the actuator mates with the electro-
mechanical switching mechanism. Upon opening of the
movable guard, the actuator key mechanically rotates a
cam mechanism forcing the NC electrical contacts to
open the safety circuit.
For machine applications with residual motion after shut-
down, key actuated interlocks are available with a sole-
noid latch which, in conjunction with a time-delay,
motion detector, position sensor or other machinery sta-
tus monitor, can delay access to hazardous areas until
safe conditions exist.
What are positive-break safety interlocks?
4
Actuator key
typically mounted to
movable guard
Positive-break switch
mechanism, typically
mounted to guard
frame
Four optional
actuator key
entry locations
(one on back
of unit)
FIGURE 1 3
FIGURE 2
Conventional versus Positive-Break Contacts
MACHINE GUARD CLOSED
MACHINE GUARD OPEN
MACHINE GUARD CLOSED
MACHINE GUARD OPEN
CONVENTIONAL NORMALLY-CLOSED CONTACTS
OPEN BY RESILIENT MECHANICAL MECHANISM.
CONTACTS MAY NOT OPEN DUE TO SPRING FAILURE
OR WELDED CONTACTS.
POSITIVE-BREAK NORMALLY-CLOSED CONTACTS
FORCED TO OPEN BY A NON-RESILIENT MECHANICAL MECHANISM.
MOVABLE
MACHINE GUARD
ACTUATING A
CONVENTIONAL
LIMIT SWITCH
WITH
SPRING-DRIVEN
CONTACTS
MOVABLE
MACHINE GUARD
ACTUATING A
SAFETY INTERLOCK
SWITCH WITH
POSITIVE-BREAK
CONTACTS 4
FIGURE 3
Conventional limit switches are typically designed to
use a spring force to open normally-closed electrical
contacts. Such designs are subject to two potential fail-
ure modes:
.
Spring failure
.
Inability of the spring force to overcome stuck or
welded contacts.
When actuated, either situation may result in an unsafe
condition due to failure to open normally-closed con-
tacts. Consequently, such designs are not certified or
recognized as suitable for safety applications.
SCHMERSAL offers several limit switches designed
with positive-break contacts in both snap-acting and
slow-action models for use in safety applications.
Are conventional electromechanical limit switch-
es
designed with positive-break contacts?
5
Devices which feature a positive-break design carry the
following internationally-recognized (IEC) safety symbol:
These designs meet the international requirements
established for such safety interlock switches.
How can I recognize positive-break safety
interlock switches?
6
Typically, the positive-break symbol
can be found on products as part of
product specification identification,
as shown below. 5
A positive linkage switch actuator is designed to elimi-
nate possible slippage between the actuator and its
mounting shaft. Examples of such designs are pinned,
square and serrated shafts (see Figure 4, below).
What is meant by a positive linkage switch
actuator, and why is it recommended for safety
applications?
Positive-mode mounting assures that an electro-
mechanical safety interlock switch is positively-actuated
when equipment or machinery shut-down is desired.
Safe Positive-Mode Mounting (Figure 5)
When mounted in the positive-mode, the non-resilient
mechanical mechanism which forces the normally
closed (N.C.) contacts to open is directly driven by the
safety guard. In this mounting mode, opening the safety
guard physically forces the N.C. contacts to open when
the guard is open.
What is positive-mode mounting and why is it
essential in safety applications?
8
FIGURE 4
Pinned
Square
Actuator
Positive Linkage
Serrated
Safety
Guard
Closed
Contacts
Closed
Contacts forced
open by non-
resilient
mechanical member
Safety
Guard
Open
FIGURE 5
POSITIVE-MODE INSTALLATION
7 6
Unsafe Negative-Mode Mounting (Figure 6)
When mounted in the negative-mode, the force applied
to open the normally-closed (N.C.) safety circuit contacts
is provided by an internal spring. In this mounting mode
the N.C. contacts may not open when the safety guard is
Open. (Here welded/stuck contacts, or failure of the
contact-opening spring, may result in exposing the
machine operator to a hazardous/unsafe area of the
machinery.)
Positive-mode installation is especially important when
using single-piece safety interlock switches. This instal-
lation mode takes full advantage of the devices posi-
tive-break design using the external force applied by
the safety guard to open the N.C. contacts.
Safety
Guard
Closed
Contacts
Closed
Contacts open
by internal
spring force
Safety
Guard
Open
FIGURE 6
NEGATIVE-MODE INSTALLATION
When mounted in the negative-mode (see Figure 6
above), single-piece safety interlock switches can be
easily defeated/circumvented by the operator often
simply by taping down the switch actuator when the
safety guard is open.
I