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System Installation
Monitoring Series-Connected Safety Interlocking Switches
When monitoring two individually mounted safety switches (as shown in Figure
3-13), a faulty switch will be detected if it fails to switch as the guard opens. In
this case, the Module will de-energize its output relays and disable its reset func-
tion until the input requirements are met (i.e., the faulty switch is replaced).
However, when a series of safety interlocking switches is monitored by a single
Module, the failure of one switch in the system may be masked or not detected at
all (refer to Figure 3-14).
Series-connected interlock switch circuits may not meet OSHA Control Reliability
or ISO13849 (EN 60954-1)Safety Category 4 requirements because of the poten-
tial of an inappropriate reset or a potential loss of the safety stop signal. A multi-
ple connection of this type should not be used in applications where loss of the
safety stop signal or an inappropriate reset can lead potentially to serious injury
or death. The following two scenarios assume two positive-opening safety
switches on each guard:
1) Masking of a failure. If a guard is opened but a switch fails to open, the
redundant safety switch will open and cause the Module to de-energize its
outputs. If the faulty guard is then closed, both Module input channels also
close, but because one channel did not open, the Module will not reset.
However, if the faulty switch is not replaced and a second "good" guard is
cycled, opening and then closing both of the Module's input channels, the
Module considers the failure to be corrected. With the input requirements
apparently satisfied, the Module allows a reset. This system is no longer
redundant and, if the second switch fails, may result in an unsafe condition
(i.e., the accumulation of faults results in the loss of the safety function).
2) Non-detection of a failure. If a good guard is opened, the Safety Module de-
energizes its outputs (a normal response). But, if a faulty guard is then
opened and closed before the good guard is re-closed, the failure on the
faulty guard is not detected. This system also is no longer redundant and may
result in a loss of safety if the second safety switch fails to switch when need-
ed.
The systems in either scenario do not inherently comply with the safety standard
requirements of detecting single faults and preventing the next cycle. In multiple-
guard systems using series-connected safety switches, it is important to periodi-
cally check the functional integrity of each interlocked guard individually.
Operators, maintenance personnel, and others associated with the operation
of the machine must be trained to recognize such failures and be instructed to
correct them immediately.
Open and close each safeguard separately while verifying that the Module outputs
operate correctly throughout the check procedure. Follow each safeguard closure
with a manual reset, if needed. If a contact set fails, the Module will not enable its
reset function. If the Module does not reset, a switch may have failed; that switch
must be immediately replaced.
This check must be performed and all faults must be cleared, at a minimum, dur-
ing periodic checkouts. If the application can not exclude these types of fail-
ures and such a failure could result in serious injury or death, then the series
connection of safety switches must not be used.
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MM-TA-12B Muting Module
Instruction Manual
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