WTR2-640
Chapter 2: Functional Description
15 of 46
Fail Safe Operation
If a safety device or circuit should fail, the design must be such that the failure causes a “Safe”
condition. As an example, a safety switch must be a normally open switch. The switch must be
held closed with the device it is to protect. If the switch fails, it will go to the open condition,
tripping out the safety circuit.
At no time should the safety device fail and allow the operation to continue. For example, if a
safety switch is guarding a motor, and the safety switch fails, the motor should not be able to
run.
Safety Device Lock-Outs
Some safety devices disconnect electrical energy from a circuit. The safety devices that are
used on these temperature control units are primarily concerned with electrical power
disconnection and disabling of moving parts that may need to be accessed during the normal
operation of the machine.
Some of the safety devices utilize a manual activator. This is the method of initiating the safety
lock out. This may be in the form of a plug, lever, or a handle. Within this lockable handle,
there may be location for a padlock. Personnel servicing the equipment should place a padlock
in the lockout handle.
In addition to the safety devices listed above, these temperature control units are equipped with
a line cord plug. This allows the operator or maintenance personnel to unplug the tcu from its
power source and tag it out. The plug can then be tagged with any number of approved
electrical lockout tags available at most electrical supply stores.
Always disconnect and lockout all electrical power and pneumatic (i.e. compressed air)
sources prior to servicing or cleaning the temperature control unit. Failure to do so may
result in serious injury. No one but the person who installed the lockout may remove it.
