Automation Canada - Machine Safety - 13

Therefore, a structure of Cat 3 should be considered. This isn't much
more expensive than Cat 2, and it can be easy to create with currently
available components. In addition, its redundancy meets the principles
of control reliability in certain North American standards.
In this example, the parameters that must be followed include:
* Cat 3 structure
* A diagnosis of Low to Medium
* A MTTF from Medium to High
DIAGNOSTIC: HOW TO ASSESS THE LEVEL OF DIAGNOSIS
MTTFd
In a safety system, MTTFd is the calculation of how oſten a piece of
equipment or safety system fails in a way that threatens the safety of
workers, the environment or equipment. MTTFd is critical to the
determination of the performance level of a safety system.
Contactor manufacturers like NOARK provide the B10d values for
components, which indicates the number of cycles where 10% of the
component fails to danger. MTTFd can easily be determined by the B10d
value.
For example, if a motor is a 5HP, at 460 volts 3
phase, the number of operations per day is 200,
5 days weeks, and the control voltage is 24 VDC.
The contactor that should be chosen is an
Ex9CAD11B, from the Ex9CA series that's
specially designed to meet the standard IEC
60947-4-1 and IEC 60947-5-1 (mirrored contacts
and mechanically linked contacts).
In this example application, two safety contactors can be used with
linked contacts to evaluate the position of the power contacts via an NC
auxiliary contact. This type of assembly achieves the requirements for an
average diagnostic coverage range of 90% to 99%.
CANADIAN AUTOMATION
The construction is such that if the power contacts are stuck or welded,
the NC auxiliary contact attached won't change state. This ensures that
the integrity of the logic of the circuit responsible for supervising the
contactors is maintained. The relays must see a closed loop before a
restart can happen. In the industry, this is called an external device
monitoring (EDM) function. This is usually performed by the safety
controller or a safety light curtain.
VOLUME 4, ISSUE 2
13
Automation Canada - Machine Safety

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