Matakuliah
Tahun
Versi
: Mechatronics 2
: 2005
: 1.0/0
Week 11
SYSTEM SAFETY
1
Learning Outcomes
• By the end of this session, students will
understand the safety in building a PLC
based system.
2
System Integrity
• Environmental effects may cause premature
failure of the system.
• Complying with existing standards, e.g. National
Electrical Manufacturers Association (NEMA)
and International Electrotechnical Commissions
(IEC). Some issues among others
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–
–
–
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Accidentally body contact
Falling Dirt
Wind blows dust
Falling liquid, lights splash
Submersion
3
Equipment Temperature
Consideration
• The life of an electronic device depends on the
temperature at which it is operated under
(inversely proportional).
• Ensuring that the equipment is not overheated
or exceeding the manufacturer’s specification is
important.
• Any protective enclosure will raise the
temperature of the PLC. During the design
process of the enclosure, measurement should
be taken under the worst case conditions.
• The use of cooling fans require screens/ filters to
clean the air being drawn into the enclosure.
4
Fail Safe Wiring & Programming
• Use more than 1 switch to start the
system, e.g. 2 switches being pressed
simultaneously to start will prevent the
system to be accidentally started.
5
Unsafe Start/Stop Program 1)
Improved Start/Stop Program using Overriding Stop 1)
1)
Hackworth & Hackworth Jr.
6
Safety Interlocks
• Personnel are prevented from making any
physical contact with the equipment while it is
active (energized).
• Maintenance personnel however needs to have
access on the equipment.
• Monitoring access points ensuring that they are
not opened while energized is important (or the
equipment cannot be energized while personnel
is inside).
• Interlocks is used to provide this monitoring
method.
7
Safety Interlocks
• Most commonly used interlocks :
– Interlock switch.
– Pressure sensitive mat switch.
– Light curtains.
8