12-8
Series 90-30 PLC Installation and Hardware Manual
–
August 2002
GFK-0356Q
12
Designing For Safety
A good design should not only function properly and efficiently, but must also
protect personnel
and equipment from harm
. Although some basic guidelines are found in the “Installation”
chapter of this manual, it is not possible to cover every aspect of safety because of the diversity of
applications. Additionally, it is not practical for this manual to try to cover all the possible codes
and regulations that may apply to your locality or type of equipment.
You have the ultimate
responsibility to consult applicable safety codes for your locality, or that pertain to the
particular type of equipment you are designing, and ensure that your design complies with
these standards.
In the United States, the National Electric Code (NEC) has been adopted by
many localities. The United States Occupational Safety and Health Administration (OSHA)
regulations also contain many safety regulations that apply to all industrial equipment in the United
States. In the absence of local regulations, the NEC and OSHA regulations should be followed
when designing your system, in addition to the information contained in this manual. OSHA
regulations can be accessed on-line at www.osha.gov. Some key safety issues are outlined below:
Protection From Electrical Shock
Proper wiring design, including grounding and circuit protection issues, should be followed.
Personnel should be prevented from accidentally coming in contact with hazardous voltages. Also,
unauthorized personnel should be prevented from gaining access to high voltage cabinets and
panels. Interlock circuits are often used for this purpose.
Fire Prevention
The guidelines in the NEC and OSHA regulations protect against fires, especially those caused by
faulty electrical design.
Protection From Mechanical Hazards
Personnel should be protected from physical hazards, such as moving mechanisms like conveyors
or index tables or mechanical pinch points. The use of interlocked safety gates, light curtains,
safety mat switches, dual hand buttons, physical barriers (guards), etc. can be used for this purpose.
See the applicable section of the OSHA regulations for details.
Protection From Electrical Failure
In the event of a system component failure, the design should be an acceptable “fail-safe” one in
which the failure does not cause a safety hazard such as a runaway condition or a disabling of
emergency stop circuits. Emergency stop and other safety circuits should consist of hard-wired
components that tend to fail in a harmless manner.
For example, in a Master Control Relay (MCR) circuit, use series-wired, normally closed
Emergency Stop pushbutton switches and interlocks to control an electro-mechanical Master
Control Relay (see next figure). This relay should directly disable motor starters, PLC output
