Rockwell Automation Safety Guidelines for the Application, Installation, and Maintenance of Solid-State Control - Manual (Page 16)

Publication SGI-1.1 - August 2009
16 Section 3: Application Guidelines
power from equipment before working on or near exposed circuit parts.
(See NFPA 70E, Part II.)
It should not be assumed that a shock hazard does not exist simply
because a solid-state circuit operates at low voltage levels. Standing on a
wet floor or working in a damp location can lower a person's body
impedance to the extent that off-state current from low voltage also
presents an electrical shock hazard.
If it is necessary to work on energized equipment, the guidelines detailed
in section 5.2 for Preventive Maintenance should be followed. In addition
to the specific procedures for personnel safety, care is needed when
making measurements in energized systems. First, there is a possibility of
damage to delicate instruments due to off-state current. Second, the
off-state current can lead to false conclusions when using sensitive
instruments to check for "contact continuity."
Precautions should be taken to prevent the off-state current of an output
device which is in the off-state from energizing an input device.
When a device (solid-state or electromechanical) that can produce a
leakage current in the off-state is used to provide the input to a solid-state
control, the precautions explained in section C.2.3 apply.
3.6 Avoiding Adverse
Environmental Conditions 3.6.1 Temperature
Solid-state devices should only be operated within the temperature ranges
specified by the manufacturer. Because such devices generate heat, care should
be taken to see that the ambient temperature at the device does not exceed the
temperature range specified by the manufacturer.
The main source of heat in a solid-state system is the energy dissipated in the
power devices. Since the life of the equipment can be increased by reducing
operating temperature, it is important to observe the manufacturer's
"maximum/minimum ambient temperature" guidelines, where ambient refers
to the temperature of the air providing the cooling. The solid-state equipment
must be allowed to stabilize to within the manufacturer's recommended
operating temperature range before energizing control functions.
When evaluating a system design, other sources of heat in the enclosure which
might raise the ambient temperature should not be overlooked. For example,
power supplies, transformers, radiated heat, sunlight, furnaces, incandescent
lamps, and so forth should be evaluated.
In instances where a system will have to exist in a very hot ambient
environment, special cooling methods may have to be employed. Techniques
that are employed include cooling fans (with adequate filtering), vortex coolers,
heat exchanges, and air conditioned rooms.
Over-temperature sensors are recommended for systems where special cooling
is employed. Use of air conditioning should include means for prevention of
condensing moisture.