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- August 2005 -
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Programmable Logic Controller
---- Installation ----

By Eugene Kowch, P.I.D. Consultants Inc.

Remember to allow for other items, like a Lutron interface unit, to be mounted in the same PLC enclosure.


There are many things to consider when installing a programmable logic controller (PLC).

First, the PLC will require a proper enclosure and output loads, such as room lights, should have their own relay panel. The relay panel is installed above or below the PLC enclosure depending on the output wiring of the PLC. Mount the PLC horizontally to provide proper ventilation. You cannot mount the PLC vertically, upside down, or on a flat horizontal surface. Provide a minimum clearance of 40mm between the PLC and all sides of the enclosure.

Remember to allow for other items, like a Lutron interface unit, to be mounted in the same PLC enclosure. There should also be at least 80mm of clearance between the PLC and wiring that runs parallel to the unit.

PLCs can be secured to an enclosure by using mounting rails. They are approximately 35mm high, with a depth of 7mm. If you mount the PLC on a rail, do consider using end brackets on each side of the PLC. The end bracket helps keep the PLC from sliding horizontally along the rail, reducing the possibility of accidentally pulling the wiring loose.

The ground terminal on the PLC must be connected to a good common ground reference (Earth ground). One method of providing an adequate common ground reference is connection to the incoming power system ground. Most PLCs have no internal fuses for the input power circuits, so external circuit protection is needed to ensure the safety of personnel and safe operation of the equipment itself. When operating the PLC from 110/125 VAC, it is only necessary to fuse the line (L) lead; it is not necessary to fuse the neutral (N) lead.

Most connections, indicators, and terminals on a PLC are located on its front panel. The communication ports are located on front of the PLC as are the option card slots and the mode selector switch.

 

The PLC terminals are divided into two groups. Each group has its own terminal block. The outputs and power wiring are on one block, and the input wiring is on the other. In some instances, it may be desirable to remove the terminal block for easy wiring. The terminal block is designed for easy removal with just a small screwdriver. Minimum wire sizes, color coding, and general safety practices should comply with appropriate electrical codes and standards for your area. Each terminal connection of the PLC can accept one 16 AWG wire or two 18 AWG size wire. Avoid running DC wiring close to AC wiring where possible.

The PLC systems are designed to be powered by 110/125 VAC. Electrical power in some areas where the PLCs are installed is not always stable and storms can cause power surges. Due to this, powerline filters are recommended for protecting the PLCs from power surges.

Evaluate any installations where the ambient temperature may approach the lower or upper limits of the PLC specifications. If you suspect the ambient temperature will not be within the operating specification for the PLC system, measures such as installing a cooling/heating source must be taken to get the ambient temperature within the range of specifications.

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