PLC installation considerations


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For a PLC installation, these are the factors that necessarily taken into account:

  1. Environmental conditions of the physical environment where it will be located.
  2. Distribution of components in the cabinet that will contain them.
  3. Correct wiring and power.

Environmental conditions of the physical environment where it will be located:

Normally-except express indication-, the environment where the PLC is located must be meet the following physical conditions:

  • Absence of vibrations, blows, etc.

  • Protection from direct exposure to sunlight or heat sources intense, as well as temperatures that exceed 50-60 degrees Celsius, approximately.

  • Discard places where the temperature drops, at some point, through below 5 degrees Celsius or where sudden changes can give origin to condensations.

  • Discard environments where relative humidity is below 20% or above 90%, approximately.

  • Absence of dust and saline environments.

  • Absence of corrosive gases.

  • Environment free of flammable gases -for security reasons-.

  • Avoid placing it next to high voltage lines, the distance being variable depending on the value of said tension.

Distribution of components in the cabinet that will contain them.

It is the norm that the PLC is placed in a metal cabinet. Before choosing it, it has to know if this cabinet needs a built-in fan to force the ventilation of the air, because the ambient temperature exceeds the specified temperature, or to incorporate a heat generating element, if condensation problems are anticipated.

The cabinet is chosen in the right size so that it contains in a clear way not only the PLC but all the elements that are next to it, so that can perform a correct job in wiring and maintenance operations.
The elements that are next to the PLC can be:

  • switch or power switches,
  • the corresponding protections,
  • relays, contactors, etc.,
  • power supplies,
  • lint strips,
  • cabling channels, etc.

The PLC can be placed in different positions; but, in general, it is situated vertically on DIN rail or perforated plate.

Regarding its distribution, the following considerations are taken into account:

  • The heat dissipating elements -mainly the PLC and the sources of feeding- are located in the upper part of the cabinet, in order to facilitate dissipation of heat generated to the outside.

  • Electromechanical elements - relays, contactors, etc. - are generators of magnetic fields due to their coils; Because of this, it is recommended Keep them away as much as possible. The transformers, meanwhile, will be located at the greatest possible distance from any part of the PLC.

Correct wiring and power

For a correct wiring it is necessary to take into account some minimum rules, among the that are found:

  • Separate the cables that conduct DC from the AC, to avoid interference.

  • Separate the cables from the inputs of the outputs.

  • If possible, separate the analog I / O conductors from the digital ones.

  • The power cables that feed contactors, power supplies, etc., run through a different channel of the I / O cables.

  • The power and I / O cables run through a different tube or channel; it is recommended between both groups of cables a minimum distance of 30 cm, if they run parallel.

  • Stable tension of the adequate value and exempt, as far as possible, from induced peaks by other devices of the installation.

  • Protections against overloads and short circuits, by means of switches magneto-thermal, fuses, etc., as well as against grounding, for the middle of differential switches.

  • Earth cable of the appropriate value and properly signalled by driver yellow green. If the installation does not have it, it is necessary to enable one, exclusively for PLCs, of -about- 3 to 5 ohms.