CONTINUATION OF SKILL ENHANCEMENT (NOVEMBER 07, 2014) Control Devices A large amount of the electrical equipment that is installed by the electrical contractor or facilities maintenance personnel involves control elements such as programmable controllers, push buttons, pilot lights, limit switches, photo electric sensors, solenoid valves, and the like. The techniques employed for physically mounting these items are reasonably similar to those for other electrical equipment. However more precision and attention to detail is required. More than any other electrical equipment, control deices that is spread out over a wide area need to be integrated into a single system. For the installer, it is analogous to buying bus bar, overcurrent devices, indicating lights, enclosures, etc., and having to assemble to create a piece of switchgear. Location and mounting. Programmable logic controllers (PLCs) and most sensors and pilot devices are intended for use in harsh environments and are available in enclosures that meet any requirements. However, the installer should check that these items are suited for environment in which they are to be located. Most sensors should be mounted on a bracket that is sturdy enough to maintain the item in a fixed position, yet have slots or other provisions for making fine adjustments of locations when required. This will greatly simplify the check-out and fine-tuning of the finished control system. Wiring. Single-conductors or multiconductor control cable(usually 2.0 sq. mm) is extensively used for wiring between control components. Techniques in installation and termination of these conductors are similar to that of power cable. Increasingly ,however, co-axial and twisted-pair cable are being used as more of the control devices being employed include micro-computers and solid-state components. More attention must be given to the quality of the connections that are made on these conductors. Use of proper tools and techniques is required. Fiberoptic cable that will be used increasingly in the future will demand even more attention to splicing and terminations. Identification. Because of the large number of conductors that interconnect the various components of a control system, it is necessary that the individual wires be identified at all locations in which they terminate. Usually, a wire-numbering scheme is established on construction drawings. Standard practice requires that self-adhesive or clip-on numbering devices conforming to the scheme be attached to each of the conductors. This will greatly simplify check-out of the system and allow trouble shooting and modification at a later date. Checkout. Since a typical control system is made up of many separate field-installed- and wired control components, a much more thorough checkout of the finished system is required than is the case of a power system. A ringout of deenergized system is usually carried out by the installer to insure that continuity exists and that items are connected to the proper terminals according to the drawings. A simulated test of the energized system is usually done in conjunction with the designers of the system.This is intended to catch miswiring that was not detected in the ringout, errors in the design, and changes that must be made to accommodate equipment added during the construction phase of the project. Such a check will greatly reduce the time required for the actual startup of the facility.
Posted on: Sat, 08 Nov 2014 12:08:00 +0000
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