Wiring Enclosures Support

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					                 Wiring Enclosures & Support
(Note: All the mentioned tables in this course refer to, unless otherwise specified, Low
   Voltage Electrical Installation Handbook, by Johnny C.F. Wong, Edition 2004)

                                    Chapter 4

                                  Electrical Installation II                               1
               Protection of cables against
               Electromechanical & Mechanical
   Electromechanical forces may be caused by fault
   Mechanical forces may be caused by accidental
    drilling of holes on a wall, the conduits own weight,

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   Rigid Steel Conduits
    – Excellent mechanical protection
    – Good protection against fire
    – Can be used as circuit protective conductor (cpc)
    – Resistant to ultra-violet radiation from sunlight
    – BS4568-1 classifies conduits by:
     Light gauge and heavy gauge (CoP permits only heavy
      gauge conduits)
     Classes 1, 2, 3 & 4

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   Rigid Insulating Conduits / PVC conduits
    –   Ambient temperature is a major consideration
    –   Overall cost saving
    –   Separate circuit protective conductor must be used
    –   Weight reduction compared with steel conduits

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   Flexible Conduit (Metallic / PVC)

    – Used in applications involving constant movement, e.g.
      connection to a pump
    – Separate circuit protective conductor must be used
    – Where exposed to damp situation, it should have metallic
      pattern with PVC oversheath and IP rating not less than IP54
      (Table 4.16)

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   Supports:

    –   Horizontally surface-mounted
    –   Vertically surface-mounted
    –   For rigid steel conduit, refer to Table 4.5
    –   For rigid insulating conduit, refer to Table 4.7

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   Cable capacity
    - Selection principle: Conduit Factor ≥ ∑ (Cable Factors)

   Refer to Tables 4.1 to 4.4
    – Solid and stranded cables
    – Short run (3m and no bend) and long run (>3m or with bends)
    – Adaptable boxes to be installed at least every 10m straight run.

   Example:
    – How many 2.5 mm2 1-C PVC cables can be accommodated in a 20 mm
      conduit 10m long incorporating 2 bends?

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   Solution:
    From Table 4.4, conduit factor for a 20mm conduit, 10m long
    incorporating 2 bends is 141.
    Let the max. no. of cables that can be drawn in be x, and from
    Table 4.3, cable factor for 2.5mm2 1/C PVC cable is 30, then

    30x ≤ 141  x ≤ 4.7

    Therefore, we select x = 4

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   Construction
    – Steel trunking (screw-on-lid)

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   PVC (or insulating) trunking (clip-on-lid)

                        Electrical Installation II   10
   There are other trunking systems also of PVC steel
    construction, e.g. 2- and 3- compartment trunkings
    (telephone and / or data + power):-
    – Raised floor trunkings (flush with raised floor panels)
    – Underfloor trunkings
      (flush with floor screed)
    – Skirting trunkings

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   Classification

    – The higher the Class, the better the mechanical protection,
      e.g. Class 3 steel trunking  heavy protection on both
      inside and outside of trunking

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   Can Be Used as Circuit Protective Conductor (cpc)
    – Steel trunkings may be used as cpc, whereas for PVC
      trunkings, separate cpc must be used.

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   Bends and supports

    – Support requirements for steel trunking and insulating
      trunking refer to Tables 4.13 and 4.15

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   Cable Capacity
    - Selection principle: Trunking Factor ≥ ∑ (Cable Factors)
     – Refer to Tables 4.10, 4.11 and 4.12
     – For sizes and types of cable and sizes of trunking other than those given
       in the table, the no. of cables drawn into a trunking should be ≤ 45%
     – Space factor is the percentage occupancy of space inside the trunking
   Example
    – What is the max. number of 10 mm2 1-C PVC cables that can be
      accommodated in a 100mm x 50mm trunking?
      What if a space factor of 0.45 is applied?

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              Cable Supports
   Cable Clips
    – For surface-mounted PVC-insulated PVC-sheathed non-
      armoured cables  10 mm.sq.

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               Cable Supports
   Cable Cleats / Saddles
    – As above but for > 10 mm.sq. cables and all armoured cables.

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                Cable Supports
   Cable Trays and Ladders
    – Suitable for almost any mounting method but for heavier
      cables than those accommodated in a trunking.
    – The mechanical strength of cable ladders is better than cable

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