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Chapter 4

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. Protection of cables against Electromechanical & Mechanical Stresses.

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Chapter 4

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  1. 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

  2. Protection of cables against Electromechanical & Mechanical Stresses • Electromechanical forces may be caused by fault currents • Mechanical forces may be caused by accidental drilling of holes on a wall, the conduits own weight, etc. Electrical Installation II

  3. Conduits • 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 Electrical Installation II

  4. Conduits • 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 Electrical Installation II

  5. Conduits • 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) Electrical Installation II

  6. Conduit • 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 Electrical Installation II

  7. Conduit • 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? Electrical Installation II

  8. Conduit • 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 Electrical Installation II

  9. Trunkings • Construction • Steel trunking (screw-on-lid) Electrical Installation II

  10. Trunkings • PVC (or insulating) trunking (clip-on-lid) Electrical Installation II

  11. Trunkings • 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 Electrical Installation II

  12. Trunkings • 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 Electrical Installation II

  13. Trunkings • Can Be Used as Circuit Protective Conductor (cpc) • Steel trunkings may be used as cpc, whereas for PVC trunkings, separate cpc must be used. Electrical Installation II

  14. Trunkings • Bends and supports • Support requirements for steel trunking and insulating trunking refer to Tables 4.13 and 4.15 Electrical Installation II

  15. Trunkings • Cable Capacity - Selection principle: Trunking Factor ≥ ∑ (Cable Factors) • Refer to Tables 4.10, 4.11 and4.12 • 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? Electrical Installation II

  16. Cable Supports • Cable Clips • For surface-mounted PVC-insulated PVC-sheathed non-armoured cables  10 mm.sq. Electrical Installation II

  17. Cable Supports • Cable Cleats / Saddles • As above but for > 10 mm.sq. cables and all armoured cables. Electrical Installation II

  18. 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 trays. Electrical Installation II

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