electrical installation ii1 wiring enclosures & support (note: all the mentioned tables in this...

Electrical Installation II 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


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.


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


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


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)


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


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?


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


Trunkings

Construction– Steel trunking (screw-on-lid)


Trunkings

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


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


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


Trunkings

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

gs, separate cpc must be used.


Trunkings

Bends and supports

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


Trunkings

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?


Cable Supports

Cable Clips– For surface-mounted PVC-insulated PVC-sheathed non-

armoured cables 10 mm.sq.


Cable Supports

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

cables.


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 ii1 wiring enclosures & support (note: all the mentioned tables in this...

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