Conduit
● Screwed conduit of the welded type to BS 4568 should be used.
● Surface conduits should be supported and fixed by means of distance saddles spaced and located within 300mm of bends or fittings.
● Runs must be earthed.
● The conduit system should be completely erected before cables are drawn in.
● A space factor of at least 40% should be provided.
● The inner radii of bends should never be less than 2.5 times the outer diameter of the conduit..
● Conduit systems should be designed so that they can be sealed against the entry of dust and water.
Nevertheless, ventilation outlets should be provided at the highest and lowest points in each section of the system. These will permit the free circulation of air and provide drainage outlets for any condensation that may have accumulated in the runs.
● To maintain the fire resistance of walls, ceilings and floors, any opening made in them should be made good with materials to restore the fire integrity of the particular building element.
Trunking
● Steel trunking must comply with BS 4678
● Fittings must be used to ensure that bend radii are adequate.
● As with steel conduit, steel trunking may be used as a protective conductor provided it satisfies the IEE Wiring Regulations, it may not be used as a combined protective and neutral (pen) conductor.
● A space factor of at least 45% should be provided.
● Supports should be spaced at distances and ends should not overhang a fixing by more than 300mm.
● Trunking should not be installed with covers on the underside. Covers should be solidly fixed in passage through walls, floors and ceilings.
● On vertical runs internal heat barriers should be provided to prevent air at the topmost part of the run attaining excessively high temperatures.
Cable installation into PCC module with Digital Master Control (DMC) cubicle and wall mounted changeover contactor box adjacent to set.
Segregation of Circuits
Segregation of cables of different circuits will prevent electrical and physical contact. Three circuits are defined in the Regulations. They are:
● 1 LV circuits (other than for fire alarm or emergency lightning circuits) fed from the main supply system.
● 2 Extra low voltage or telecommunication circuits fed from a safety source (e.g. telephones, address and data transmissions systems).
● 3 Fire alarm or emergency lighting circuits.
Where it is intended to install type 1 cables in the same enclosure as telecommunication system which may be connected to lines provided by a public telecommunications system authority, the approval of that authority is necessary. Cables used to connect the battery chargers of self contained luminaries to mains supply circuits are not deemed to be emergency lighting circuits.
Cable Trays
The most common method for installing cables is by clipping them to perforated trays. The trays should be galvanised or protected with rust preventing finishes applied before erection. Cleats or clips should be of galvanised steel or brass.
Cables should be laid in a flat formation. The maximum spacing for clips and cleats should be 450mm.
Tray supports should be spaced adequately, usually about 1200mm.
Steel supports and trays should be of sufficient strength and size to accommodate the future addition of approximately 25% more cables than those originally planned.
Digital Master Control Cubicle (DMC) installed with a water companies Switchboard Suite. Access to the DMC is all front entry.
C A B L E A N D W I R I N G Section D
Trenches
Trenches within plant rooms and generator halls should be of the enclosed type with concrete slab or steel chequer plate covers. (See Fig. D5).
Fig. D5 Trench Construction
Trench bends should be contoured to accommodate the minimum bending radius for the largest cable installed.
Trenches should be kept as straight as possible. The bottoms should be smoothly contoured and arranged to fall away from the engine plinths so that water and oil spillages do not accumulate within the trenches but are drained away to a common catchment pit.
Trenches external to the building are often back-filled which should be consolidated before the cable is installed. This ensures that there is no further ground settlement. Back filling should be made in even layers.
Laid Direct in Ground
Where armoured and sheathed cables are run external to the buildings and laid direct in the ground, they should be laid on a 75mm deep bedding medium.
Every cable in the layer should be protected by interlocking cable tiles (to BS2484).
The separation distances between HV and LV cables in trenches or laid direct in the ground should be between 160mm and 400mm, depending on the space available.
Cables passing under roads, pavements, or building structures should be drawn through ducts and must be of a type incorporating a sheath and/or armour which is resistant to any mechanical damage likely to be caused during drawing-in. The ducts should be laid on a firm, consolidated base. The ends of the ducts should always be sealed by plugs until the cables are installed.
No more than one cable should occupy a ductway, providing a number of spare ways for future cables (say 25% more than those initially required).
Cable Termination
The termination of any power cable should be designed to meet the following
requirements:-● Electrically connect the insulated cable conductor(s) to electrical equipment
● Physically protect and support the end of cable conductor, insulation, shielding system, and the sheath or armour of the cable
● Effectively control electrical stresses to give the dielectric strength required for the insulation level of the cable system.
It is only necessary on LV systems to apply tape from the lower portion of the terminal lug down onto the conductors extruded insulation. The tape should be compatible with the cable insulation. An alternative method is to use heat shrinkable sleeves and lug boots.
Where cables are connected direct to busbars which are likely to be operating at higher temperatures than the cable conductors, high temperature insulation in sleeve or tape form is used.
Screened MV cables must be terminated at a sufficient distance back from the conductor(s) to give the creepage distance required between conductor and shield.
It is recommended that heat shrink termination kits is used on 11kV XLPE cables. These incorporate stress control, non-tracking and weatherproof tubes, cable gloves and termination boots.
Glands
Polymeric cables should be terminated using mechanical type compression glands to BS6121. The material of the gland must be compatible with the cable armour. Where the glands terminate in non-metallic gland plates they must be fitted with earth tags. Where glands are to be screwed into aluminium or zinc base alloy plates, use cadmium plated glands.
The gland must be capable of withstanding the fault current during the time required for the cable protective device to operate. Where a circuit breaker is used the fault clearance time could be near one second.
It is good practice to fit PVC or neoprene shrouds over armoured cable glands, particularly in outdoor applications.
Connections to Terminals
Power cable conductors are usually terminated in compression type cable lugs using a hydraulic tool. The hexagonal joint appears to be the most popular crimp shape for conductors over 25mm2. Insulated crimped lugs are used on the stranded conductors of small power and control cables. Soldered lugs and shell type washer terminations are now seldom specified.
Cable Tails
Cable tails from the gland to the terminals of the equipment should be sufficient length to prevent the development of tension within them. Allowance should be made for the movement of cables connected to the terminal boxes of any plant mounted on vibration isolators. In these circumstances, and where connections to the main switchboard are in single core armoured cable, or in multicore, unarmoured cable, it is usual to terminate in a free standing terminal box mounted as close as possible to the plant. Flexible connections, e.g. in single-core, PVC insulated or PVC/XLPE insulated and PVC sheathed cables, are then used between this floor mounting box and the plant terminals. The connections should be generously looped.
Control panel
Note:
If flexible cable is used between control panel, remote panel and generator, a load terminal box is un-neccessary Flexible cable should be used
Alternator terminal box
Load terminal box must be used if connecting cable is PVC/SWA/PVC
Multicores run between set and DMC cubicle.
DMC control cubicle
DMC control cubicles do not require back accessibility. All access is from front of cubicle.
Output power to changeover/ATS cubicle.
Fig. D6 Cable Connections – Cable Tails
E A R T H I N G Section D
The generating set and all associated equipment, control and switchgear panels must be earthed before the set is put into operation. Earthing provides a reference for system voltages to:
● Avoiding floating voltages
● Prevents insulation stress
● Allows single earth faults to be detected
● Prevents touch voltages on adjacent components Provision is made on the set and within the panels for connection of an earth continuity conductor. It is the responsibility of the installers to ensure that the system is correctly earthed with references to IEE Wiring Regulations in countries where these apply, or to the local wiring regulations where they do not.
There are a number of different earthing