Conduit Wiring Systems
● Screwed conduit of the welded type to BS 4568 or local code 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 (in UK), 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.
D30
Generation
Cable and Wiring
Section D
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. Normally these can be installed in galvanically segregated trunking. 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.
Note: Plastic cable ties must not be used for securing power cables.
Digital Master Control Cubicle (DMC) installed with a water companies Switchboard Suite. Access to the DMC is all front entry.
Generation
Cable and Wiring
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
Depth of cables to comply with local codes.
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 HV 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.
D32
Generation
Cable and Wiring
Section D
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
Generation
Earthing
General
During the design of a generator installation, consideration must be given to the earthing of the electrical system. The key reasons for earthing are:
● To limit and stabilise the electrical potential of any part of the installation to a pre-determined level with respect to earth.
● To ensure that the voltage between any phase conductor of the system and earth does not exceed the phase-to-neutral voltage of the system.
● To ensure that the neutral point voltage does not rise significantly above earth potential.
● To provide a path for fault current to enable overcurrent protection to operate in the event of a fault.
● To ensure that extraneous conductive components nearby remain at or around earth potential.
Earthing requirements can be broadly divided into two categories:
i) System earthing requirements – such as neutral earthing, provided to maintain the security of the electrical system.
ii) Equipment earthing – such as equipotential bonding, provided for ensuring the safety of humans and animals.
The application of system earthing depends largely upon the type of electrical system that is being designed. This will also be influenced by the characteristics of any other electrical system that may be connected to the generator, such as a utility supply. Equipment earthing is applied more universally and is less influenced by the electrical system characteristics.