Cables

In some plants they keep the cables of off-circuit loads connected to the network? What is the reason?

In order to disconnect a load, it is recommended to disconnect consumers from cables rather than disconnecting cables from feeding panel. Therefore the cables will remain in the circuit and their capacitance will help to compensate the network reactive power. And this will result to the higher load factor. Note that this is recommended for long cables feeding heavy consumers i.e. long cables feeding off-shore plant from on-shore plant. This will also avoid transient voltage responses caused by cable capacitance capacity while switching the power on and off .

How do you compare the current capacity of Aluminium cables with that of copper cables?

The current capacity of the Aluminium cable is approximately 77.8% of that of copper cable with the same size.

What type of cable shall be used at VSD output?

To feed a drive from a VSD panel (Variable Speed Drive), the cables must be shielded to avoid electromagnetic interfaces. Use cable with braided shields. Cables with foil shields have the shielding effect which is worse by a factor of five. Braid shields are better than foil for minimizing low frequency interference and have lower DC resistance.

What percentage of circuit harmonic current must be considered in neutral sizing?

To consider harmonic currents in sizing of neutral conductor, the third and higher order harmonic circuit in the neutral conductor shall be taken as 100% of the highest load-generating harmonic currents on any phase.

What restrictions on voltage drop shall be considered for cable sizing?

The cross sectional area of every current carrying conductor shall be such that the voltage drop between the point of supply for the low voltage electrical installation and any point in the electrical installation does not exceed 5% of the nominal voltage at the point of supply.

Where the voltage point of supply is the low voltage terminals of a substation located on the premises containing the electrical installation and dedicated to the installation, the permissible voltage drop may be increased to 7%.

This limit need not to apply to stand-alone systems, that are designed such that the combination of the output voltage from the source, together with the voltage drop within the installation, does not result in the utilization voltage more than a total of 11% below the nominal supply voltage under normal operating condition at equipment and appliances intended to operate at low voltage.
Some projects specify values below as restrictions for maximum permissible voltage drop.

Motors

• 5% of nominal nominal supply voltage at full load
• 15% of nominal supply voltage during motor starting

Feeder circuits

• 5% of nominal voltage at full load

Lighting circuits

• 2% of nominal supply voltage between source and lighting board
• 3% of nominal supply voltage between the lighting board and the most distance fixture

Switchboard feeders

• 1% of nominal supply voltage from main switchgear to MCC distribution board

Generator/transformer feeder

• 2% of nominal supply voltage between generator/transformer terminals and switchboard

Extra-low voltage electrical installation

AS/NZS 3000, 7.5.7

The drop in voltage at any point in an extra-low voltage electrical installation shall not exceed 10% of the nominal value when all live conductors are carrying the circuit-operating current. This requirement need not apply where electrical equipment is specially designed for operate with a voltage drop greater than 10%.

What colours are recommended for wiring in a plant?

AS/NZS 3000, section 3.8 nominates below colouring.

• Protective earth: green/yellow
• Equipotential bonding: green/yellow
• Neutral: black or light blue
• Active: Any colure rather than green, yellow, green/yellow, black or light blue. This standard recommends red or brown for single phase and red, white and dark blue for three-phase.

What bending radius is permitted for cables routing?

The bending Radius recommended by the cable manufacturer shall be observed. Where manufacturer’s information is not available, the following minimum internal radius may be considered suitable.

• Unarmoured sheathed cables: 6 times the cable diameter.
• Armoured sheathed cables: 12 times the cable diameter

What measures need to be taken to reduce the effects of low frequency interfaces?

When laying cables, separate control cables from power cables by using spacing or different routes. Lay power cables in trefoil formation in preference to a flat formation. Control cables routes should not be parallel to busbars or power cables as far as possible. Control cables should be laid away from inductances and single-phase transformers.

Loops should be avoided in circuit arrangements. Prefer a radial configuration to a ring configuration for DC auxiliary supply circuits. Avoid the protection of two different DC circuits by the same miniature circuit breaker. Avoid parallel connection of two coils located in separate cubicles. Locate all wires of the same circuit in the same cable. When different cables have to be used, they should be laid in the same rout.

Additional measures need to be considered where gas-insulated switchgear is used. The concrete reinforcement grids should be connected to the earthing system at various points, especially in the floor.

Does higher temperature insulation permit a higher current-carrying capacity (V-75 or V-90)?

The use of the higher temperature insulation compounds does not permit a higher current-carrying capacity. AS/NZS 3008.1.1 recommends 75ºC for current carrying capacity calculations.

Is there any guidance on selecting cable for DC consumers in Australian standards?

For DC systems, the Australian standards provide no guidance and Cables for DC consumers are selected based on current-carrying capacity and voltage drop considerations only.

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