The continuous current rating of a cable is determined by the ability of the cable to dissipate the heat generated by the current passing through its conductor. It depends on a number of parameters, but the most important are the:
For standard cables such as PVC building wire, XLPE SDI or circular cables (all rated 0.6/1kV), the ratings have all beencalculated and are tabulated in Australian Standard AS/NZS 3008.1.1. This standard includes various circuit configurations such as single and three phase and various installation arrangement such as “in air”, “direct buried in ground”, “in ducts”, etc.
A current which flows in a conductor will cause a voltage drop over the conductor’s length. This voltage drop is due to the resistance of the conductor. The Wiring Rules AS/NZS 3000:2000 states a maximum limit for voltage drop of 5% for low voltage systems. Therefore there is a simple calculation which can be done that relates the percentage volt drop, the cable length, and the voltage drop factor for a particular cable. This calculation and voltage drop factors (in mV/A.m –milliVolt per Amp metre) are given in AS/NZS 3008.1.1. The formulae for how much current is:
l = (5/100) x V x (1000/L x Vc)
l = cable current that produces the maximum voltage drop
V = system voltage (ie normally 415V for 3 phase and 240V for single phase)
Vc = mV/A.m volt drop factor for cable from AS/NZS 3008.1.1
L = circuit length
Note: In addition to performing this voltage drop calculation, the continuous current rating of the cable must be checked.
A current which flows in a conductor will cause a voltage drop over the length of conductor due to its resistance. The cable size can be determined to find Vc as shown below:
Vc = (5/100) x V x (1000/L x l)
When Vc is found, it is then necessary to look up the appropriate table of volt drop factors in AS/NZS 3008.1.1 to find a cable size which has a lower voltage drop factor than that just calculated. After obtaining the cable size, the continuous current rating must again be checked.
If the cable size meets or exceeds the required current, I, then this is the answer. If not, the cable size must be increased until the current, I, is obtained.
The answer is dependent on the State in which the mine is located.
In NSW, the system is that approval is needed from the Department of Mineral Resources for compliance to the NSW Coal Mines Regulation Act, 1982.
In QLD, the system is that approval is needed from the Department of Minerals and Energy for compliance to the Queensland Coal Mining Act, 1925-1990. This Act stated that cable made and in conformance with Australian Standards 1802, 2802 and 1972 is deemed suitable to be used subject to the cable being installed, operated, maintained and overhauled in accordance with the provisions of the Act.
For cables made to other Standards, then specific approval must be obtained from the Inspector of Coal Mines at the Department. As our equipment is of an electrical nature, the approval would come from the Principal Electrical Inspector.
In Western Australia, Western Power (formerly SECWA) that hold jurisdiction in this area and authorises approvals.
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For assistance, contact any of NPE’s Australian branches.
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