How to Determine the Rating of a Cable?
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:
- Conductors’ DC resistance;
- Thermal resistance of the insulating sheathing materials; and
- Ambient conditions of the environment where the cable is installed (for example the surrounding air temperature).
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.
How to Measure the Amount of Current a Cable Can Carry Over a Particular Distance?
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)
Where:
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.
How to Determine the Size of Cable Required for a Particular
Current of a Distance?
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.
Which Cables are Approved for Use in Mines?
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.
