Current calculation method

For buried cables, the user may choose from various cyclic and other calculation methods.

For cables in air the conductor temperature follows changes in load current sufficiently rapidly so that the usual daily cycles do not permit peak loads greater than the steady state value.

Transient current step and emergency rating acc. IEC 60853 are both only possible for one system. All other parallel systems are considered to be under steady-state load conditions.

0Continuous load (IEC 60287)The calculation is done according to the newest edition of the IEC 60287 standards for steady-state conditions. The term steady state is intended to mean a continuous constant current (100% load factor) just sufficient to produce asymptotically the maximum conductor temperature, the surrounding ambient conditions being assumed constant. The external thermal resistance of touching cables or ducts is calculated acc. to the corresponding equations listed in IEC 60287-2-1 whereas for non-touching cables of the same system, the grouping factor Feq is being used.
1Cyclic rating using load factor (Neher-McGrath 1954)In order to evaluate the effect of a cyclic load upon the maximum temperature rise of a cable system, Neher observed that one can look upon a heating effect of a cyclical load as a wave front that progresses alternately outwardly and inwardly in respect to the conductor during the cycle. He further assumed that, with the total joule losses generated in the cable equal, the heat flow during the loss cycle is represented by a steady component of magnitude plus a transient component, which operates for a period of time during each cycle. The transient component of the heat flow will penetrate the earth only to a limited distance from the cable, thus the corresponding thermal resistance will be smaller than its counterpart which pertains to steady-state conditions. Neher evaluated constants empirically to best fit the temperature rises calculated over a range of cable sizes to measured data. IEC 60853 and Neher-McGrath approach for sinusoidal loads can be considered to be equivalent for cable diameters of up to 100 mm.
2Cyclic rating using load factor (Heinhold 1999)The calculation of the cyclic rating is basically the same as the method by Neher-McGrath. However, the type of load curve can be chosen to be sinusoidal, rectilinear or median (being neither sinusoidal nor rectilinear).
3Cyclic rating using load factor (Dorison 2010)In the majority of practical cases, the load variations will exhibit a more complex pattern than the one described by a daily load cycle. For example, loading of cables is usually much lighter during the weekend than during the weekdays. For deeply buried cables, the yearly load variations will play a significant role because of the very long time constants at great depths. The method allows for daily, weekly and yearly load variations. The cable diameter influences the diameter of the area affected by load variations. This is particularly important for larger cables and for cables in duct. The method calculates the characteristic diameter by the use of bessel functions. This method works basically acc. to Neher-McGrath but is not based on empirical data and thus suitable for all cable diameters and equivalent to IEC 60853 approach for sinusoidal loads.
4Cyclic rating using rating factor M (IEC 60853)The cyclic rating factor is denoted by M, and is the factor by which the permissible steady-state rated current (100% load factor) may be multiplied to obtain the permissible peak value of current during a daily (24 h) or longer cycle such that the conductor attains, but does not exceed, the standard permissible maximum temperature during this cycle. A factor defined in this way has the steady-state temperature, which is usually the permitted maximum temperature, as its reference. The cyclic rating factor depends only on the shape of the daily cycle and is thus independent of the actual magnitudes of the current. The loss-load factor (μ) of the daily current cycle is determined first by decomposing the cycle into hourly rectangular pulses. The temperature responses of the cable and soil to the complete cycle of losses can be found by adding together the response to each hourly rectangular pulse, having regard to the time period between each pulse and the time of maximum temperature. Detail of the load cycle is needed over a period of only 6 h before the time of maximum temperature, and earlier values can be represented with sufficient accuracy by using an average. The loss-load factor μ provides this average.
5Transient emergency rating (IEC 60853)The methods for calculating the emergency ratings apply to cables buried in the ground, either directly or in ducts, and to cables in air. Provision is made for incorporating the transient caused by a sudden application of voltage (i.e. the transient due to dielectric loss). The method is intended for emergency loads not greater than about 2.5 times rated full load current (100% load factor). The procedure for calculating the short time rating of single circuits is based on a knowledge of the conductor temperature transient. Considering an isolated buried circuit carrying a constant current I1 applied for a sufficiently long time for steady-state conditions to be effectively reached. Subsequently, from a time defined by t=0, an emergency load current I2 (greater than I1) is applied. If I2 is applied for any given time t, the question is how large may I2 be so that conductor temperature does not exceed a specified value, taking into account the variation of the electrical resistivity of the conductor with temperature. The effect of dielectric loss is neglected in this treatment, but is taken into account at the end of the calculation.
6Transient current step (IEC 60853)The transient temperature response of a cable to a step-function of current in its conductor depends on the combination of thermal capacitances and resistances formed by the constituent parts of the cable itself and its surroundings. The methods applies to cables buried in the ground, either directly or in ducts, with max 168 current steps within the coming week. This method assumes that the voltage and current had been applied for a sufficiently long time for the conductor temperature rise to have reached a steady state.