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In many medium pressure single core cable in the global, in order to avoid wastage of eddy current, and ensure the cable can withstand a certain degree of tension, armored by aluminium wire armouring. Because in the cable installation in practical operation, armour and shield, are to be grounded. Power system abroad mostly belongs to a category system, the neutral point is directly grounded, in this system, the larger the short-circuit current, the cable is short circuited, shielding and armoring can short-circuit current is large, the current into the earth, and the cable from being damaged. Therefore, calculation of short circuit current aluminum wire armoring, is a skilled engineering and technical personnel necessary. In the IEC standard formula, no aluminum wire armoured short-circuit current, only copper tape shielding and copper wire shielding. But, still can according to IEC standard and reference to other domestic and foreign data, calculate and deduce the aluminum wire armoured short-circuit current, and obtain accurate stand the test of practice of aluminium wire armoured short-circuit current data, play an important role in R & D, and the cable design, manufacture process.

Cable in any current carrying part, the method for calculating the rated short-circuit current, usually assumed in short duration, the heat retention in the carrier fluid internal (i.e. adiabatic heating). In short, some of the heat passes adjacent materials, is in fact the short-circuit current can be bigger, the so-called considering non adiabatic effect. In the whole process of short-circuit, non adiabatic method is effective.

Compared with the adiabatic method, calculated using the non adiabatic method, shielding layer, support layer and less than 10mm2 of the conductor (especially as a shielding line), which allows the short circuit current will increase considerably.

Aluminum wire armoured diabatic short-circuit current calculation are as follows:

**First, consider the coefficient of non adiabatic effects**

** **Due to internal aluminum wire armoring is PVC protective layer, while the external use cloth binding, and then extruded PVC outer sheath surrounding medium parameters, therefore only need to consider the PVC protective layer and non-woven fabric. In the formula, sigma 2, sigma 3-- aluminum wire armoring layer around the medium heat (J/Kom3)

Also: PVC protective layer of σ 2=1.7 × 106 J/Kom3

Non-woven fiber σ 3=2.0 × 106 J/Kom3

And: P 2, P 3 aluminum wire armoring layer around the medium resistance (Kom/W)

PVC protective layer P 2=6.0Kom/W

Non-woven fiber ρ 3=6.0Kom/W

F - aluminum wire armoring surrounding non-metallic materials and the imperfect of imperfect thermal contact factor, F=0.5

σ 1-- shield, shield or armor layer specific heat, J/Kom3 aluminum wire σ 1=2.5 × 106 J/Kom

All parameters are calculated:

ε =1.158 non adiabatic factor

**Second,formulas for calculation of short circuit current, adiabatic process:**

Among them, S-- cable shielding cross-sectional area, using YJV72 12/20kV 1 × 500 as an example, S=60*2.5^2*0.7854=295mm2

IAd aluminum wire armoured shielded short-circuit current

The beta temperature coefficient, 228

Ultimate short-circuit temperature θ F, θ f=250 °

Theta I circuit temperature =90 ℃, theta I

Sigma C 20 ℃ conductor of heat capacity, 2.5 × 106 J/Kom3

The resistivity of conductor - 2020 ℃, 2.8264x10-8 Ω.M. T is the short time (S) and 1S

Then, the adiabatic process allows the short-circuit current (1 seconds): =28.87kA

**Third,Calculation of short circuit current , non adiabatic effects**

Based on the above calculation process,

Non adiabatic process allows the short-circuit current (1 seconds): =1.158*28.87=33.43kA

Through the above calculation can be seen, the non adiabatic short-circuit current aluminum wire armoured compared to adiabatic short-circuit current in fact has greatly increased. IEC949 (1988) standard has been assumed that the calculation of the worst conditions, namely, in fact in the calculation has considered the margin, of course the calculation results of rated short circuit current is safe. The above calculation was basically in accordance with the IEC949 (1988) formula in the standard copper wire shielding, but considering the surrounding medium aluminum wire armoured with surrounding medium different from the copper wire shielding, and the corresponding parameters of copper raw formula into the aluminum wire.

In addition, referring to the experience of other countries, the final circuit temperature metal shielding layer can reach to 350 ℃, for the sake of safety, but also have the option to 300 DEG C, in order to improve the safety level Chinese grid, the actual calculated according to 300 ℃ calculation. The metal shielding are against copper or copper wire, aluminum wire armoured should vary with the copper shield, because tolerance to high temperature than the aluminum copper. We refer to the final circuit temperature aluminum conductor in actual calculation, calculation of short circuit current at 250 ℃, so it can ensure the actual short-circuit armoured aluminium wire is not overloaded and safety problems.

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