Carrying power through metal-enclosed bus systems 28/999
mm ¥ 12.7 mm or (2'' ¥ 1/2'' ) or any other equivalent flat size (Tables 30.4 or 30.5). This formula is also drawn in the form of curves as shown I in Figure 28.5, sc ¥ t (Isc in kA) versus final temperature. A From these curves the minimum conductor size can be easily found for any fault level, for both aluminium and copper conductors and for any desired end temperature. As in the above case
The welded portion, such as at the flexible joints*, should also be safe up to this temperature. Welding of edges is essential to seal off flexible ends to prevent them from moisture condensation, oxidation and erosion of metal. Tin or lead solder starts softening at around this temperature and should not be used for this purpose. For joints other than flexibles it is advisable to use oxyacetylene gas welding or brazing for copper and tungsten inert gas (TIG) or metal inert gas (MIG) welding for aluminium joints.
ÊI ˆ 100 = 1.166 ¥ sc 100 Ë A ¯
Note In case of copper also, the end temperature is considered as 185∞C only. Although this metal can sustain much higher temperature than this, without any adverse change in its mechanical properties, merely as a consideration to Table 32.3, and to safeguard other components, insulations and welded parts etc., used in the same circuit.
or or
Ê 50 000 ˆ 100 = 1.166 ¥ Á ˜ 100 Ë A ¯
2
Ê 50 000 ˆ 100 = 1.166 ¥ Á ˜ 100 Ë A ¯
2
A = 50 000 ¥ 100
¥ (1 + 0.00403 ¥ 85) ¥ 1
¥ 1.34255
1.166 ¥ 1.34255
625.6 mm2 for pure aluminium or
617.6 mm2 for alloys of aluminium
(assuming a20 = 0.00363) The standard size of aluminium flat nearest to this is 50.8
*Welding of flexible joints should preferably be carried out with high-injection pressing (welding by press heating), eliminating the use of welding rods.
t = 0.0799 for an operating temperature at 85∞ C and end temperature on (28.2) fault at 185∞ C
Therefore, for the same parameters as in Example 28.2
50 ¥ 1 625.8 mm 2 0.0799 A small difference, if any, between this and that calculated above may be due to approximation and interpolation only. This minimum conductor size will take account of the heating effects during the fault, irrespective of the current rating of the conductor. This much conductor size is essential for this fault level even for very low current ratings. However, the required conductor size may be more than this also, depending upon the continuous current it has to carry, as discussed later.
IS BN :
A=
K. C. Ag ra w al
Au th or :
Example 28.2 Determine the minimum conductor size for a fault level of 50 kA for one second for an aluminium conductor. Assuming the temperature rise to be 100∞C and the initial temperature of the conductor at the instant of the fault 85∞C then
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I sc ¥ A
(28.1)
where qt = temperature rise (in ∞C) Isc = symmetrical fault current r.m.s. (in Amps) A = cross-sectional area of the conductor (in mm2) µ20 = temperature coefficient of resistance at 20∞C/∞C, which as in Table 30.1 is 0.00403 for pure aluminium and 0.00363 for aluminium alloys and 0.00393 for pure copper q = operating temperature of the conductor at which the fault occurs (in ∞C) k = 1.166 for aluminium and 0.52 for copper t = duration of fault (in seconds)
10 4 1.166 ¥ 1.34255 ¥ 10 6
Generalizing,
2
◊ (1 + µ 20 q ) ◊ t
t =
¥ 1.34255 ◊ t
= 0.0799 (Isc is in kA)
To determine the minimum size of conductor for a required fault level, Isc, to account for the thermal effects one can use the following formula to determine the minimum size of conductor for any fault level: ÊI ˆ q t = k ◊ sc 100 Ë A ¯
I sc A
or
2
Example 28.3 If the conductor is of copper then, assuming the same parameters,
Ê ˆ 100 = 0.52 ¥ Á 50 000 ˜ 100 Ë A ¯ or
A = 50 000 ¥
2
¥ (1 + 0.00393 ¥ 85) ¥ 1
Ê 0.52 ˆ ¥ 1.33405 ˜ Á 100 ¯ Ë 100
= 416 mm2
Copper is two thirds the size of aluminium for the same parameters. The melting point of copper at almost 1083∞C (Table 30.1) is approximately 1.5 times that of aluminium at 660∞C. These melting points are also located on the nomograms in Figure 28.6. Refer to nomograms (a) and (b) for aluminium and (c) for copper conductors. The same area can also be obtained from the copper curves of Figure 28.5. Assuming the same end temperature at 185∞C, then corresponding to the operating curve of 85∞C, I sc t = 0.12 (28.3) A and for the same parameters as in Example 28.3, 50 1 = 0.12 A or A 416.7 mm2
