The electrical properties of HC copper were standardised in 1913 by the International Electrotechnical Commission which defined the International Annealed Copper Standard (IACS) in terms of the following properties at 20°C:
Volume conductivity sv = 58 MS/m
Density d = 8890 kg/m3
Temperature coefficient of resistance a = 0.00393/°C
It follows from the first two of these two values that:
Volume resistivity rv = 1/ sv = 1.7241 mWcm
Mass conductivity sm = sv /d = 6524 Sm2/kg
Mass resistivity rm = 1/ sm = 153.28 mWkg/m2
These values correspond to 100% IACS. However, the lower oxygen and impurity levels of modern HC coppers have led to higher typical values of density and conductivity, 101.5% of the IACS value being typical of the conductivity of modern HC copper in the annealed condition.
The conductivity of copper is decreased by cold working and may be 2 to 3% less in the hard drawn condition than when annealed. Thus standards for hard drawn HC copper products should stipulate a minimum conductivity requirement of 97% IACS compared with 100% IACS for annealed products.
An approximate relationship between tensile strength of cold worked copper and its increase in electrical resistivity is:
P = T/160
Where P = % increase in electrical resistivity of cold worked copper over its resistivity when annealed.
T = tensile strength, N/mm2
As temperature increases, the conductivity of all metallic materials decreases with the corresponding increase in resistivity, according to the formula:
where rT1 = resistivity at temperature T1, mWcm
rT2 = resistivity at temperature T2, mWcm
bT1T2 = the temperature coefficient of resistivity for the temperature range T1 to T2, per°C
The value of b itself changes with temperature, but for small temperature ranges, the value of b and T1 is usually taken as a constant over the range. Its value at any temperature above -200°C is taken as
where T is expressed in degrees Celsius.
Hence the value of
b20 = 0.003947 per °C.
Resistance is related to resistivity by
where R = resistance, (mW)
b = volume resistivity, (mWcm)
l = length of conductor, (cm)
A = cross-sectional area of conductor, (cm2)
It follows that the resistance of a metallic conductor also rises with temperature. Thermal changes of resistance can be calculated in a similar way to thermal changes of resistivity, but a different coefficient, a, is used.
Hence
where RT1 = resistance at temperature T1, W
RT2 = resistance at temperature T2, W
aT1T2 = temperature coefficient of resistance for the temperature range T1 to T2, per °C
Like b, a itself changes with temperature but for small temperature ranges its value at T1 can be taken as constant over the range. Its value at any temperature T (°C) above -200°C is taken as
Copper and copper alloys have properties which make them ideal for many types of contacts from light electronics applications to very heavy duties. The ranges of compositions and properties of the coppers, copper alloys and copper-based sintered materials, and the duties for which they are suitable are described in the CDA booklet Copper in Electrical Contacts (Available on CD-ROM 'Megabytes on Copper II').
Available forms
HC copper conductors are obtainable in bar, strip, rod or tube form. For busbar applications, the most common forms supplied are bar, rod or tube and these are normally supplied in the hard condition. In this condition they offer greater stiffness, strength and hardness and have a better surface finish. Because of the practical difficulty of straightening uncoiled hard material it is normally supplied in straight lengths, coiled material being limited to the smaller sizes.
The maximum length of material available with the advent of continuous casting methods is dependent on a supplier's plant capabilities rather than the piece weight of a billet or wirebar. The following notes can be used as a guide to what is available at the present time.
For the reason given above these are drawn straight in the final stages of manufacture. The maximum length attainable is therefore limited by the length of the drawbenches on which they are produced. For sizes up to 100 mm x 25 mm, lengths up to 9 m, and for 200 mm x 37.5 mm lengths up to 5 m can be obtained. Rods up to 50 mm dia. can be supplied in lengths up to 9 m. Larger diameters are available but because of the limited reductions to which they can be subjected with normal commercial equipment hardness variations across the section will be obtained.
Because of the difficulty in producing long lengths free from surface blemishes and the handling problems encountered as the bar or rod weight increases with size and lengths, it is normal practice for lengths supplied to be around 3 - 4 m.
Seamless, high conductivity copper tubes, complying with the requirements of BS 1977 can be supplied in a range of sizes covering outside diameters of 1 mm up to 610 mm in wall thicknesses of 0.3 mm to 27 mm. Clearly, all combinations of wall thickness and outside diameter are not available, because it is not possible to produce the extremes of thickness in tubes at the extremes of the outside diameter range.
The maximum lengths available depend upon the tube dimensions specified. As a general rule, tubes in the size range 108 mm o.d. to 610 mm o.d. are supplied in 6 m lengths. Sizes smaller than 108 mm o.d. can be produced up to 10 m long.
It is usual practice to supply tubes in the as-drawn condition, M, or alternatively, in the annealed condition, O. However, other tempers can be supplied by arrangement.
Mandrel or bar drawing of tube is virtually obsolete and all the sizes indicated above are manufactured by plug drawing processes. Thus, bore and outer surface finishes are good and dimensional tolerances can be maintained over the whole length.
These generally take the form of hard drawn angle or channel sections produced by extrusion and drawing. Larger sizes can be fabricated from large sheets or plate by shearing and bending.
BS 159 for busbars requires the dimensions of flat and round bars to be within the tolerances in BS 1432, BS 1433 and BS 1977.
If necessary, material can be supplied to closer tolerances than those quoted in the respective British Standards. Obviously these involve a higher initial cost, but this may be offset by the savings accrued from reduced or eliminated machining operations normally carried out to ensure a good contact surface and fit.
The benefit to users of a range of preferred sizes is obvious and designers using copper should be aware of this desirable and growing trend. BS 1432 and BS 1433 list the recommended sizes.
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