Electrolytic Tough Pitch Copper – Cu-ETP, Cu-ETP1

Electrolytic Tough Pitch (ETP) copper is the most widely used grade of copper in electrical applications all over the world and represents over half of global copper use. It is extensively used in electric power systems, electrical installations in homes, offices and industry and all kinds of electrical and electronic equipment.

ETP1 is the highest purity tough pitch copper with a maximum impurity content of 0.0065% (compared to ETP with a maximum of 0.0355). The very high purity means that ETP1 is used for the most demanding electrical applications such as the transmission of power and signals.

Cu-ETP1 and Cu-ETP are made from the highest purity copper cathodes Cu-CATH1 (CR001A) and Cu-CATH2 (CR002A), designated in EN1978:1998.

History

The term ‘tough pitch’ originates from the time when molten copper, after refining, was cast into ingot moulds. During refining the copper was oxidised to remove impurities then reduced by hydrogen to give the correct oxygen level. To monitor this process, a small sample was taken and the solidification surface observed. If the surface sunk there was too much oxygen; if it was raised there was too much hydrogen. If it was level (correct pitch), the oxygen was correct, and the properties good; in other words ‘tough’, hence tough pitch.

A level of 0.02% to 0.04% oxygen is maintained in ETP copper to oxidise the remaining impurities to oxides which would otherwise dissolve in the copper forming solid solutions thereby reducing conductivity. Oxides have little effect on conductivity.

Heat Treatment

It may be necessary to stress relieve ETP copper to reduce the possibility of distortion or cracking after machining or cold working. This may be done at 150 to 200^oC but does not soften the copper. If this is required then a full anneal at 400 to 650^oC is needed. Copper cannot be hardened by heat treatment and it does not become harder with time. It does not have a ‘shelf life’.

Properties

A summary of the properties from the annealed (soft) to the hard condition is shown.

• Tensile strength: 220-385 N/mm²
• Proof strength: 60-325 N/mm²
• % Elongation: 55-4
• Hardness (HV): 45-155
• Electrical conductivity: 100-101.5% IACS
• Thermal conductivity: 394 W/m^oC

Fabrication

This copper will suffer from steam (hydrogen) embrittlement when heated in a hydrogen (reducing) atmosphere.

Machining

While ETP copper like other pure coppers cannot be regarded as a free-machining material, it is not difficult to machine, especially in the work-hardened condition. The machinability rating is 20% (free-machining brass is 100%).

Resistance to Corrosion

All coppers corrode at negligible rates in unpolluted air and water due to the formation of a protective oxide surface layer. Copper artefacts have been found in nearly pristine condition after being in the earth and under the sea for thousands of years. However, copper is susceptible to more rapid attack in the presence of ammonia, sulphur, hydrogen sulphide and mercury. All coppers are virtually immune to stress corrosion cracking.

Resistance to Softening

Softening is time and temperature dependent and it is difficult to estimate precisely the time at which it starts and finishes. It is usual, therefore, to consider the time to ‘half-softening’, i.e. the time taken for the hardness to fall by 50% of the original increase in hardness caused by cold reduction. In the case of ETP copper, this softening occurs at temperatures above 150^oC. It has been established experimentally that such copper would operate successfully at a temperature of 105^oC for periods of 20-25 years, and that it could withstand short-circuit conditions as high as 250^oC for a few seconds without any adverse effect.

Creep

Creep is slow deformation of a material with time at temperature and must be allowed for in the design of any component above room temperature.

ETP copper does not creep at room temperature (unlike high conductivity aluminium) under normal stresses. However, creep must be considered in the temperature range above 150^oC though this temperature is well above the usual operating temperature of busbars.

Applications

Due to the combination of high conductivity, good corrosion resistance, workability and aesthetics, ETP copper is widely used for:

• Busbars
• Motor windings and components
• Transformer windings and components

ETP1, with its higher purity, is used for:

• Automotive cables
• Signal cable braid shielding

Available Forms

ETP copper is available in bar, plate, sheet, strip, tube and wire.

Specifications

Below are the specifications for Europe, US and Asia. Note that for USA and Asia, some compositions are not identical. For equivalent standards from other countries visit the Copper Key website.

• UK: C100 (ETP1) C101 (ETP) (British Standard BS designation). UK Standards are superseded by European Standards.
• Europe: CW004A (ETP) CW003A (ETP1) (European Standard EN designation).
• USA: C11000 (ETP) C11040 (ETP1) (American Society for Testing and Materials ASTM designation).
• Japan: C1100 (ETP) (Japanese Industrial Standards JIS designation).

Further information on ETP copper, and other conductivity materials, is available at the Copper Alloys Knowledge Base.

Application Example 1: Energy Efficient Motor

Motors are relatively cheap to buy but expensive to run; they can consume electricity to the equivalent of their capital cost within the first 300 hours of operation, a mere 3 weeks of continuous use. By good design and use of more high conductivity ETP copper, the running costs can be reduced by up to 5%, a large operating cost saving and a reduction in CO2 emissions over a 10-year life. Die-cast copper rotors made from Cu-ETP or Cu-OF increase the efficiency further by reducing rotor losses. Motors with copper rotors operate at a lower temperature which extends their service life.

Cutaway of a high-efficiency motor with a die-cast copper rotor. (Courtesy of Siemens.)

Application Example 2: Busbar

In electrical power distribution, a busbar is a metallic strip, tube or bar that conducts electricity within a switchboard, distribution board, substation, battery bank or other electrical apparatus. Its main purpose is to conduct a substantial current of electricity, and not to function as a structural member.

The usual choice for copper busbars is tough pitch copper because of its availability, good conductivity, strength, ductility and resistance to normal atmospheric corrosion. However if welding or brazing is required then a copper deoxidised with a small amount of phosphorus should be used and if high temperatures are likely then silver bearing coppers are the best choice.

High precision copper busbars. (Courtesy of H V Wooding Ltd.)

Quick facts

Properties
In the annealed (soft) to hard condition the alloy has the following combination of properties:

• Tensile strength: 220-385 N/mm²
• Proof strength: 60-325 N/mm²
• % Elongation: 55-4
• Hardness (HV): 45-155
• Electrical conductivity: 100-101.5% IACS
• Thermal conductivity: 394 W/m^oC

Applications
ETP

• Busbars
• Motor windings and components
• Transformer windings and components.

ETP1 (higher purity):

• Automotive cables
• Signal cable braid shielding.

Available Forms

• Bar
• Plate
• Sheet
• Strip
• Tube
• Wire