Case Study - 13A plug

Requirements for a pin

The requirements for a pin will determine the materials properties we should be interested in. Let's look at a few:

• Overheating: the pins must not get too hot, or there will be the risk of fire. The heating comes from the current which is drawn by the appliance and the resistance of the conductor (=I²R losses). We therefore want low electrical resistance in the pin.
• Firm fit: the plug will be inserted/removed many times during its life. If the material wears too much, the plug will be loose in the socket. The wear resistance of a material depends on its strength, so we want high material strength.
• Low cost: a plug has to be cheap, so we need to keep the material and processing costs down. Although the processing costs will depend to some extent on what material we choose, we want low material cost.

We have 3 different requirements to meet, so we are going to have to compromise in some way. We know that we're only interested in metals because the pin must be a good conductor - so let's look at the metals on the 2 selection charts we've seen so far...

Resistivity - Cost The materials with the lowest values of resistivity are aluminium, brass, copper and gold. All these are commonly used as conductors in practice, although clearly gold is a bit too expensive for use in a plug! Also, although aluminium is a good conductor, it is not suitable for a removal plug because it develops an insulating oxide layer. So, how to choose between copper and brass? The strength-toughness chart helps to answer this...

Strength - Toughness We can see from this chart that brass has higher strength and hence better wear resistance than copper. So, brass is used for the pins because it is the best compromise between the three competing needs for low cost, good electrical resistivity and good wear resistance. Brass is an alloy of copper; it is common for alloys to have higher strength than the pure metal. The higher strengths in the copper bubble only come from 'cold working' which would be expensive, so brass is even more attractive than it first appears. Finally, why not use steel, which is much stronger (and cheaper) than either brass or copper?

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