Important Design Questions

Reconsider the electric plug, and again concentrate on the plug body, pins and cable clamp. Before considering what the materials are, and why they have been chosen, consider the following questions:

• What are the function and requirements of each component (electrical, mechanical, aesthetic, ergonomic, etc.)?
• What is the function of the plug and how does it work?
• What is each part made of and why?
• What manufacturing methods were used to make each part and why?
• Are there alternative materials or designs in use, or can you propose improvements?

It may seem that the answers to most of these are quite obvious, and yet if we look globally the design of plugs is far from universal, suggesting that in fact there are many different potential answers to these questions.

That being said, for our purposes we can consider the following to be correct.

The plug should:

• Enable the user to provide an electrical path from the socket to the appliance
• Prevent an electrical path being formed between the user and the mains!
• Provide a rigid set of pins for location in the socket
• Be sufficiently tough to prevent failure upon dropping
• Be resistant to the use environment (e.g. temperature, moisture, etc.)
• Prevent or enable the user to fit the cable to the plug
• Be aesthetically pleasing and easy to grip
• Satisfy the requirements of the British Standards

As you can imagine, each of these specifications place constraints on which materials can be used.

Choosing the Right Materials

The safe and efficient functioning of a plug and cable depends on the ability of the various parts to conduct electricity. Clearly our first step in choosing materials must be to consider the need for electrical conductivity in each part. The parts of the plug can be divided into those which should be good conductors, those which should be good insulators and those for which conductivity is not a major factor as outlined below.

If we were to base our choice entirely on this, the selection of materials available would be vast. However, there is one factor which in many cases is actually the most important – cost. Whilst we cannot choose a cheap material that will not meet a minimum standard, it is normal practice to choose a material that may not be the absolute best, but achieves a reasonable standard of performance at an acceptable price – component designers do not become rich by choosing unnecessarily expensive materials!

Hence, as well as considering the electrical conductivity when choosing materials, we also need to consider the cost. What is the easiest way of considering cost at the same time as making sure we have suitable conductivity?

One way of selecting the best materials would be to look up values for the conductivity (or values for the resistivity – it’s opposite) in tables for various possible materials: good conductors would have low values of resistivity whereas good insulators would have high values. From a list of suitable materials, we could then choose those which are fairly cheap.

However, this method is time consuming, and the designer may miss materials which they simply forgot to consider. A much easier method is to plot materials on a chart of electrical resistivity against cost.

This kind of graph – such as the one shown in the image - is called a materials selection chart. Note how materials of each class (e.g. metals) form clusters marked here by the shaded regions. The chart makes it easy to identify cheap conductors (bottom left) and cheap insulators (top left).

Figure 3 selection chart of various materials. Public domain

We can now see why, most of the non-conducting parts in the plug are made from polymers - they have high resistivity. All the plug components requiring good conductivity are metallic - there is no other choice!

Plug Body

If we look at the resistivity-cost chart wood seems to be a cheaper choice for the plug body than polymers. So why isn't wood used in practice? It is cheaper, and its insulation properties should be more than adequate to protect the user.

The answer is to do with production of the end product. The plug body is a complex 3D shape. It is no good having the perfect material if we can't actually form it to the desired shape, so we need to examine the processing options for various materials. Making an item the shape of a plug out of wood is possible, but it would take a skilled craftsperson a considerable time, massively increasing the cost of the end product.

If we look at the table below, we notice the following:

1. Polymers can be shaped by various moulding processes.
2. Wood can only be machined into the shape of a plug, which would not be practical for mass production.
3. The processing options are greater for thermoplastics than for thermosets.

This explains why we don’t use wood, but we are then left with another question: out of all the polymers available, we are only two commonly used?

The answer now lies in the end use of the plug. To hold the pins securely and to protect the conducting parts, the plug body must be sufficiently strong and stiff. Plugs are also likely to suffer impacts, so toughness may also be a factor in material choice. A materials selection chart showing strength against toughness for various materials allows us to compare various polymers and we might be able to see why other polymers are not suitable.

Strength/Toughness

As you can see from the strength/toughness chart, the strength of polymers is relatively low compared to other materials. However, good design of the moulding shape is able to provide sufficient strength to support the pins. Correct shaping is also important for providing sufficient rigidity for the pins as the stiffness of polymers is relatively low (not shown here).

Figure 5 Strength/toughness chart for various materials. Public domain

In practice, ABS and urea formaldehyde are both used for plugs. The toughness of ABS is a lot greater than that of urea formaldehyde (remember these are logarithmic axes) - this means it can withstand a greater impact before any damage is caused.

Other materials: Nylon appears to combine the strength of urea-formaldehyde with the toughness of ABS, so why isn't it used for plug bodies? Looking back at the resistivity-cost chart, we can see that it is much more expensive than ABS or urea-formaldehyde, although this may not be the only reason.

So why are 2 different polymers used?

Although both ABS and urea-formaldehyde are used for plugs, they are in fact used for slightly different applications. In addition, ABS is used for one-piece moulded plugs which prevent access by the user whereas urea-formaldehyde is used for two piece plugs that can be fitted by users. From the selection chart and processing information we can see why:

ABS is used for one-piece plugs because thermoplastics can be easily joined after moulding. ABS is also much tougher and as a result is ideal for pre-fitted plugs which might suffer impact during service, for example those fitted to vacuum cleaners.

As a thermoset, urea-formaldehyde is stronger (and stiffer) than ABS so it is ideal for a two-piece construction where each half must be individually stronger. The lower toughness means that urea-formaldehyde plugs should only be used for fixed appliances where the plug is unlikely to suffer impact, for example those fitted to fridges or washing machines where not only is the plug unlikely to be removed very often, but it is also usually protected from accidental impact as it is located behind a large appliance.

Plug Pins

The requirements for a pin will determine the materials properties we should be interested in – obviously the first requirement is that it needs to conduct electricity well, so we will not investigate conductivity any further here. However,  there are several other requirements that we need to try and meet; 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 (P=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, which means we are almost certainly 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 below helps to answer this.

We have now established that the pins will be made from brass, but how will they be made? The selection of a suitable process involves several stages; here we will follow the approach taken elsewhere in the unit, so let's start with material compatibility.

Selection Stages

Summary

There is at least one more important factor which affects all manufacturers equally - standards. If you look on the back of any plug in the UK you should see BS1363 or 1363A - this means it conforms to a particular British Standard. Standards are important in protecting consumers by ensuring products meet a minimum safety level. BS1363 sets levels for plug performance including strength, toughness, and fire-resistance - all of which may a limit a designer in their choice of materials and processes.

Most product analysis tends to ignore many of these complications, but remember that real-life designers must check their decisions in much more detail than we have done here.