Exercise 1. Choose the correct answer

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There are currently 15 types of electri- cal plugs in use today, each of which has been assigned a letter by the US Department of Commerce International Trade Admin- istration (ITA), starting with A and moving through the alphabet. Let’s consider some most widely spread of them.

Type A

This class II ungrounded plug with two flat parallel prongs is pretty much standard in most of North and Central America. It is known as NEMA 1-15 and was invented in 1904 by Harvey Hubbell II. The plug has two flat 1.5 mm thick blades, measuring 15.9–

18.3 mm in length and spaced 12.7 mm

apart. Type A plugs are generally polarized and can only be inserted one way because the two blades do not have the same width. The blade con- nected to neutral is 7.9 mm wide and the hot blade is 6.3 mm wide. This plug is rated at 15 amps.

Type B

This class I plug is designated as American standard NEMA 5-15. It has two flat 1.5 mm thick blades, spaced 12.7 mm apart, measuring 15.9–

18.3 mm in length and 6.3 mm in width. It also has a 4.8 mm diameter round or U-shaped earth pin, which is 3.2 mm longer than the two flat blades, so the device is grounded before the power is connected. The cen- tre-to-centre distance between the grounding pin and the middle of the im- aginary line connecting the two power blades is 11.9 mm. The plug is rated at 15 amps.

Type A and B plugs have two flat prongs with (often, but not always) a hole near the tip. These holes aren’t there without a reason. If you were to take apart a type A or B socket and look at the contact wipers that the prongs slide into, you would find that in some cases they have bumps on them. These bumps fit into the holes so that the outlet can grip the plug’s prongs more firmly.

This prevents the plug from slipping out of the socket due to the weight of the plug and cord. It also improves the contact between

the plug and the outlet. Some sockets, how- ever, do not have those bumps but just two spring-action blades that grip the sides of the

plug pin, in which case the holes are not necessary.

Type A and B plugs are not insulated (i.e. the pin shanks do not have a black covering towards the plug body) and the outlets are not recessed into the wall, which means that if a plug is pulled halfway out, its prongs are still connected to the socket. Type A and B sockets are potentially dan- gerous, since the distance between the socket and a partially pulled-out

plug is big enough to touch the pins with your fingers or with a metal ob- ject such as a teaspoon.

Type C

Type C is probably the single most widely used international plug.

This two-wire plug is ungrounded and has two round prongs. It is popularly known as the Europlug which is described in CEE 7/16. The plug has two 4 mm round pins, measuring 19 mm in length on centers spaced 18.6 mm apart at the base and 17.5 mm apart at the tip. The two pins have 10 mm long insulated sleeves. They converge slightly, but they are relatively flexible, which allows the plug to mate with any

socket that accepts 4.0–4.8 mm round con- tacts on 17.5–19 mm centers. The plug is generally limited for use in class II applica-

tions that require 2.5 amps or less. It is, of course, unpolarized. It is commonly used in

all countries of Europe except in the United Kingdom, Ireland, Cyprus and Malta. It is al- so used in various parts of the developing world.

Whereas type C plugs are very com- monly used, this is not the case for type C sockets. This kind of socket is the older and ungrounded variant of socket types E, F, J, K or N. Nowadays most countries demand grounded sockets to be installed in new

buildings. Since type C sockets are un- grounded, they are being replaced by type E, F, J, K or N (depending on the country).

Type F

Plug F is known as CEE 7/4 and commonly called “Schuko plug”, which is the acronym of “ Schu tz ko ntakt”, a German word meaning “pro- tection contact” or “safety contact”. The plug was designed in Germany shortly after the First World War. It goes back to a patent (DE 370538) granted in 1926 to Albert Büttner, a Bavarian manufacturer of electrical accessories.

Type F is similar to C except that it is round and has the addition of two grounding clips on the side of the plug. The plug has two 4.8 mm round pins, measuring 19 mm in length on centers spaced 19 mm apart. The distance between either of the two earthing clips and the middle of the imaginary line connecting the centers of the two power pins is 16 mm.

Because the CEE 7/4 plug can be inserted in either direction into the socket, the Schuko connection system is unpolarized (i.e. line and neutral are connected at random). It is used in applications up to 16 amps.