Overview: The rules for British Plugs - Simplified

SIMPLIFIED RULES FOR BRITISH PLUGS

The British system for plugs and sockets started before World War II at a time when there was a shortage of copper for cables. The 'Ring Main' system was devised to reduce the amount of copper needed in cables inside each house or office.

The 'Ring Main' system to the socket outlets inside each house, apartment or office uses a cable with three cores - Line Neutral and Earth. The line and neutral are 2.5 mm2 and the earth is 1.5 mm2. The cable goes from the Consumer Unit (a small switchboard in each house etc) to each of the socket outlets in turn and then back to the Consumer Unit and connected to the start of the cable - in parallel. The power can go either way round the 'Ring Main'. The plugs and sockets used must comply with British Standard BS1363 or its equivalent in Hong Kong, Malaysia, Singapore etc.

The protective device inside the consumer unit is usually rated at 30 or 32 Amps - this may be either a fuse or a miniature circuit breaker. If a portable appliance is plugged into one of the socket outlets it can receive power at more than 30A before the protection operates. However, the maximum rating allowed on such appliances is only 13A.

If a small appliance, such as a radio, is plugged in it may only take 0.5A and so may only have a small cable between it and the socket - sometimes only 0.75 mm2 or 0.50 mm2 in cross-section. The problem then arises that if there is a short circuit or other fault in the appliance then a large fault current would flow through the cable. The cable may be unable to handle this current and a fire could start that could cause damage to property and injury or death to the people in the property. To prevent this a small fuse is put INSIDE the plug. This fuse is made in accordance with standard BS1362 and has a characteristic to operate quickly during a short circuit. The purpose of the fuse is to protect the cord between the appliance and the socket.

Part of the principle of the ring main system is that the earth circuit must be able to take a current that is guaranteed to blow the fuse in the plug if there is a sudden short circuit. The test requirement during production of 25A earth continuity is specified to ensure that this will happen. If the device does not have any conducting metal parts it may be classified as Class II and need not have an earth connection.

The fuses may be rated 3A, 5A, 10A or 13A. However they are not highly sensitive to overload current and in fact will not operate for half an hour even at 1.6 x the rated current. Their primary purpose is to protect against short circuit currents. The fuses are designed to give the minimum temperature rise when operating at their normal rating but still respond quickly upon the sudden occurrence of a short circuit.

The presence of the fuse inside the plug causes a problem! The fuse gets warm or hot in normal operation - the word 'fuse' means 'melt'. It must get hot - and this is normal. When the fuse gets hot so must the plug itself. The plug then gets even hotter because of the electrical connections inside the plug. For a non-rewireable plug the connections are usually crimped. The British Standard limits the maximum temperature rise in the plug to 52oC above ambient. The fuse has already taken up part of the permit temperature rise. That is why it is so important to make the best possible crimps and keep the temperature rise at these joints to a minimum.

If a non-rewireable plug is used on an appliance such as a radio taking less that 0.5A the plug itself will not get very hot. In this case it will never have to take 13A and so the heat generated in the connections (I2R) will be negligible.

However, if the plug is to supply a washing machine that may take up to 13A and has a motor that starts and stops the I2R loss will be much much greater.

A plug may have to supply an appliance such as a fan that has a motor - there is a current surge when it first starts but then the current settles down to a much lower level. In these cases it is allowed to have a fuse of higher rating than the cord supplying the fan.

If a non-rewireable plug is fitted to an appliance that will not normally take a large current it need not be tested for temperature rise at the full current of 13A with the I2R losses that his would give. In these cases the plug may be rated at less than 13A but it may have a fuse greater than the cord rating. The advantage to the manufacturer of doing this is that it is easier to achieve a successful Type Test against the standard BS 1363. However, for non-rewireable plugs the plug base must be marked with the rating of the fuse to be fitted - not the rating of the plug when it was Type Tested and as shown on the Licence. Common ratings for British '13A' plugs are 6A, 10A and 13A. The customer must decide what plug rating and what fuse rating he needs for his intended duty.

Plugs for use in Britain must be of a design approved by one of the nominated certification bodies such as ASTA BEAB Certification Services. The plugs are first 'Type Tested' in a laboratory. The production and test of the plugs in the factory must then be under regular surveillance by the certification body and must be subject to Annual Type Tests to confirm that no change in design or performance has been made. The plugs should not be 'CE' marked because they are in accordance with national rather than European regulations. In the UK these are called the 'Plugs and Sockets Regulations'. However, when they are used in conjunction with a connector the resulting 'cordset' should be 'CE' marked if it is to be sold separately (this marking may be on the packaging).

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