to the 16th Edition IEE Regulations
   
   
   
 
 

chapter 5
Earthing

chapter 6
Circuits

Circuits
  6.1. - Basic requirements for circuits 6.4 - Industrial socket outlet circuits
  6.2 - Maximum demand and diversity 6.5 - Other circuits
  6.3 - BS1363 socket outlet circuits 6.6 - Circuit segregation


6.3.1 - The fused plug 

In many situations there is a need for socket outlets to be closely spaced so that they are available to feed appliances and equipment without the need to use long and potentially dangerous leads. For example, the domestic kitchen worktop should be provided with ample sockets to feed the many appliances (deep fat fryer, kettle, sandwich toaster, carving knife, toaster, microwave oven, coffee maker, and so on) which are likely to be used. Similarly, in the living room we need to supply television sets, video recorders, stereo players, table lamps, room heaters, etc. In this case, more outlets will he needed to allow for occasional rearrangement of furniture, which may well obstruct access to some outlets.

If each one of these socket outlets were wired back to the mains position or to a local distribution board, large numbers of circuits and cables would be necessary, with consequent high cost. The alternative is the provision of fewer sockets with the penalties of longer leads and possibly the use of multi-outlet adaptors. Because the ideal situation will have closely-spaced outlets, there is virtually no chance of more than a small proportion of them being in use at the same time, so generous allowance can be made for diversity. Thus, cables and protective devices can safely he smaller in size than would he needed if it were assumed that all outlets were simultaneously fully loaded.

Thus a ring circuit protected by a 30 A or 32 A device may well feed twenty socket outlets. It follows that judgement must be used to make as certain as possible that the total loading will not exceed the protective device rating, or its failure and inconvenience will result. Two basic steps will normally ensure that a ring circuit is not overloaded.

1. - Do not feed heavy and steady loads (the domestic immersion heater is the most obvious example) from the ring circuit, but make special provision for them on separate circuits.

2. - Make sure that the ring circuit does not feed too great an area. This is usually ensured by limiting a single ring circuit to sockets within an area not greater than one hundred square metres.

Fig 6.3 Plug and socket to BS 1363

We have already indicated that a 30 A or 32 A fuse or circuit breaker is likely to protect a large number of outlets. If this were the only method of protection, there could be a dangerous situation if, for example, a flexible cord with a rating of, say, 5 A developed a fault between cores. {Figure 3.13} shows that a 30 A semi-enclosed fuse will take 5 s to operate when carrying a current of almost 90 A, so the damage to the cord would be extreme. Because of this a further fuse is introduced to protect the appliance and its cord. The fuse is inside the BS 1363 plug, and is rated at 13 A or 3 A, although many other ratings up to 12 A, which are not recognised in the BSS, are available.

A plug to BS 1363 without a fuse is not available. The circuit protection in the distribution board or consumer's unit covers the circuit wiring, whilst the fuse in the plug protects the appliance and its cord as shown in {Fig 6.4}. In this way, each appliance can be protected by a suitable fuse, for example, a 3 A fuse for a table lamp or a 13 A fuse for a 3 kW fan heater.

Whilst the installer of the wiring is seldom concerned with the flexible cords of appliances connected to it, he must still offer guidance to users. This will include fitting 3 A fuses in plugs feeding low rated appliances, and the use of flexible cords which are of sufficient cross-section and are as short as possible in the circumstances concerned. Generally, 0.5 mm˛ cords should be the smallest size connected to plugs fed by 30 A or 32 A ring circuits. Where the cord length must he 10 m or greater, the minimum size should be 0.75 mm˛ and rubber-insulated cords are preferred to those that are PVC insulated.

This type of outlet is not intended for use at high ambient temperatures. A common complaint is the overheating of a fused plug and socket mounted in an airing cupboard to feed an immersion beater; as mentioned above, it is not good practice to connect such a load to a ring circuit, and if unavoidable, final connection should be through a fused spur outlet.

Fig 6.4 Principle of appliance protection by plug fuse

The British fused plug system is probably the biggest stumbling block to the introduction of a common plug for the whole of Europe (the 'europlug'). The proposed plug is a reversible two~pin type, so would not comply with the Regulations in terms of correct polarity. If we were to adopt it, every plug would need adjacent fuse protection, or would need to be rewired back to its own protective device. In either case, the cost would be very high.

Ring circuits fed from systems where no earth terminal is provided by the Electricity Supply Company (TT systems) must be protected by an RCD rated at 30 mA, In all installations, a socket intended to feed equipment outdoors must be individually protected by a 30 mA RCD.

Where a socket is mounted on a vertical wall, its height above the floor level or

the working surface level level must be such that mechanical damage is unlikely. A minimum mounting height of 150 mm is recommended.

 

 

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Extracted from The Electricians Guide Fifth Edition
by John Whitfield
Published by EPA Press Click Here to order your Copy

Click here for list of abbreviations