3.4.4 -  Protection from contact

Four methods of protection are listed in the Regulations.

1. Protection by separated extra-low voltage (SELV)
This voltage is electrically separated from earth and from other systems, is provided by a safety source, and is low enough to ensure that contact with it cannot produce a dangerous shock in people with normal body resistance or in livestock. The system is uncommon.

2. Protection by protective extra-low voltage (PELV)
The method has the same requirements as SELV but is earthed at one point. Protection against direct contact may not be required if the equipment is in a building, if the output voltage level does not exceed 25 V rms or 60 V ripple-free dc in normally dry locations, or 6 V rms ac or 15 V ripple-free dc in all other locations.

3. Protection by functional extra-low voltage (FELV)
This system uses the same safe voltage levels as SELV, but not all the protective measures required for SELV are needed and the system is widely used for supplies to power tools on construction sites {7.5}. The voltage must not exceed 50 V ac or 120 V dc. The reason for the difference is partly that direct voltage is not so likely to produce harmful shock effects in the human body as alternating current, and partly because the stated value of alternating voltage is r.m.s. and not maximum.

As {Fig 3.9} shows, such a voltage rises to a peak of nearly 71 V, and in some circumstances twice this voltage level may be present. The allowable 120 V dc must be ripple free.

Fig 3.9 An alternating supply of 50V may provide 141 V when the supply is rectified

{Figure 3.10} shows how a 120 V direct voltage with an 80 V peak-to-peak ripple will give a peak voltage of 160 V. The allowable ripple is such that a 120 V system must never rise above 140 V or a 60 V system above 70 V. It is interesting to note that a direct voltage with a superimposed ripple is more likely to cause heart fibrillation {3.4.2} than one which has a steady voltage. Unlike the SELV system, functional extra low voltage supplies are earthed as a normal installation. Direct contact is prevented by enclosures giving protection to IP2X (which means that live parts cannot be touched from outside by a human finger - see {Table 2.4}) or by insulation capable of withstanding 500 V r.m.s. a.c. for one minute.

Fig 3.10 Increased peak value of a direct voltage with a ripple

It must be quite impossible for the low voltage levels of the normal installation to appear on the SELV system, and enclosures/insulation used for their separation must be subjected to the same insulation resistance tests as for the higher voltage. Any plugs used in such a circuit must not be interchangeable with those used on the higher voltage system. This will prevent accidentally applying a low voltage to an extra low voltage circuit.

4. Protection by limitation of discharge energy
Most electrical systems are capable of providing more than enough energy to cause death by electric shock. In some cases, there is too little energy to cause severe damage. For example, most electricians will be conversant with the battery-operated insulation resistance tester. Although the device operates at a lethal voltage (seldom less than 500 V dc) the battery is not usually capable of providing enough energy to give a fatal shock. In addition, the internal resistance of the instrument is high enough to cause a volt drop which reduces supply voltage to a safe value before the current reaches a dangerous level. This does not mean that the device is safe: it can still give shocks which may result in dangerous falls or other physical or mental problems.

The electric cattle fence is a very good example of a system with limited energy. The system is capable of providing a painful shock to livestock, but not of killing the animals, which are much more susceptible to the effects of shock than humans.