An electric shock is caused by current flowing through a body. A very small amount – between 50 and 80 mA is considered to be lethal to most human beings, although this would of course depend on the person’s health and other circumstances. In livestock the lethal current would be considerably less.
The electrical regulations are set out to provide for the safety of persons and livestock. An electric shock is one risk of injury; others are:
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Excessive temperatures likely to cause burns, fire and other injurious effects.•
Mechanical movement of electrically actuated equipment, in so far as such injury is intended to be prevented by electrical emergency switching or by switching for mechanical maintenance of non-electrical parts of such equipment.•
Explosion.This can be found in Chapter 13 of BS 7671 (Regulation 130-01-01).
Regulation 130-02-01 tells us that persons and livestock shall be protected so far as is reasonably practical against dangers that may arise from contact with live parts of the installation. This protection can be achieved by one of the following methods.
For direct or indirect contact:
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Preventing current passing through the body of any person or livestock.•
Limiting the current which can pass through a body to a value lower than the shock current.Testing
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For indirect contact:
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Automatic disconnection of the supply in a determined time on the occurrence of a fault likely to cause a current to flow through a body in contact with exposed conductive parts, where the value of that current is equal to or greater than the shock current.The regulations quoted mention Direct and Indirect contact.
Direct contact is electric shock received by touching a known live part which is intended to be live.
Indirect contact is electric shock received when touching exposed conductive parts made live due to a fault.
Protection can be achieved by various methods which can be found in Chapters 41 and 47 of BS 7671. The most common methods used for protection against direct contact within electrical installations are: the use of insulation, and enclosures.
Protection from electric shock from indirect contact can be by many methods.
The most common method used within a normal electrical installation is by the use of Earthed Equipotential Bonding and the Automatic Disconnection of the supply (EEBADs). Class 2 equipment (double insulated) and electrical separation (shaver socket) are also very common methods.
In a single phase system, current flow is achieved by creating a difference in potential.
If we were to fill a tank with water and raise the tank a metre or so, then connect a pipe with a tap on one end of it to the tank, when we open the tap the water will flow from the tank to the open end of the pipe. This is because there is no pressure outside of the pipe – the higher we raise the tank the greater the pressure of water and therefore the greater the flow of water.
Current flow is very similar to this. If we think of voltage as pressure, then to get current to flow we have to find a way of creating a difference in pressure.
This pressure in an electrical circuit is called potential difference and it is
achieved in a single phase system by pegging the star point of the supply transformer to earth. The potential of earth is known to be at 0 volts.
If we place a load between a known voltage and earth, the current will flow from the higher voltage through the load to earth. If we increase the voltage, then more current will flow, just as more water would flow if we increased the height of the water tank.
The problem we have with electricity is that if we use our body to provide the current with a path to earth it will use it, and possibly electrocute us at the same time.
Current will not flow unless it has somewhere to flow to – that is, from a high pressure to a lower pressure, possibly zero volts but not always. It is also possible in some instances to get different voltages in an installation, particularly during a fault where volt drops may occur due to loose connections, high resistance joints and different sizes of conductors. We must also remember that during a fault it will not only be the conductors that are live, but any metalwork connected to the earthing and bonding system, either directly or indirectly. It is highly likely that a shock by indirect contact could be received between pipe work at different voltages.
In any installation, protection must be in place to prevent electric shock.
The protection we use against direct contact is self-explanatory and we can only prevent unintentional touching of live parts – if a person is intent on touching a live conductor, we can only make it difficult for them, not impossible.
Protection against indirect contact is a different problem altogether and we can achieve it by different methods. Firstly, if there is a fault to earth all of the metal work connected to the earthing system, whether directly or indirectly, would become live. In the first instance we need to ensure that enough current will flow through the protective device to earth to operate the protective device very quickly. This is achieved by selecting the correct type of protective device, and ensuring that the earth fault loop path has a low enough impedance to allow enough current to flow and operate the device in the required time.
On its own this is not enough and that is where the equipotential and supple-mentary bonding is used. The basic principle is that, if one piece of metal work becomes live, any other parts that could introduce a potential (voltage) difference also become live at the same potential. If everything within the building is at the same potential, current cannot possibly flow from one part to another via a person or livestock.