Safety in electrical systems

1. Safety in electrical systems Electricity distribution and safety measures: Insulation, fuses, earths, and circuit breakers

2. The electricity supply we rely on so much in our homes is extremely dangerous. We therefore need to take great care in its use, and provide several different levels of protection. A typical house will have a 240V electricity supply capable of providing 15kW. This is needed to allow the use of electric showers (8kW) electric heaters (1 – 3 kW each), electric cookers (4kW or more) etc. A simple calculation shows why the voltage used for mains electricity is so high. Remember Power = Volts * Current 15 kW = 240V * 60A - or- 12V * 1250A - or - 36 kV * 0.4A Distributing the necessary power at low voltage would require extremely thick wires which would be both impractical and expensive. Distributing the power at very high voltages would be very unsafe, and require impracticably high standards of insulation. However very high voltages (415 kV) are used for overhead lines where people are not exposed to the hazard. Electricity is dangerous

3. Supply connections and Earths Electricity sub-station Your house • All domestic electrical appliances will work with only two wires the Live wire (Brown or Red) and the Neutral wire ( Blue or Black). The electrical supply in normal operation will only flow through these two wires. The third wire, if used, is the Earth or Ground (Green or Yellow/Green). L Fuses,Consumer Unit & Meter N E Substation earth Local earth

4. 1 3 2 N Three phase electricity distribution • Mains electricity is distributed as three ac signals of the same voltage but with different phases, as shown in the diagrams below, and in the photo of overhead electric wires. The neutral line is connected at your local electricity sub-station.

5. Here you can see: • High voltage (180kV) distribution via pylons – 3 wires each side • Medium voltage (30kV) distribution via poles – three wires • Transformer • Local distribution (240V) via poles – four wires – three phases plus neutral.

6. Insulation, Earths, Fuses and Circuit Breakers Techniques to provide safety in using electricity

7. 1: Insulation protects people • Insulation protects us from the electricity. • All cables used to carry electricity consist of a metal core of one or more wires, covered by a layer of insulating material. • All electrical fittings – switches, sockets, etc. are made of a plastic or ceramic insulator. (sometimes the outside is metal) • Electrical sockets have an insulating shutter that closes when the plug is removed. Also the conductors are placed deep inside the socket so they cannot be touched directly. • All equipment that does not have a secure earth must be double-insulated. This means that even if the case is removed no electrical connections are exposed.

8. 2: Fuses protect equipment • If a fault develops in a piece of equipment it can draw too much current. This leads to overheating due to the increased power, and poses a risk of fire. Without something to stop the current flow either the equipment or the supply wiring would burst into flame. • A fuse is a thin piece of wire in a carrier. If the current increases too much it gets hot and melts. This stops the current from flowing. The fuse is placed in the live connection. All modern electrical equipment has a fuse in the mains plug. FUSE

9. FUSES • The problem here is that the fuse on each product needs to be rated at a slightly higher current that the products need for normal operation. So my fridge which needs 2 to 3 amps to run has a fuse rated at 5 amps. If an electrical fault occurs such as a faulty motor the fuse blows. • If the electrical insulation breaks down the metal case of the fridge is connected to the electrical circuit. When I touch it the fuse will only blow if I get five amps through my body.  (After a second or two I would not care – death is a great tranquillizer!)

10. 3: Earths protect people • The earth on the appliance is connected to any metal part and the earth in the socket is connected to a water pipe, earth spike, or any conductor sunk into the ground. Should the fridge casing now be accidentally connected to the electrical supply the current will flow from the supply to the casing, then down to earth and back to the power station.  The power station has a similar earth system connected to it allowing the current return to its source. Because the fridge casing is a good conductor of electricity large currents will flow thus blowing the fuse in a fraction of a second. If the metal case of an appliance is effectively earthed no hazardous voltages can appear on it.

11. .. If they are correctly wired!

12. Circuit breakers (MCB’s and RCCB’s) • Electromechanical relays can be used to disconnect the supply if the current being drawn exceeds their rated capacity. These are called circuit breakers. (MCB’s) • They behave in the same way as fuses, except that they can be reset by the switch.

13. RCCB’s or RCD’s • Modern safety systems are available which compare the flow of current through the live and  neutral wires. If they are not the same they cut off the electrical supply. These circuit breakers are called Residual Current Circuit Breakers. They are available as wired in devices, and also in plugs and adaptors to provide additional protection e.g. for hedge trimmers or lawnmowers. • Protection: a mismatch of 30mA will trip the breaker within 40msec. This is sufficient to provide protection from dangerous electric shocks.

14. Shock hazards • Your body from an electrical point of view is like a bag of conductive jelly. Most of the resistance is in the skin, and this is why electric shocks cause burns. • In most cases where electric shock can be dangerous there are no good electrical connections to the body. For example when you touch a live wire the current flows through your skin, inside your body to your feet, and through your shoes, carpet, floor etc to ground. Usually the resistance is so high you only feel a slight tingle. • (Don’t try this at home boys & girls!)

15. Electric shock • The time a real problem can occur is if you are touching both live and neutral or earth at the same time. If you can, unplug or take the fuses out before working on electrical equipment. Then test with a neon screwdriver or similar to make sure there are no live connections.

16. Current paths through your body A good rule is to keep one hand behind your back when working with electrical connections that could be live. This prevents the possibility of a circuit path across your chest (left). Dry shoes are usually an effective insulator, so the path down your body is much less likely (unless you are struck by lightning)

17. Patient / operator safety • Special care needs to be taken when a patient or operator has connections made to their body. For example when a patient in hospital is having an ECG with good electrical connections to their chest, or EEG with good connections to their skull the possibility exists of fatal shocks. Clearly extreme measures need to be taken to prevent this, and the ideal is for equipment to use only low voltage supplies that are totally isolated from the mains – ie using batteries. • Signal connections can then be made to mains operated equipment via non-conductive optical fibers.

18. Electrical safety outdoors • A second hazard situation exists when electrical equipment is to be used out of doors, where damp conditions can drastically reduce resistance of skin, clothing and footwear, and where the operator is usually standing directly on the ground. • Gardening equipment – hedge trimmers, chainsaws, strimmers, lawnmowers etc can all create a major shock hazard especially if the connecting cable is accidentally damaged. They MUST be protected by an RCD breaker directly in the line – either as a fixed plug or as a plug-in adaptor.