Chapter 8 Basic Electricity Chapter 8—Basic Electricity 8-23 wire may be dangerously hot, creating a fire. This is called an overloaded circuit. Common causes of overloads are: • Too many devices on the circuit • Devices working harder than they are supposed to • Damaged or worn out devices • Current surge when a motor starts A short circuit causes a sudden excessive draw well beyond the capacity of the circuit. It happens if the resistance of the loads is removed, or there is a direct unrestricted current flow to ground, conductors touching grounded metal, or the source conductor is touching neutral. Fuses and circuit breakers To protect against any circuit carrying too much current, every circuit needs a fuse or circuit breaker. It is wired in series so all the current in the circuit goes through it. These are automatic switches that open if the current goes higher than their rating. Fuses are one time only devices. When they blow, they must be replaced. Circuit breakers can be reset and used repeatedly. Never used an oversized fuse or circuit breaker; this defeats the purpose of the fuse or breaker and may create a dangerous situation. You should find out why the circuit is drawing too many amps, causing the fuse or breaker to open. Shock protection The electric shock from a 120-volt circuit is dangerous and can be fatal. A shock occurs when a person becomes part of an electric circuit. The severity of shock depends upon: • Amount of current • Type of voltage (AC or DC) • The path the electricity takes through the body • Amount of voltage • Time duration of the shock • Condition of the skin To protect from shock, electricity must be kept in the wire. Install the circuits so that in case the electricity escapes, it does no harm. A shock is when an electric current passes through the body, causing spastic contraction of muscles. If it goes through the heart, it may kill. Shocks happen when you touch a wire carrying lots of volts and some other part of your body is touching something at ground potential. Bare feet, especially when wet, are very dangerous. A dry human body is pretty resistant to electric flow, but a sweaty or otherwise wet one is a very good conductor. Rubber soled shoes insulate your body from ground and prevent the formation of a complete circuit. Good insulation keeps the electricity inside the wire, but insulation may crack with age or high temperature and may also wear off if it is touching a moving part. So even if it was wired right at first, with age it may go bad and let electricity leak out, presenting a shock hazard. To be sure that the electricity has an easy way to get home without going through you, install a ground fault protection system (GFI- Ground Fault Interrupter). Grounding provides a direct low resistance path from the circuit to ground.
NORA Oilheat Technicians Manual
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