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NORA Oilheat Technicians Manual

Chapter 8 Basic Electricity Figure 8-12: Switching arrangements (various setups) power available from the engines even if they were not using it. Electric switches open to stop the flow of electricity just like the drawbridge opens to stop the flow of traffic. Switches can be manually or automatically operated. For a switch to safely control a load on a typical oilheat system, it must be located in the source or hot line (L1), never in the neutral line! Switches in the neutral line can turn the load on and off but will not allow work to be done on the load without the risk of electric shock. There are many different kinds of switches. The most common switch is a single pole, single throw switch, or SPST for short. They are either on or off. This is a swinging gate (or drawbridge) that can make an air break in a conductor when it is open and connect the break when it is closed. A SPST switch turns everything in a single circuit on and off. There can be many SPST switches in one hot line. 8-12 Basic Electricity Another type of switch is a single pole double throw switch, or SPDT. This switch is used to turn the electricity on or off in one or the other of two separate circuits. This would be like a Y intersection. You can decide to drive on one road or the other. Some heat/cool thermostats have SPDT switches. Double pole single throw (DPST) switches can make or break two separate circuits at the same time. The two circuits being switched can be off the same voltage or different voltages. This would be like a drawbridge that had a street and a railroad track on it. The main breaker at the electric panel is a DPST. Double Pole Double Throw (DPDT) switches re-direct the power of two separate supply lines to two different circuits. See Figures 8-12. The contacts of automatically operated switches in wiring diagrams are shown in their normal (at rest) position when the unit


NORA Oilheat Technicians Manual
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