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NORA Oilheat Technicians Manual

Chapter 8 Basic Electricity Figure 8-13: Warp switch, often used as a safety switch in controls Firgure 8-14: Warp switch locking burner out on safety Figure 8-13 Figure 8-14 Chapter 8—Basic Electricity 8-13 Bimetal Spring Reset Button Spring Wires Contact Blades Heater Wires Contacts Insulators Heater Wires Contact Blades Spring Reset Button Wires Spring Contacts Insulators Bimetal is not operating. Contacts on automatically operated switches are classified as normally open (NO) or normally closed (NC). The determination is made by the position of the contacts when the device is either not energized or not sensing the condition it is designed to sense. Manual switches are operated by hand. Examples of manual switches in oilburner circuits are the stair switch, the on/off switch at the burner, the manual/auto fan and thermostat switches and the reset button on the primary control and motor. The reset button is an example of a manual switch that also is switched automatically. Automatic sensing switches respond to a change in conditions such as: temperature, pressure, flow, liquid level, light and humidity. Some switches open when the temperature, pressure, light, or liquid level rises and some open when one of these things falls. If the switch “makes” (closes) on fall, this is called “makes on fall” or “breaks (opens) on rise.” If a switch opens as a result of a rise in the sensed condition, this is called a direct acting (DA) switch. If it opens on fall in the sensed condition, it is a reverse acting (RA) switch. Heat-only thermostats and high limits are direct acting switches since they open on the rise in temperature. Cooling thermostats, fan off switches in fan-limit controls, low-water cutoffs, and reverse acting aquastats are reverse acting because they open on the fall of the sensed condition. Timers–bimetals & warp switches As you will learn in the primary control chapter, one of the things an oilburner circuit must do is shut itself off on “safety” in case flame is not established after the burner has been energized. In many of these controls, there is a timer switch that uses small electrical heaters in combination with bimetals. Figures 8-13 and 8-14 shows how it works. A bimetal is composed of two different strips of metal fused together. One end of the strip is secured and a small electric heater is placed under it. As it heats up, the two metals expand at different speeds. The


NORA Oilheat Technicians Manual
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