Switch Symbols
Switch symbols are employed to represent many common control sensing devices. There are four basic symbols: normally open (NO), normally closed (NC), normally open held closed (NOHC), and normally
closed held open (NCHO). To understand how these switches are drawn, it is necessary to begin with how normally open and normally closed switches are drawn (Figure 2 – 17, page 18). Normally open switches are drawn with the movable contact below and not touching the stationary contact. Normally closed switches are drawn with the movable contact above and touch- ing the stationary contact.
The normally open held closed and normally closed held open switches are shown in Figure 2 – 18 (page 19). Note that the movable contact of the nor- mally open held closed switch is drawn below the stationary contact. The fact that the movable contact is drawn below the stationary contact indicates that the switch is normally open. Since the movable contact is touching the stationary contact, however, a complete
circuit does exist because something is holding the contact closed. A very good example of this type of switch is the low pressure switch found in many air conditioning circuits (Figure 2 – 19, page 19). The low pressure switch is being held closed by the refrigerant in the sealed system. If the refrigerant should leak out, the pressure would drop low enough to permit the con- tact to return to its normal open position. This would open the circuit and de-energize coil C, causing both C contacts to open and disconnect the compressor from the power line. Although the schematic indicates that the switch is closed during normal operation, it would have to be connected as an open switch when it is wired into the circuit.
The normally closed held open switch is shown open in Figure 2 – 18. Although the switch is shown open, it is actually a normally closed switch because the movable contact is drawn above the stationary con- tact, indicating that something is holding the switch open. A good example of how this type of switch can be used is shown in Figure 2 – 20 (page 20). This circuit is a low water warning circuit for a steam boiler. The float switch is held open by the water in the boiler. If the water level should drop sufficiently, the contacts will close and energize a buzzer and warning light.