Electrical pressure switches provide on/off contact closures for use with equipment or an alarm or shutdown system. They should have one of three standard case designs, depending on the electrical classification where they are installed:
• Explosionproof (NEMA 7)
• Weather-resistant (NEMA 3 & 4)
• General purpose (NEMA 1)
Electrical pressure switches should have snap-acting, dual, single pole double throw (SPDT) or double pole double throw (DPDT) contacts. The contacts should be rated to supply the operated device with a minimum of 10 amperes at 115 volts AC and 5 amperes at 28 volts DC. For potentially corrosive environments and for intrinsically safe systems, hermetically sealed switch contacts should be specified. The engineer should study the switch specifications to verify that its ratings are compatible with the application. Switch contacts should be open in the alarmed condition. Terminal blocks or terminal strips should be provided. Dead front or shrouded terminal blocks are acceptable.
The electrical conduit connection should have a minimum diameter of ½ inch. If the switch contacts are handling milliamp signals, the contacts may be specified with a gold or other conformal coating to minimize oxidation. This usually adds only about 20 dollars to the cost of the switch.
The pressure element may be a Bourdon tube, a bellows diaphragm, or a spring disk. Spring disks are preferred if the application permits a relatively large dead band (about 7% to 8% of the differential pressure range). Otherwise, specify Bourdon tubes for set pressures above 100 psig and bellows diaphragms for set pressures less than 100 psig. The pressure switch setpoint should be in the middle third of the range. Proof pressure should be higher than the maximum process pressure.
Electrical pressure switches are available with either a fixed or an adjustable dead band between the setpoint and the reactivation point (see Figure 400-7). Close differential switches are generally factory set at 0.5% to 1% of the span. On double adjustment switches, both the set and reactivation points can be adjusted. The minimum differential varies from 2% to 8% of the span. The type of switch depends on the application.
Electrical pressure switches should have internally adjustable setpoints with calibrated scales. Dual control electrical pressure switches are available with two independent switches in the same housing.
Selection of the adjustable range for a specific installation should consider both the setpoint actuation accuracy and the life factor. For greatest accuracy, the setpoint should fall in the upper half of the range. For the most favorable component life, it should be in the lower half. The usual compromise is to specify a setpoint in the
middle third of the range.