Electrical Symbols
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Symbols 21 HVAC Industry Standards In order to read and understand an electrical diagram you need to know how different controls and loads are drawn. 22 SYMBOLS SINGLE POLE SINGLE THROW SPST 23 SYMBOLS SINGLE POLE SINGLE THROW NORMALLY OPEN SPST - NO 24 SYMBOLS SINGLE POLE SINGLE THROW NORMALLY CLOSED SPST - NC 25 SYMBOLS DOUBLE POLE SINGLE THROW DPST – 2 NO 26 SYMBOLS SINGLE POLE DOUBLE THROW SPDT 27 SYMBOLS DOUBLE POLE DOUBLE THROW DPDT 28 Relays or Contactors R Normally Closed Contact R Normally Open Contact Relay Coil Relays C Contactors Contactor Coil C 29 Relays L2 120v M 24v c 30 Relays L2 Large Current 120v M 24v Small Current c 31 Relays or Contactors THE CONTACTS OF A RELAY OR CONTACTOR ARE DRAWN IN A DE-ENERGIZED CONDITION 32 Pressure Switches High- Pressure Switch SPST Opens on Rise (Safety Control) Low-Pressure SPST Opens on Drop Switch (Safety Control) 33 Pressure Switches Air Pressure SPST Closes on Drop (Operating Control) Switch SPST Closes on Rise Fan Cycling (Operating Control) Pressure Switch 34 Pressure Switches Differential Air Pressure Switch SPST Closes on Rise (Differential) Differential Refrigerant Pressure SPST Opens on Rise (Differential) Switch 35 Pressure Switches THE CONTACTS OF A SAFETY CONTROL ARE DRAWN IN THE NORMAL RUN POSITION, USUALLY CLOSED 36 Temperature Switches SPST Closes on Drop (Operating Control) Heating Defrost Thermostat Thermostat SPST Closes on Rise (Operating Control) Return Water Cooling Thermostat Temperature Controller 37 Temperature Switches SPST Opens on Rise (Safety Control) Overtemperature Discharge Gas Limit Switch Thermostat SPST Opens on Drop (Safety Control) Freeze Protection Thermostat 38 Temperature Switches SPDT Temperature (Safety or Operating) Low-Water Temperature Cutout Control Klixonâ Bi-Metal Temperature Overtemperature (Safety) Cutout Control 39 Temperature Switches THE CONTACTS OF AN OPERATING CONTROL ARE DRAWN OPEN, WITH NO CALL FOR COOLING, HEATING 40 Humidity Control SPST Humidistat Closes on Drop (Operating Control) SPST De-Humidistat Closes on Rise (Operating Control) 41 Time Delay Time Delay Relay Delays Closing Time Delay Relay (Mechanical/Thermal) (Electronic) (Electronic) Time Delay Relay Time Delay Relay (Mechanical) Delays Opening (Mechanical/Thermal) (Electronic) Time Delay Relay (Thermal) 42 Time Delay THE CONTACTS OF A TIMER ARE DRAWN IN THE READY-TO-START POSITION OR AT TIME ZERO ON A BAR CYCLE DIAGRAM NOTE: BLACK DENOTES CLOSED CIRCUITS 43 Manual Switches SPST Normally Open Manually Operated Push Button SPST Normally Closed Manually Operated Momentary Push Button Push Button SPDT Switch Single-Pole Double-Throw Switch 44 Flow Switches Flow Switch SPST Flow Switch Opens on Rise (Safety Control) Sail Switch SPST Flow Switch Closes on Rise (Safety and Operating) Thermal Dispersion Flow Switches 45 Enthalpy Switches Electronic Enthalpy Control SPDT Enthalpy Control (Operating Control) Mechanical Enthalpy Control 46 Capacitors Run Capacitor Run Capacitor (Marked Terminal) Dual Run Capacitor Start Start Capacitor Capacitor 47 Fuses One Time Fuse (Fusible Link) Fuse - Fusible Link One Time Fuse 48 Transformers 49 230 VAC 24 VAC Fused & Non Fused Secondary COMM NF F 24 VAC Multi Tap Primary 575 480 380 230 200 C 50 Dual Secondary Transformer Dual Secondary With Center Tapped Transformer 51 Transformers Secondary Volt Amps Voltage VA RATING TOTAL AMP DRAW OF A CIRCUIT = SECONDARY VOLTAGE 75VA = = 3.1 AMPS 24V 52.