If you want to know, you have to test. The professional hvacr service technician/installer uses electrical test instruments (ETIs) to diagnose, test, and measure equipment performance, controls, circuitry, and wiring. From troubleshooting direct digital control (ddc) systems, to setting a thermostat heat anticipator, to verifying proper voltage at a transformer, servicers/installers are knee-deep in situations that demand the use of ETIs every day.
The use of an ETI involves the ability to recognize what is being tested or measured, and to interpret that information and its relationship to overall system operation.