Electrical and Controls: Loads, Sequencing, and Component Testing
The Electrical and Controls domain tests whether you can read a ladder diagram, calculate control-circuit loading, and follow a sequence of operation to the failed component instead of guessing at parts.
Ohm's law and control loading
- E = I × R; P = V × A. On a 24 V circuit, VA ÷ 24 gives amps.
- Size and judge transformers at no more than about 80% of rated VA. Overloaded transformers still read near 24 V open-circuit but sag under load, causing chattering contactors and random lockouts.
- Series circuits share current; parallel circuits share voltage. Every safety switch in a control string is in series, so one open device kills the whole call.
Sequencing and component checks
- Typical gas sequence: call for heat → board proves limits → inducer → pressure switch → ignition → main valve, with flame sensing proving the trial for ignition.
- Capacitors: replace when more than 10% below rated microfarads, or bulging/leaking. Discharge before testing.
- PSC motors: common-to-start plus common-to-run should approximately equal start-to-run.
Worked example
A 40 VA, 24 V transformer carries a control board at 6 VA, a thermostat at 3 VA, a contactor at 8 VA, and a reversing valve at 12 VA: 29 VA, or 72.5% loaded - acceptable. Adding a 10 VA zone damper brings it to 39 VA, 97.5% of rating, which is not acceptable; the fix is a second transformer or a relay-driven damper, not a bigger fuse.
Traps that cost marks
- Reading 24 V with a high-impedance meter and concluding the circuit can carry current; load-test instead.
- Assuming a transformer can be loaded to 100% of nameplate VA.
- Jumping a safety to "see if it runs" instead of proving it.
- Checking only one side of a switch; verify both legs under load.