A most basic electrical circuit will have three components
- A source of energy. It could be a battery or an electrical outlet in the wall
- A load. A load does useful work. A load could be a light bulb or a motor
- Conductors. A conductor would be your wires to transfer the electrical energy to your load
While not entirely needed, most circuits will include a switch. A switch will stop the flow of electricity through the conductor to the load.
As you can see, with an open switch, power cannot get past the switch to turn on the lights.
With the switch in the closed position, power is allowed to flow through the circuit and turn the lights on.
What this means on a service call
If the load does not run, one part of the circuit is not doing its job. The source, the conductors, the switch, or the load itself has failed, is open, or is not connected.
Start with the path. Power leaves the source, travels through a conductor, passes through a switch or a control, reaches the load, and returns to the source through a conductor. A break anywhere on that path stops the load. A light bulb stays dark and a motor stays still when the path is open, even if the source is good.
On a furnace or air conditioner call, the switch in this lesson may be a thermostat, a limit, a pressure switch, or a relay contact. The load may be a blower motor, an inducer motor, a contactor coil, or a gas valve. Do not guess which part failed. Follow the path in order, from the source toward the load, and find where power stops.
A closed switch does not prove the whole circuit is good. It only proves that one switch is closed. A loose wire, a broken conductor, or a failed source can still leave the load dead. Check each part in turn, and write down what you find at each step so you do not recheck the same part twice.
Numbers and measurements you will see
Use the voltages that belong to the circuit you are working on. A control circuit on HVAC equipment is commonly 24V. Line power to the equipment is commonly 120V. Check the rating on the equipment and on the part before you test, and use the meter setting that fits that circuit.
- Source voltage at the source. With the circuit powered, the meter should read the source voltage, for example 24V on a control circuit or 120V on a line circuit.
- 0V at the load when the switch is open. An open switch stops power from reaching the load, so the load sees no voltage.
- Source voltage at the load when the switch is closed and the path is complete. If the load still does not run at full source voltage, the load itself is suspect.
- Source voltage across an open switch. A meter across an open switch in a powered circuit will often read source voltage, because the open switch is the break in the path.
- Continuity with the power off. A good conductor and a closed switch should show continuity. An open switch, a broken wire, or a loose connection should not.
- Current only when the circuit is complete. Current flows when the switch is closed and the path is complete. No current flows through an open switch, even if voltage is present at the source.
Read the meter at the part you are testing, not only at the equipment disconnect. A reading at the source tells you about the source. A reading at the load tells you whether power reached the load.
A common mistake
Replacing the load first is the common mistake. A technician replaces a motor or a bulb, the new part also does not run, and the real fault is still in the circuit.
Check the source, the conductors, and the switch before you condemn the load. A motor that never gets power cannot run, and a new motor will not fix a broken wire or an open switch. Test for voltage at the load. If voltage never arrives, move back toward the source until you find where it stops. If full source voltage arrives and the load does not run, then look hard at the load.
A second mistake is testing only one point and stopping. One good reading at the source does not prove the conductors are good. One reading at the load does not prove the switch is good. Test the source, test each side of the switch, and test at the load, so each part is proved in turn.
Check it in the field
Caution: live and powered work can hurt you. Treat a circuit as live until you prove it is dead with your meter. Keep your hands clear of bare conductors and terminals. If you are not sure a test is safe, stop, turn the power off, and get help. Do not work on a live circuit with wet hands, and do not bypass a switch or a safety control to force a load to run.
- Turn the power off before a continuity or resistance check. Continuity and resistance checks are done with the power off.
- Prove the meter works on a known live source, then prove the circuit is dead, then check continuity through the conductors and through the switch in its closed position.
- Restore power only for a voltage check, and only if you are trained to work on that circuit. Measure source voltage at the source first.
- Measure voltage on each side of the switch. With the switch open, expect no voltage at the load side. With the switch closed, expect source voltage to pass through to the load side.
- Measure voltage at the load. If source voltage is present and the load does not run, the load needs further testing. If source voltage is not present, move back toward the source and find the open part.
- Turn the power off again before you repair a wire, tighten a connection, or replace a part. Recheck after the repair, and confirm the load runs when the switch is closed and stops when the switch is open.