BASIC ELECTRICAL FOR THE HVAC TECHNICIAN
14. Troubleshooting
Troubleshooting is not guessing. It is a systematic process
of finding where a circuit stops operating as expected.
Throughout this course you have learned how to identify
sources, switches, electrical paths and loads. You have also
learned how to use a schematic and a meter to determine what
is happening in a circuit.
This final section brings those skills together into a
practical troubleshooting process.
Troubleshooting Is a Process
Troubleshooting is the process of determining why a circuit
is not operating as expected.
The goal is not to guess which component is bad. The goal is
to understand what the circuit is supposed to do, determine
what it is actually doing, and then use the schematic and
your meter to locate the difference.
Know what should happen before you measure what is happening.
A meter reading becomes useful when you can compare it
with the reading you expected to find.
Safety Before Troubleshooting
Electrical troubleshooting may require measurements on
energized circuits. Those measurements should only be
performed when they are necessary and when you understand
how to perform them safely.
Never perform an electrical test that you do not
understand or are not comfortable performing.
When working in a classroom or laboratory, ask your
instructor for assistance whenever you are unsure how
to proceed.
De-energize the circuit before making wiring changes,
moving conductors, or performing resistance or continuity
measurements.
YOUR BASIC TROUBLESHOOTING MODEL
Source → Switch → Path → Load
Most of the circuits in this course can be broken into four
basic parts. When a circuit does not operate, identify each
part before deciding where the problem is located.
Where does the electrical energy for this circuit
come from?
Line voltage, transformer secondary, battery, etc.
What device controls whether the circuit is complete?
Manual switch, thermostat, relay contact, etc.
Is there a complete electrical path between the
source and load?
Conductors, terminals, connections and contacts.
What device is supposed to use the electrical energy?
Lamp, relay coil, transformer primary, contactor, etc.
Start with the source.
If the proper source voltage is not available, there is
no reason to begin replacing switches or loads farther
down the circuit.
TROUBLESHOOTING PROBLEM #1
My Meter Does Not Seem to Be Working
Before using a meter to troubleshoot a circuit, make sure
the meter itself is operating properly.
1
Inspect the Meter
Check the display and battery indication.
If the meter indicates a weak battery, replace
the battery before continuing.
2
Inspect the Meter Leads
Check the leads and probes for visible damage.
Make sure the leads are plugged into the correct
meter terminals for the measurement you are
about to make.
3
Select Resistance (Ω)
With the probes not connected to an energized
circuit, select the resistance function.
4
Touch the Probes Together
The meter should indicate continuity and a
resistance very close to zero ohms.
A small resistance such as 0.1 or 0.2 Ω may
simply be the resistance of the meter leads.
5
Separate the Probes
The meter should indicate an open circuit.
Many digital meters display
OL or a similar over-range
indication.
Never measure resistance on an energized circuit.
Resistance and continuity measurements are made with
the circuit de-energized.
TROUBLESHOOTING PROBLEM #2
My Training Board Has No Power
Do not immediately assume that a circuit breaker has tripped.
First determine where power is present and where it is lost.
First Question
Is power available at the source?
Divide the Circuit
Troubleshooting becomes much easier when you divide a large
problem into smaller questions.
1
Disconnect the Training Board
Unplug the board before inspecting its wiring
or making any changes.
2
Verify the Source
If you have been trained and authorized to
measure receptacle voltage, verify that the
proper AC voltage is available at the receptacle
according to your laboratory procedures and the
meter manufacturer’s instructions.
In the classroom, ask your instructor for
assistance if you are unsure.
3
Follow the Power
If the source has the proper voltage, determine
whether that voltage reaches the incoming power
connection of the training board.
4
Find Where the Condition Changes
Continue through the circuit using the schematic.
You are looking for the point where the electrical
condition changes from what it should be to what
it should not be.
Find the last point that is correct.
If voltage is correct at one point but incorrect at the
next point in the circuit, you have greatly reduced the
area in which the fault can be located.
If a Circuit Breaker Trips
A tripped circuit breaker is not simply an inconvenience.
It may indicate that something is wrong with the circuit.
Do not repeatedly reset a tripped breaker.
Disconnect the training board and inspect the circuit
before restoring power.
In the projects completed earlier in this course, an incorrect
connection could create a very low-resistance path between
L and N. That is a short circuit and may
cause the circuit protection to operate.
Compare your wiring with the schematic and look for incorrect
connections, loose conductors, damaged insulation or other
conditions that could have caused the breaker to trip.
In the classroom, notify your instructor before resetting
the circuit protection.
TROUBLESHOOTING PROBLEM #3
I Received an Electrical Shock
STOP WORK
Receiving an electrical shock is not a normal part
of electrical troubleshooting.
1
Stop Work Immediately
Do not continue troubleshooting or operating
the circuit.
2
Disconnect Power if It Can Be Done Safely
Do not touch exposed conductors or equipment
in a way that could result in another shock.
3
Notify the Instructor or Supervisor
In a classroom or laboratory, immediately
report what happened.
4
Do Not Resume Work
The circuit should not be energized again until
the cause of the shock has been identified and
corrected.
Electrical shock can cause injury.
Follow your laboratory, workplace or emergency procedures
for evaluation after an electrical shock. Seek appropriate
medical assistance when required by the circumstances,
symptoms or applicable procedures.
Investigating the Cause of an Electrical Shock
Once the circuit has been safely de-energized, the cause
must be identified before work resumes.
| Question | What to Check | Notes |
|---|---|---|
|
Was the circuit energized when you thought it was de-energized? |
Verify the procedure used to disconnect and verify power. |
|
| Was exposed energized metal accessible? |
Inspect terminals, connections and conductor placement. |
|
| Was a conductor connected incorrectly? | Compare every connection with the schematic. | |
| Was insulation or a test lead damaged? |
Inspect conductors, meter leads, probes and components. |
|
|
How did your body become part of the electrical path? |
Identify the points of electrical potential involved. |
PUTTING EVERYTHING TOGETHER
The Eight-Step Troubleshooting Process
1
Understand the Complaint
What is the circuit supposed to do, and what
is it actually doing?
2
Study the Schematic
Identify the source, switches, electrical path
and loads.
3
Inspect the Circuit
Look for loose connections, incorrect wiring,
damaged components and other obvious problems.
4
Verify the Source
Determine whether the required source voltage
is actually available.
5
Follow the Circuit
Use the schematic and appropriate meter
measurements to determine where the circuit
stops behaving as expected.
6
Identify the Fault
Use the measurements you have made to determine
what has actually failed or been wired incorrectly.
7
Correct the Problem
De-energize the circuit and make the required
repair or wiring correction.
8
Test the Complete Circuit
Restore power using the appropriate procedure
and verify that the complete circuit now operates
correctly.
Troubleshooting Is Not Parts Changing
Do not replace switches, relays, transformers,
thermostats or other components simply because the
equipment does not operate.
MEASURE FIRST.
Use the schematic to determine what should be happening.
Take measurements to determine what is actually happening.
The difference between the two helps you locate the fault.
Final Course Reminder
The components used in HVAC/R equipment may become more
complicated, but the basic electrical principles do not change.
Every operating circuit requires a source and a
complete electrical path.
Switches and controls determine when a path is
complete.
Loads use electrical energy to perform work.
Schematics show how the circuit is supposed to operate.
Your meter tells you what the circuit is actually doing.
Good troubleshooting compares what should be happening
with what is actually happening.
Source → Switch → Path → Load
When a circuit seems complicated, return to the basics.
Identify these four parts and work through the circuit
systematically.