Electrical Principles and Circuit Diagnostics
Introduction
This chapter covers the essential electrical fundamentals for the Red Seal exam for appliance service technicians. You must master these concepts not only to pass the exam but also to effectively diagnose faults in the field. Electrical diagnostics represent approximately 30% of the tasks in your trade, and exam questions reflect this reality.
2.1 Fundamental Electrical Quantities
2.1.1 Voltage (V)
Voltage is the electrical potential difference between two points in a circuit. It is expressed in volts (V) . It is the "pressure" that pushes electrons through the circuit.
Standard residential voltage in Canada: 120/240 V single-phase, 60 Hz
Control voltage (thermostats, timers): 24 V AC (step-down transformer)
Motor voltage: 120 V or 240 V depending on the configuration
> For the exam: Voltage is always measured between two points. A voltage measurement is taken in parallel with the circuit, never in series.
2.1.2 Current (I)
Current is the flow of electrons in a conductor. It is expressed in amperes (A) . Current is measured in series with the circuit using an ammeter or a clamp meter.
Alternating current (AC): changes direction periodically (60 cycles/second in Canada)
Direct current (DC): flows in one direction only (electronic circuits, control boards)
2.1.3 Resistance (R)
Resistance is the opposition to the flow of current. It is expressed in ohms (Ω) . In appliances, resistance is found in heating elements, motors, and solenoids.
Factors affecting resistance:
Length of the conductor (longer = more resistance)
Cross-sectional area (larger = less resistance)
Temperature (increases with heat for most metals)
Type of material (copper, aluminum, nichrome)
2.1.4 Power (P)
Power is the rate at which electrical energy is converted into another form of energy (heat, motion). It is expressed in watts (W) .
Essential formulas:
P = V × I (power = voltage × current)
P = I² × R (power = current² × resistance)
P = V² ÷ R (power = voltage² ÷ resistance)
2.1.5 Ohm's Law
Ohm's Law is the fundamental relationship between voltage, current, and resistance:
V = I × R
| Quantity to Find | Formula | Example (V=120 V, R=12 Ω) |
|---|
| Voltage (V) | V = I × R | V = 10 A × 12 Ω = 120 V |
| Current (I) | I = V ÷ R | I = 120 V ÷ 12 Ω = 10 A |
| Resistance (R) | R = V ÷ I | R = 120 V ÷ 10 A = 12 Ω |
2.2 Electrical Circuits
2.2.1 Series Circuit
In a series circuit, components are connected end-to-end, forming a single path for current.
Characteristics:
Current is identical throughout the circuit
Total voltage is the sum of individual voltages
Total resistance = sum of resistances: R_total = R₁ + R₂ + R₃...
If one component is open, the entire circuit stops working
Practical application: The heating elements in some dryers are wired in series to achieve different power levels.
2.2.2 Parallel Circuit
In a parallel circuit, components are connected between the same two points, providing multiple paths for current.
Characteristics:
Voltage is identical across each branch
Total current = sum of currents in each branch
Total resistance is calculated: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃...
If one branch is open, the others continue to function
Practical application: The 120 V supply circuits in a range (elements, oven, clock, lights) are in parallel.
2.2.3 Calculating Equivalent Resistance
For two resistors in parallel, the simplified formula is:
R_total = (R₁ × R₂) ÷ (R₁ + R₂)
> Exam example: Two heating elements of 20 Ω each are in parallel. R_total = (20 × 20) ÷ (20 + 20) = 400 ÷ 40 = 10 Ω. The total current at 240 V will be I = 240 ÷ 10 = 24 A.
2.3 Measuring Instruments and Diagnostics
2.3.1 The Digital Multimeter
The digital multimeter is the primary diagnostic tool for the appliance service technician. It measures voltage (AC and DC), current, and resistance.
Safety and best practices:
Always start with the highest measurement range
Check the condition of test leads and probes
Never measure resistance on a live circuit
Use the "continuity" function (buzzer) to check fuses and connections
To measure a component's resistance, disconnect it from the circuit (isolate at least one end)
2.3.2 The Clamp Meter
The clamp meter measures current without interrupting the circuit. It is essential for:
Measuring a motor's starting current
Checking the load of a heating element
Detecting leakage current
> Exam trap: The clamp must surround only one conductor (never both wires of a circuit together, as the magnetic fields would cancel each other out).
2.3.3 Systematic Diagnostic Procedure
72.Check the power supply: confirm voltage is present at the outlet (120 V or 240 V)
73.Check the power cord: continuity of each conductor
74.Check the appliance fuses/circuit breakers
75.Test the safety components: door switches, thermostats, pressure switches
76.Test the load components: elements, motors, solenoids
77.Check the control board (if applicable): input and output voltages
2.4 Electrical Components Specific to Appliances
2.4.1 Heating Elements
Heating elements convert electrical energy into heat through Joule heating. They are made of nichrome (nickel-chromium alloy) which withstands high temperatures.
Calculating element power:
At 240 V, a 20 Ω element produces: P = V² ÷ R = 240² ÷ 20 = 57,600 ÷ 20 = 2,880 W
Diagnostic tests:
Continuity test: expected resistance (typically between 10 and 50 Ω depending on power)
Insulation test: resistance between the element and ground (must be infinite or very high, > 1 MΩ)
Visual test: hot spots, deformation, cracks
2.4.2 Motors
Motors in appliances are primarily:
Universal motors (series): vacuum cleaners, blenders, dryer blowers
Induction motors: oven fans, dishwasher pumps
Brushless DC motors: modern front-load washing machines
Motor windings:
Run winding: lower resistance
Start winding: higher resistance
Start capacitor: provides the phase shift needed for starting
Testing windings:
Measure resistance between each terminal and the common terminal
Compare with manufacturer values
Check insulation between windings and the housing
2.4.3 Solenoids and Water Valves
Solenoids (water inlet valves) control water flow in washing machines and dishwashers. They operate at 120 V AC or 24 V DC depending on the model.
Characteristics:
Typical coil resistance: 500 to 2,000 Ω
Continuity test: the coil must show finite resistance
Operation test: check water flow with voltage applied
Check water pressure (minimum 20 psi, maximum 120 psi)
2.4.4 Thermostats and Thermistors
Mechanical thermostats (bimetallic):
Open or close the circuit based on temperature
Test: continuity at room temperature (normally closed) or when heated (normally open)
Verify calibration with a reference thermometer
Thermistors (electronic temperature sensors):
Resistance varies with temperature
NTC (negative temperature coefficient): resistance decreases as temperature increases
PTC (positive temperature coefficient): resistance increases as temperature increases
| Type | Behavior | Typical Application |
|---|
| NTC | Resistance ↓ as T ↑ | Oven temperature sensor, washing machine |
| PTC | Resistance ↑ as T ↑ | Motor protection, refrigerator start relay |
> For the exam: You will be given a resistance value at room temperature (e.g., 10 kΩ at 25 °C) and you must determine if the sensor is faulty by comparing it with the measured value.
2.5 Canadian Electrical Code — Relevant Provisions
The Canadian Electrical Code, Part I (CE Code) (C22.1) applies to the installation of appliances. Although the appliance service technician is not an electrician, you must be familiar with the following rules:
2.5.1 Rule 8-200 — Load Calculations
Rule 8-200 of the Canadian Electrical Code deals with branch circuit load calculations. For appliances:
Each fixed appliance must have its own branch circuit if its power rating exceeds 1,500 W
120 V circuits are limited to 80% of their rated capacity for continuous loads
A 15 A, 120 V circuit can support a maximum load of 1,440 W (15 A × 120 V × 0.8)
2.5.2 Rule 26-700 — Appliances
Rule 26-700 specifically covers appliances:
Appliances must be installed in accordance with the manufacturer's instructions
Connections must be made with approved connectors
Appliances must be grounded in accordance with Rule 26-700(2)
2.5.3 Grounding and Bonding
Rule 10-200: grounding of electrical installations
Rule 10-400: grounding of appliances
Appliances must be connected to ground by a grounding conductor
The grounding conductor must be green or green with yellow stripes
> Exam trap: The grounding wire normally carries no current. It only serves in the event of a fault. Current measured on the ground wire indicates an insulation fault.
2.5.4 Rule 26-744 — Connection of Ranges and Dryers
Electric ranges and dryers at 240 V must be connected with:
An approved cord and plug (NEMA 14-30 for new installations)
A separate grounding conductor (4 wires: 2 phases, neutral, ground)
Older 3-wire installations (without separate ground) are no longer permitted for new installations
2.6 Electrical Safety
2.6.1 The Dangers of Electric Current
The human body is a conductor. The effects of current depend on:
The intensity of the current
The path through the body
The duration of exposure
The frequency (60 Hz alternating current is more dangerous than direct current)
| Intensity | Effect on the Human Body |
|---|
| 0.5 to 1 mA | Slight perception |
| 5 to 10 mA | Painful shock, loss of muscle control |
| 10 to 20 mA | Possible respiratory paralysis |
| 50 to 100 mA | Ventricular fibrillation (lethal) |
| > 100 mA | Cardiac arrest, severe burns |
2.6.2 Safety Rules for the Technician
153.Unplug the appliance before any internal work
154.Verify the absence of voltage with a voltage tester or multimeter
155.Use insulated tools conforming to CSA standards
156.Never bypass safety devices (door locks, thermostats)
157.Follow lockout/tagout procedures in commercial environments
158.Verify the ground connection before returning the appliance to service
159.Never work alone on live circuits
2.6.3 Arc Flash Protection
Arc flashes can reach temperatures of 20,000 °C. To protect yourself:
Wear appropriate personal protective equipment (PPE)
Use Class 0 insulated gloves (1,000 V)
Work de-energized whenever possible
Maintain a safe distance (Table 2 of CSA Z462)
2.7 Troubleshooting Common Faults
2.7.1 Appliance Completely Dead
Diagnostic procedure:
170.Check the wall outlet (is voltage present?)
171.Check the circuit breaker or fuse in the panel
172.Test the power cord (continuity)
173.Test the appliance's internal fuse (if present)
174.Check the program selector or main switch
175.Check the connections at the terminal block
Common causes:
Tripped circuit breaker (overload or short circuit)
Blown internal fuse
Damaged power cord (internal break)
Defective door switch (safety interlock)
2.7.2 Appliance Works but Does Not Heat
Diagnostic procedure:
183.Check voltage at the element terminals (240 V or 120 V depending on the model)
184.If voltage is present, test the element's continuity
185.Check the thermostats in series with the element
186.Check the relay or control triac on the electronic board
187.Check the temperature sensor (thermistor)
Verification calculation:
A 3,500 W element at 240 V should have a resistance of:
R = V² ÷ P = 240² ÷ 3,500 = 57,600 ÷ 3,500 = 16.46 Ω
If the measurement gives 0 Ω: short circuit (the element is burned out and may trip the circuit breaker)
If the measurement gives ∞ Ω: open circuit (the element is broken)
2.7.3 Motor Does Not Start
Diagnostic procedure:
195.Check voltage at the motor terminals
196.Check the start capacitor (capacitance test)
197.Test the windings (resistance)
198.Check the start relay (on refrigerators)
199.Check the bearings (mechanical binding)
Capacitor test:
Discharge the capacitor before handling (short the terminals with a resistor)
Use a multimeter with capacitance function
Compare the measured value with the rated value (± 10%)
A swollen or leaking capacitor is always defective
2.7.4 Water Leak (Washing Machine, Dishwasher)
Electrical checks:
207.Test the water inlet valves (coil continuity)
208.Check voltage at the valve terminals during the fill cycle
209.Check the pressure switch (water level switch)
210.Check the level sensor (pressure transducer)
2.8 Interpreting Electrical Schematics
2.8.1 Standard Symbols
| Symbol | Component |
|---|
| ——— | Conductor |
| ——— | Connection |
| ——— | No connection (crossing) |
| — | — | Single-pole switch |
| —( )— | Solenoid / coil |
| —/\/\/— | Resistor / heating element |
| — | > | — | Diode |
| —(M)— | Motor |
| —[ ]— | Fuse |
| —(T)— | Thermostat |
| —(P)— | Pressure switch |
2.8.2 Reading a Schematic
The electrical schematic is read from left to right and top to bottom. The line (hot) conductor (L) is typically at the top, and the neutral conductor (N) at the bottom.
Key points:
Identify the power source (120 V or 240 V)
Follow the control circuit (low voltage) separately from the power circuit
Locate normally open (NO) and normally closed (NC) contacts
Identify safety devices in series in the circuit
2.8.3 Wiring Diagrams vs. Schematic Diagrams
Wiring diagram: shows the actual physical position of components
Schematic diagram (ladder diagram): shows the logical operation of the circuit
> For the exam: You will be presented with a schematic and must identify the defective component based on the described symptoms. Practice following the circuit step by step.
2.9 Practical Calculations for the Exam
2.9.1 Calculating Appliance Current
Formula: I = P ÷ V
Example: A 5,500 W dryer at 240 V
I = 5,500 ÷ 240 = 22.92 A
Verification: The required circuit breaker must be ≥ 22.92 A × 1.25 = 28.65 A, therefore a 30 A breaker.
2.9.2 Calculating Energy Consumption
Formula: E = P × t (energy = power × time)
Example: A 3,000 W oven used 2 hours per day
E = 3,000 W × 2 h = 6,000 Wh = 6 kWh per day
2.9.3 Calculating Heating Element Resistance
Formula: R = V² ÷ P
Example: 4,000 W element at 240 V
R = 240² ÷ 4,000 = 57,600 ÷ 4,000 = 14.4 Ω
2.9.4 Calculating Total Circuit Power
Series circuit: P_total = P₁ + P₂ (if resistances are identical, power is divided by 2 for each element)
Parallel circuit: P_total = P₁ + P₂ (power adds up)
> Exam trap: Two 2,000 W elements in series on 240 V do not produce 4,000 W. Each element receives 120 V, so it produces P = 120² ÷ R. If R = 14.4 Ω (for 2,000 W at 240 V), each element produces 120² ÷ 14.4 = 1,000 W. Total = 2,000 W, not 4,000 W.
2.10 Standards and Certification Organizations
2.10.1 CSA (Canadian Standards Association)
Appliances sold in Canada must be certified CSA or by an equivalent accredited organization (cUL, cETL). Certification guarantees compliance with Canadian safety standards.
2.10.2 Relevant Standards
CSA C22.2 No. 64: Electric cooking appliances
CSA C22.2 No. 112: Clothes drying appliances
CSA C22.2 No. 120: Refrigerators and freezers
CSA C22.2 No. 1335: Dishwashing appliances
CSA C22.2 No. 1335.1: Laundry washing appliances
2.10.3 Appliance Nameplates
The nameplate on the appliance indicates:
The model and serial number
The rated voltage (V)
The frequency (Hz)
The rated current (A) or power (W)
The type of refrigerant (for refrigeration appliances)
> For the exam: Nameplate values serve as the reference for calculations. An appliance rated 120 V/15 A has a maximum power of 1,800 W.
2.11 Advanced Troubleshooting of Electronic Boards
2.11.1 Basic Electronic Components
Resistor: limits current, identified by color bands
Capacitor: stores energy, filters voltages
Diode: allows current to flow in one direction only
Triac: electronic switch for alternating current
Relay: electromechanical switch controlled by a coil
2.11.2 Testing Components on the Board
Resistor:
Measure with the multimeter in ohmmeter mode
Compare with the value indicated by the color bands
Typical tolerance: ± 5% (gold), ± 10% (silver)
Diode:
Diode mode on the multimeter
Expected threshold voltage: 0.5 to 0.7 V (silicon)
Infinite voltage in the reverse direction
Capacitor:
Capacitance mode on the multimeter
Check the rated value (µF, nF, pF)
A shorted capacitor displays 0 µF or very low resistance
2.11.3 Replacing Electronic Boards
Unplug the appliance before any work
Photograph the connections before disassembly
Verify part number compatibility
Follow electrostatic discharge (ESD) precautions
Wear an anti-static wrist strap if available
2.12 Final Verification and Commissioning
2.12.1 Safety Tests Before Returning to Service
293.Insulation test: measure resistance between each live conductor and ground (must be > 1 MΩ)
294.Ground continuity test: resistance < 1 Ω between the ground prong of the plug and the chassis
295.Polarity test: verify that line and neutral are correctly connected
296.Functional test: run a complete cycle of the appliance
2.12.2 Documentation
Record the measurements taken
Note the parts replaced and their part numbers
Indicate the root cause of the fault
Recommend preventive measures
Pitfalls to Avoid
304.Measuring the resistance of a component still connected to the circuit: the measurement will be skewed by parallel paths. Always isolate at least one end of the component.
305.Confusing voltage and current: voltage is measured in parallel, current in series. An incorrect multimeter connection can damage the instrument.
306.Forgetting to check the power supply first: 50% of service calls are due to a faulty outlet or tripped circuit breaker.
307.Neglecting the insulation test: a heating element can show normal continuity but have a ground fault. Always test between the element and the chassis.
308.Confusing 120 V and 240 V circuits: in a 240 V circuit, both wires are live (no neutral). Measuring 120 V between one wire and ground is normal.
309.Forgetting to discharge capacitors: start capacitors can retain a lethal charge. Always discharge them before handling.
310.Ignoring nameplate values: calculations must be based on rated values, not assumptions.
311.Bypassing safety devices: never shunt a door lock or safety thermostat to "test" the appliance.
312.Using a multimeter on the wrong range: always start with the highest range to avoid overloading the instrument.
313.Not checking the ground connection: a ground fault can make an appliance dangerous even if it operates normally.
Summary
Ohm's Law (V = I × R) and the power formulas (P = V × I, P = I² × R, P = V² ÷ R) are the fundamental mathematical tools of electrical diagnostics.
Series circuits divide voltage; parallel circuits divide current. Most appliance circuits are in parallel.
The multimeter measures voltage (in parallel), current (in series), and resistance (de-energized).
Heating elements are tested for continuity and insulation. Their resistance is calculated with R = V² ÷ P.
Motors are tested by measuring winding resistance and checking capacitors.
The Canadian Electrical Code, Part I (Rules 8-200, 26-700, 26-744) governs appliance installation.
Safety is paramount: unplug, verify absence of voltage, use insulated tools, never bypass safety devices.
Systematic diagnostics always follow the same order: power supply, cord, fuses, safety devices, load components, electronic board.
NTC thermistors decrease in resistance as temperature increases; PTC thermistors increase.
The nameplate provides the reference values for all verification calculations.
Final safety tests (insulation, ground, polarity) are mandatory before returning an appliance to service.
This chapter covers the essential concepts for passing the "Electrical" section of the Red Seal exam. Review the formulas, practice the calculations, and memorize the diagnostic procedures. Good luck with your preparation!
