Electronics
18%of exam
Electrical
14%of exam
Mechanical
14%of exam
HVAC & Refrigeration
12%of exam
Computers & Networking
12%of exam
Spatial Reasoning
12%of exam
Math Reasoning
10%of exam
Safety
8%of exam
Quick Facts
- Exam
- USPS 955
- Format
- 2 Parts
- Part 1
- 120 Q Take-Home
- Part 2
- 60 Q + Spatial
- Time
- ~2 hr 15 min
- Pass Score
- 70 Minimum
- Fee
- Free
- Proctor
- PSI Testing Center
- Validity
- ~2 Years
Logic Gate Memory Cues
AND needs all; OR needs one
AND: all inputs 1OR: any input 1NOT: flips the inputXOR: inputs must differ
Analog vs Digital Signals
Analog
- Continuous signal range
- Infinite value gradations
- More sensitive to noise
Digital
- Discrete on/off states
- Binary 0s and 1s
- More resistant to noise
Continuous vs discrete values
Test Tool Picker
- Measure voltage, current, resistance→Multimeter(General purpose)
- View a waveform shape→Oscilloscope(Signal timing)
- Check motor winding insulation→Megohmmeter(Insulation resistance)
- Measure current, no contact→Clamp meter(Live circuit safe)
Logic Gate Truth Tables
- AND
- 1 only if all inputs 1
- OR
- 1 if any input 1
- NOT
- Inverts single input
- NAND
- 0 only if all inputs 1
- NOR
- 1 only if all inputs 0
- XOR
- 1 if inputs differ
- XNOR
- 1 if inputs match
Analog & Semiconductor Devices
- Diode
- Allows current one direction
- Transistor
- Amplifies or switches signals
- Resistor
- Limits or controls current
- Capacitor
- Stores energy as charge
- Op-amp
- High-gain differential voltage amplifier
- Semiconductor
- Conducts under controlled conditions
- Rectifier
- Converts AC to DC
- LED
- Emits light when powered
Schematics & Test Instruments
- Multimeter
- Measures voltage, current, resistance
- Oscilloscope
- Displays signal waveform shape
- Megohmmeter
- Tests insulation resistance value
- Clamp meter
- Measures current without contact
- Schematic symbol
- Represents a component type
- Continuity test
- Confirms a closed path
Ohm's Law Triangle
Cover unknown: V top, I×R bottom
V = I × RI = V ÷ RR = V ÷ I
Series vs Parallel Circuits
Series
- Single current path
- Voltage divides across loads
- Current stays the same
Parallel
- Multiple current paths
- Voltage stays the same
- Current divides across branches
One path vs many paths
Circuit Fault Troubleshooting
- No voltage at load→Check breaker or fuse(Power source)
- Voltage present, no current→Check for open circuit(Broken path)
- Breaker trips repeatedly→Check for short circuit(Low resistance)
- Motor will not start→Check overload relay reset(Thermal trip)
- Connection is intermittent→Check terminals for corrosion(Loose contact)
- Suspect a bad component→Meter it, compare spec(Continuity check)
Ohm's Law & Power Formulas
- V = I × R
- Voltage equals current times resistance
- I = V ÷ R
- Current equals voltage over resistance
- R = V ÷ I
- Resistance equals voltage over current
- P = V × I
- Power equals voltage times current
- P = I² × R
- Power equals current squared times resistance
- P = V² ÷ R
- Power equals voltage squared over resistance
AC vs DC
AC
- Current direction alternates
- Easily transformed voltage levels
- Used for long-distance transmission
DC
- Current direction constant
- Powers batteries and electronics
- No transformer voltage stepping
Alternating vs constant direction
Circuit & Current Types
- Series circuit
- Single current path only
- Parallel circuit
- Multiple current paths exist
- Short circuit
- Unintended low-resistance current path
- Open circuit
- Broken, incomplete current path
- AC
- Current direction alternates repeatedly
- DC
- Current direction stays constant
- Three-phase power
- Three offset AC waveforms
- Ground fault
- Current leaks to ground
Relay vs Contactor
Relay
- Switches low-current control signals
- Small load switching
Contactor
- Switches high-current power loads
- Motor and heater switching
Control signal vs power load
Motors & Controls
- Motor starter
- Controls motor start, stop
- Overload relay
- Protects motor from overcurrent
- Contactor
- Switches high-power circuits closed
- Solenoid
- Electromagnetic linear motion actuator
- VFD
- Variable frequency drive control
- Single-phase motor
- One AC waveform input
Open vs Short Circuit
Open Circuit
- Broken current path
- Zero current flows
- Infinite resistance reading
Short Circuit
- Unintended low-resistance path
- Excess current flows
- Near-zero resistance reading
No path vs wrong path
Mechanical Advantage Rule
Longer lever arm means less force
MA = output ÷ inputLonger arm: more advantageTrade force for distance
Hydraulics vs Pneumatics
Hydraulics
- Incompressible liquid medium
- High force output
- Smoother, more precise control
Pneumatics
- Compressible air medium
- Faster, lighter force
- Less precise control
Liquid force vs air speed
Mechanical Fault Troubleshooting
- Belt slipping or squealing→Check tension and alignment(Power transmission)
- Excess vibration present→Check bearing wear, balance(Rotating parts)
- Gear noise or backlash→Check gear mesh, lubrication(Wear pattern)
- Hydraulic power loss→Check fluid level, leaks(Pressure loss)
- Pneumatic cylinder runs slow→Check air pressure, filters(Restricted flow)
- Chain jumps the sprocket→Check chain wear, tension(Elongation)
Gears, Belts & Mechanical Advantage
- Gear ratio
- Driven teeth ÷ driver teeth
- Belt drive
- Pulley-to-pulley power transfer
- Lever
- Multiplies force around pivot
- Pulley
- Redirects or multiplies force
- Bearing
- Reduces friction, supports load
- Mechanical advantage
- Output force ÷ input force
- Torque
- Rotational turning force applied
- Chain drive
- Sprocket-to-sprocket power transfer
Gear Up vs Gear Down
Gear Up
- Increases output speed
- Reduces output torque
Gear Down
- Decreases output speed
- Increases output torque
Speed trade vs torque trade
Hydraulics & Pneumatics
- Hydraulics
- Uses pressurized liquid power
- Pneumatics
- Uses pressurized air power
- Pascal's Law
- Pressure transmits equally through fluid
- Actuator
- Converts pressure into motion
- PSI (pressure unit)
- Pounds per square inch
- Relief valve
- Limits maximum system pressure
- Check valve
- Allows one-way fluid flow
Rigging & Power Transmission
- Sling angle
- Changes rated lifting capacity
- Working load limit
- Maximum safe lifting weight
- Shaft coupling
- Connects two rotating shafts
- Sheave
- Grooved wheel guides cable
- Keyway
- Locks gear to shaft
- Bushing
- Wear-resistant sleeve on shaft
Welding & Shop Tools
- MIG welding
- Wire-fed gas-shielded weld process
- TIG welding
- Tungsten arc, precise weld
- Arc welding
- Stick electrode weld process
- Micrometer
- Measures precise small dimensions
- Torque wrench
- Applies a measured torque
- Feeler gauge
- Measures small gap width
Refrigeration Cycle Order
Compress, Condense, Expand, Evaporate, repeat
Compressor raises pressureCondenser rejects heatExpansion valve drops pressureEvaporator absorbs heat
HVAC Fault Troubleshooting
- No cooling output→Check compressor, refrigerant charge(Low charge)
- High head pressure→Check condenser airflow, fins(Poor heat rejection)
- Low suction pressure→Check evaporator airflow, filter(Restricted flow)
- System cycles too frequently→Check thermostat, pressure switch(Control fault)
Refrigeration Cycle
- Compressor
- Raises refrigerant pressure, temperature
- Condenser
- Rejects heat, condenses refrigerant
- Expansion valve
- Drops pressure before evaporator
- Evaporator
- Absorbs heat, boils refrigerant
- Superheat
- Vapor temperature above boiling point
- Subcooling
- Liquid temperature below condensing point
HVAC Controls & Piping
- Thermostat
- Controls the temperature setpoint
- Pressure switch
- Cuts power at threshold
- Schrader valve
- Access port for refrigerant
- Piping insulation
- Prevents heat, condensation loss
- Threaded joint
- Screwed pipe fitting connection
- Soldered joint
- Heat-bonded copper pipe joint
Computers & Networking
- IP address
- Identifies a device online
- RJ45
- Copper Ethernet cable connector
- Fiber optic
- Sends data using light
- RAM
- Temporary working memory storage
- CPU
- Processes computer instructions
- USB
- Serial peripheral connection standard
- PLC
- Programmable logic control computer
- Router
- Directs network traffic paths
Spatial Reasoning Skills
- Rotation
- Object turned around an axis
- Paper folding
- Predicts a punched-hole pattern
- Mirror image
- Reversed, flipped left-right orientation
- Assembly visualization
- Parts combine into a whole
- Block counting
- Counts hidden cube faces
- Isometric view
- 3D drawing at equal angles
Math & Unit Conversions
- Area
- Length × width
- Volume
- Length × width × height
- Circumference
- π × diameter
- °F to °C
- Subtract 32, then divide 1.8
- Feet to inches
- Multiply by 12
- Watts to horsepower
- Divide by 746
LOTO Six-Step Sequence
Notify, Shutdown, Isolate, Lock, Release, Verify
Notify affected employeesShut down equipment fullyIsolate the energy sourceLock and tag deviceRelease any stored energyVerify zero energy state
Lockout vs Tagout
Lockout
- Physical lock device used
- Prevents energy release
Tagout
- Warning tag only used
- Relies on communication alone
Lock prevents; tag warns
Lockout/Tagout & Hazcom
- LOTO
- Isolate energy, then verify
- Arc flash boundary
- Safe minimum approach distance
- PPE
- Personal protective equipment gear
- SDS
- Safety Data Sheet document
- Hazcom
- Hazard Communication Standard rules
- Confined space
- Limited entry, permit-required area
Common Traps
Lockout ≠ Tagout
Lockout physically prevents operation ≠ Tagout only warns, no lock
Series ≠ Parallel Voltage
Series divides voltage across loads ≠ Parallel keeps voltage equal always
Hydraulics ≠ Pneumatics
Hydraulics uses incompressible liquid power ≠ Pneumatics uses compressible air power
Relay ≠ Contactor
Relay switches low-current control signals ≠ Contactor switches high-current power loads
Open ≠ Short Circuit
Open circuit stops all current ≠ Short circuit causes excess current
AND ≠ OR Gate
AND needs every input high ≠ OR needs just one input
Superheat ≠ Subcooling
Superheat measures the vapor side ≠ Subcooling measures the liquid side
Last Minute
- 1.V = I × R
- 2.Series: one path, divided voltage
- 3.Parallel: many paths, equal voltage
- 4.AND needs all inputs high
- 5.LOTO: isolate, lock, then verify
- 6.Hydraulics = liquid; pneumatics = air
- 7.Passing score = 70 minimum
- 8.Part 1 home; Part 2 proctored
- 9.Compressor raises pressure and temperature
- 10.PPE must match the hazard
- 11.Exam fee = $0 always
- 12.Eligibility register lasts about 2 years
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