Electrical and Instrumentation Fundamentals
What this domain tests
Electrical and Instrumentation Fundamentals covers the basic circuits, sensors, signal standards, and controllers an operator meets daily, plus the electrical safety rules that apply in a boiler room.
Core rules
- Ohm's law: volts = amps x resistance; power in watts = volts x amps. Three-phase motors use three power conductors plus a ground, with overloads and a starter or variable frequency drive.
- Sensors: thermocouples generate millivolts and suit high temperature; RTDs (100 ohm platinum) are more stable and accurate at moderate temperature; thermistors are sensitive but limited in range.
- Standard signal: 4-20 mA current loop with a live zero, so 4 mA is 0% and 20 mA is 100%, and a broken wire reads near 0 mA and is detected as a fault. A 250 ohm resistor converts the loop to 1-5 V.
- Controllers use proportional, integral, and derivative action; interlocks and permissives must be proven before the burner can light.
- Safety: lockout and tagout, arc flash boundaries, proper grounding, and intrinsic safety for hazardous areas.
Worked example
A level transmitter has a span of 0 to 100 inches of water and a 4-20 mA output. A reading of 13.6 mA is what percent of span? Percent = (13.6 - 4) / (20 - 4) x 100 = 9.6 / 16 x 100 = 60%. Level = 0.60 x 100 in = 60 inches of water. Because the loop has a live zero, 13.6 mA is a valid 60% reading, while a reading near 0 mA would mean a fault, not an empty drum.
Traps that cost marks
- Reading 0 mA as 0% instead of a broken circuit.
- Using copper wire to extend a thermocouple; matching thermocouple extension wire is required.
- Defeating an interlock to get the burner lit for a trial.
- Working on a circuit without lockout and tagout.