CSLB Trade — Electrical (C-10) — All Questions
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521 questions
You want to measure current with a DMM's internal ammeter function. Which practice is essential to avoid a dangerous fault?
- a.Leave the leads in the current jacks while also measuring voltage
- b.Connect the meter in series with the load, on the correct (fused) current jack, and never across a voltage source✓
- c.Connect the meter in parallel across the load like a voltmeter
- d.Use the 10 A jack for all measurements regardless of range
An ammeter is a near-short internally, so it must be placed in series with the load using the proper fused current jack; connecting it in parallel across voltage (c) creates a bolted fault. Leaving leads in the current jacks while probing voltage (a) is a classic cause of arc-blast, and blindly using the 10 A jack (d) can bypass the fuse or misread small currents.
A conductor with 0.2 ohms of resistance carries 15 A. How much power is dissipated as heat in that conductor?
- a.75 W
- b.45 W✓
- c.3 W
- d.0.9 W
Power in a resistance is P = I^2 x R = (15 A)^2 x 0.2 ohms = 225 x 0.2 = 45 W. Option c is I x R (a voltage, 3 V), a divides incorrectly, and d uses I x R x 0.3. Excess I^2R heating is why loose or undersized terminations overheat.
A homeowner reports a 'weird' outlet. Which instrument most quickly indicates common miswires (open ground, open neutral, reversed polarity) at a 120 V receptacle?
- a.A phase-rotation meter
- b.A megohmmeter
- c.A plug-in receptacle (three-light) tester✓
- d.A clamp-on ammeter
A plug-in three-light receptacle tester lights a coded pattern that flags open ground, open neutral, and reversed polarity instantly. A megohmmeter tests insulation, a clamp meter reads current, and a phase-rotation meter is for three-phase sequence, none of which quickly maps outlet miswires.
Two identical 60 W, 120 V lamps are accidentally wired in series across 120 V. Approximately what voltage will a DMM read across each lamp, and how will they glow?
- a.About 60 V each; both glow dim✓
- b.240 V each; both very bright
- c.120 V each; both at full brightness
- d.120 V on one, 0 V on the other
Identical lamps in series split the 120 V roughly equally, so each sees about 60 V and both glow dimly at reduced power. They cannot each get the full 120 V in series (b, c), and equal lamps do not divide 120/0 (d). Unequal loads would divide the voltage unequally.
You must measure current on a circuit feeding electronic (nonlinear) loads such as LED drivers and VFDs. Which meter feature gives an accurate reading?
- a.A true-RMS meter✓
- b.Any meter reads distorted waveforms accurately
- c.An analog swinging-needle meter only
- d.An averaging (non-true-RMS) meter
Nonlinear loads draw distorted, non-sinusoidal current, and only a true-RMS meter computes the real heating value accurately. Averaging meters (d) assume a clean sine wave and under- or over-read on distorted waveforms. An analog meter (c) is also averaging-type, and not all meters handle distortion (b).
A 6 ohm and a 3 ohm resistor are connected in parallel. What total resistance should an ohmmeter read?
- a.9 ohms
- b.18 ohms
- c.4.5 ohms
- d.2 ohms✓
Parallel: R = (R1 x R2) / (R1 + R2) = (6 x 3) / (6 + 3) = 18 / 9 = 2 ohms. The total is always less than the smallest resistor. Option a is the series sum, c the simple average, and b the product only.
During commissioning you want to confirm that a metal equipment enclosure is effectively bonded. Which check best verifies a low-impedance bond to ground?
- a.Verify the enclosure feels cool to the touch
- b.Measure 120 V from enclosure to a hot conductor
- c.Confirm the enclosure is painted for corrosion resistance
- d.Measure a very low resistance/continuity between the enclosure and the grounding-electrode/EGC point✓
A low-resistance continuity reading between the enclosure and the grounding system confirms an effective bond that can carry fault current. Reading 120 V to a hot (b) is a shock hazard test, not a bond check; paint (c) actually impairs bonding at contact surfaces; and temperature (a) tells nothing about the ground path.
In a series circuit, 24 V is applied across a 4 ohm and an 8 ohm resistor in series. What voltage will a DMM read across the 8 ohm resistor?
- a.24 V
- b.16 V✓
- c.2 V
- d.8 V
Total R = 4 + 8 = 12 ohms, so I = 24 V / 12 ohms = 2 A. Across the 8 ohm resistor, V = I x R = 2 A x 8 ohms = 16 V (the 4 ohm drops the remaining 8 V). The larger series resistor drops the larger share of voltage.
A branch-circuit breaker trips instantly every time it is reset, with nothing plugged in. Continuity from hot to ground reads near 0 ohms. What is the most likely fault?
- a.A short circuit (hot-to-ground or hot-to-neutral) in the wiring✓
- b.An open neutral
- c.Undersized service transformer
- d.Simple overload from too many appliances
Instant tripping with no load, plus a near-0 ohm hot-to-ground reading, points to a bolted short circuit providing a very low-resistance fault path. An overload (d) needs connected load and trips on a time delay, an open neutral (b) would not trip the breaker, and transformer sizing (c) does not cause instantaneous branch trips.
Three parallel loads on a 120 V circuit draw 4 A, 6 A, and 5 A respectively. What total current should a clamp meter read on the feeder conductor?
- a.5 A
- b.120 A
- c.15 A✓
- d.1.6 A
Currents in parallel branches add: 4 + 6 + 5 = 15 A on the common feeder. Option a is only one branch, b confuses voltage with current, and d inverts the sum. This is why the feeder must be sized for the combined load.
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A non-contact voltage tester (NCVT) does not light up near a conductor you must work on. Before treating the conductor as dead, what is the correct action?
- a.Trust the NCVT completely and begin work
- b.Confirm de-energization with a contact voltage tester using live-dead-live, since NCVTs can miss voltage (shielding, ganged wires, low sensitivity)✓
- c.NCVTs are infallible, so no further check is needed
- d.Assume the conductor is a neutral and cut it
An NCVT is only a first indicator; it can fail to detect voltage due to shielding, cable geometry, or dead batteries, so it must never be the sole basis for calling a conductor dead. Verify with a contact tester using the live-dead-live method before touching conductors. Trusting it blindly (a, c) or guessing it's a neutral (d) can be fatal.
A single-phase load operates at 240 V and draws 20 A at unity power factor. What is its power in kilowatts?
- a.48 kW
- b.12 kW
- c.0.083 kW
- d.4.8 kW✓
P = V x I = 240 V x 20 A = 4,800 W = 4.8 kW at unity power factor. Option b inverts, c divides V by I, and a misplaces the decimal. Confirming measured watts against the nameplate is part of functional commissioning.
Before connecting a three-phase motor to a new feeder, why would you use a phase-rotation (phase-sequence) meter on the supply?
- a.To read the exact current the motor will draw
- b.To measure insulation resistance of the feeder
- c.To determine the phase sequence (A-B-C vs A-C-B) so the motor will rotate in the intended direction✓
- d.To verify the neutral is bonded
A phase-rotation meter identifies the sequence of the three phases so you can connect the motor for correct rotation before energizing, avoiding damage from running backward (e.g., pumps, compressors). It does not measure insulation (b), load current (a), or neutral bonding (d).
Using the rule of thumb of 1 megohm minimum per 1,000 V of rating, what is the minimum acceptable insulation resistance for a 4,160 V motor?
- a.1 megohm
- b.About 4 to 5 megohms✓
- c.416 megohms
- d.0.24 megohms
The 1-megohm-per-kV guideline gives roughly 4,160 V / 1,000 = about 4.16 megohms, often rounded up (some add 1 megohm) to about 4-5 megohms minimum. Readings below this suggest degraded insulation. Option d inverts the ratio and c multiplies incorrectly.
Why does over-torquing a lug or terminal screw create a hazard, even though the connection feels 'extra tight'?
- a.Over-torque can crush strands, deform the lug, and damage threads, raising resistance and causing a hot connection or eventual failure✓
- b.Torque has no effect on connection reliability
- c.Over-torque improves conductivity and is always safer
- d.Over-torque only matters on aluminum, never copper
Both under- and over-torque are harmful: over-torquing can crack lugs, crush or shear conductor strands, and strip threads, increasing contact resistance and heat over time. That is why the NEC requires using the manufacturer's specified value with a calibrated tool. It applies to copper and aluminum alike (d), and torque very much affects reliability (b).2023 NEC §110.14(D)
A homeowner reports that throughout the house, some lights dim while others brighten, and the effect shifts as loads switch on and off. Which service-level fault best explains this?
- a.An open (loose or corroded) service neutral✓
- b.A reversed-polarity outlet in one bedroom
- c.A single tripped GFCI receptacle
- d.One burned-out light bulb
Whole-house dimming/brightening that shifts with load is the classic signature of an open service neutral: the two 120 V legs float in series across 240 V and divide unevenly. A tripped GFCI (c) or a bad bulb (d) affects only one location, and a single reversed outlet (b) does not cause house-wide voltage swings.
Two meters are both marked CAT III, but one is rated 600 V and the other 1000 V. What does the voltage number add to the CAT rating?
- a.It is the maximum working voltage to ground for that category; the meter must meet both the CAT level and the voltage of the circuit✓
- b.Nothing; only the CAT number matters
- c.It is the maximum resistance the meter can read
- d.It is the meter's battery voltage
The CAT level defines the transient/impulse withstand for a location, and the voltage rating states the maximum working voltage to ground for that category, both must suit the circuit. A CAT III 600 V meter is not appropriate where CAT III 1000 V is required. The number is not battery voltage (d) or a resistance range (c).
By mistake you clamp your ammeter around BOTH the hot and neutral conductors of a single-phase 120 V branch circuit that is drawing 8 A. What will the clamp meter most likely read?
- a.8 A
- b.120 A
- c.Approximately 0 A✓
- d.16 A
The hot and neutral carry equal and opposite currents, so their magnetic fields cancel when both are inside the jaw, giving a reading near 0 A. To read true load current you must clamp only one conductor. A non-zero reading around both would actually indicate leakage/ground-fault current.
A motor-control resistor of 15 ohms must carry 2 A during operation. What voltage drop will a DMM read across it?
- a.30 V✓
- b.7.5 V
- c.17 V
- d.0.13 V
V = I x R = 2 A x 15 ohms = 30 V. Option b divides R by I, d inverts to I / R, and c adds the two numbers, none is Ohm's law.
During commissioning of a new bathroom circuit, how should you verify a GFCI receptacle provides ground-fault protection?
- a.Press the built-in TEST button (and/or use a GFCI tester) and confirm it trips and removes power, then RESET✓
- b.Measure 120 V hot-to-neutral only and assume it works
- c.Short the hot to ground with a screwdriver to force a trip
- d.Check that the LED is lit and skip tripping it
The correct functional test is to operate the built-in TEST button (or an external GFCI tester that injects a small fault) and confirm the device trips and de-energizes the outlet, then reset it. A normal voltage reading (b) or a lit indicator (d) does not prove the ground-fault sensing works. Deliberately shorting to ground (c) is dangerous and unnecessary.
A 120 V heater is rated 600 W. What is the operating resistance of its element (assume purely resistive)?
- a.24 ohms✓
- b.5 ohms
- c.72,000 ohms
- d.0.2 ohms
Using R = V^2 / P = (120 V)^2 / 600 W = 14,400 / 600 = 24 ohms. Equivalently, I = P / V = 5 A, then R = V / I = 120 / 5 = 24 ohms. Option b is the current in amps, and c multiplies V^2 x P.