DM Electrical Exam flashcards
DM Electrical Exam · Build recall with a focused review session.
33 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Ohm's Law
- The relationship E = I × R, where E is voltage in volts, I is current in amperes, and R is resistance in ohms. Used to solve any DC resistive circuit.
- Power Formula
- For DC or resistive AC, P = E × I = I²R = E²/R, giving power in watts. In AC circuits, multiply by the power factor for real power.
- Series Circuit Rules
- Total resistance is the sum of all resistances, current is the same through every element, and voltage divides among elements in proportion to their resistance.
- Parallel Circuit Rules
- Voltage is the same across all branches, total resistance is less than the smallest branch resistance, and branch currents sum to total current.
- Three-Phase Power
- For a balanced three-phase load, real power is P = √3 × line voltage × line current × power factor. The √3 factor comes from phase-to-line relationships.
- Inductive Reactance
- X_L = 2πfL, the opposition to AC current from an inductor, in ohms. It rises with frequency, and inductor current lags voltage by 90 degrees.
- Branch Circuit Definition
- Per NEC Article 100, the circuit conductors between the final overcurrent protection device protecting the circuit and the outlet or outlets served.
- Continuous Load Rule
- A load operating at maximum current for three hours or more is continuous and must be calculated at 125% of its rating for branch circuits and feeders.
- Standard OCPD Sizes
- NEC 240.6 lists standard ampere ratings, including 15, 20, 30, 40, 50, 60, 70, 100, 125, 150, and 200 amperes for fuses and breakers.
- Working Clearance Rule
- NEC 110.26 requires dedicated space in front of electrical equipment; the minimum depth starts at 3 feet for equipment up to 150 volts to ground.
- GFCI Protection Locations
- NEC 210.8 requires ground-fault circuit-interrupter protection for receptacles in bathrooms, garages, outdoors, kitchens, and other wet or damp locations.
- Conductor Ampacity Adjustment
- NEC 310.15 requires derating for ambient temperature and for more than three current-carrying conductors in a raceway or cable, applied to table ampacities.
- Electrical Shock Thresholds
- About 1 mA is perceptible, 10 to 20 mA can cause muscle lock preventing release, and 100 mA or more through the heart can be fatal.
- Arc Flash Boundary
- The distance at which incident energy equals 1.2 cal/cm², the threshold for a second-degree burn. It is calculated from fault current and clearing time.
- Lockout Tagout Verification
- After de-energizing and locking, a qualified worker tests for zero energy with a meter rated for the system voltage and category before beginning work.
- Class C Fire
- A fire involving energized electrical equipment. A nonconductive extinguishing agent is used, or power must be removed before applying water or foam.
- Equipment Grounding Conductor
- A conductor that provides a low-impedance path from equipment enclosures back to the source, allowing fault current to trip the overcurrent device.
- NM-B Cable Restrictions
- Nonmetallic sheathed cable is permitted in dry, protected locations but is prohibited in wet locations, masonry cavities, and where subject to physical damage.
- EMT Support Intervals
- Electrical metallic tubing must be supported within 3 feet of each box and at intervals not exceeding 10 feet along the run.
- Box Fill Calculation
- NEC 314.16 assigns volume allowances to conductors, devices, and grounds, which are compared against the box's marked cubic-inch capacity.
- Conduit Fill Percentages
- Chapter 9 Table 1 permits 53% fill for one conductor, 31% for two, and 40% for three or more conductors in a raceway.
- 360-Degree Bend Rule
- No more than 360 degrees of total bends, including offsets and kicks, is permitted between pull points in a run of conduit.
- Motor Short-Circuit Protection
- NEC 430.52 sizes the branch-circuit short-circuit and ground-fault device, often up to 250% of full-load current for an inverse-time breaker.
- Motor Overload Protection
- A running overload device protects the motor from sustained overcurrent and is sized to the motor's nameplate current and service factor.
- Contactor Versus Starter
- A contactor switches load current only; a starter adds overload relays to the contactor to provide motor running overload protection.
- GFCI Versus AFCI
- A GFCI detects current imbalance to ground and protects people from shock; an AFCI detects arcing signatures and protects against fire ignition.
- Dwelling Lighting Load
- NEC 220 uses 3 volt-amperes per square foot for general lighting in dwellings, with small-appliance and laundry circuits added separately.
- Voltage Drop Formula
- Single-phase voltage drop is approximately VD = (2 × K × I × L) / CM, where K is conductor resistivity and CM is circular-mil area.
- Interrupting Rating
- The maximum fault current a device can safely interrupt. It must equal or exceed the calculated available fault current at its point of installation.
- Meter CAT Rating
- A measurement category indicating the transient overvoltage environment a meter can withstand; CAT III and CAT IV are required for service and distribution work.
- Insulation Resistance Test
- A megohmmeter applies a DC voltage and reads resistance in megohms; declining readings over time indicate moisture, contamination, or insulation degradation.
- Clamp Ammeter Use
- A clamp meter measures current by surrounding a single conductor; clamping around both supply and return conductors cancels the fields and reads near zero.
- Neutral-Ground Bond Fault
- A neutral-to-ground connection downstream of the service creates parallel return paths that can cause nuisance GFCI trips and objectionable current on grounding conductors.