Meter Basics
To measure voltage across a load, a voltmeter is connected in this relationship to the load.
What is in parallel?
One lamp is dark while the other lamps on the same switched circuit work. Start by checking this replaceable load component.
What is the lamp?
A broken wire interrupts the intended current path. This is the name of the fault.
What is an open circuit?
A steady 120-volt supply feeds a 30-ohm resistance load. This is the expected current.
What is 4 amperes?
Before replacing a failed component, record this identifying information so the replacement has the correct electrical ratings.
What are the nameplate/model information and ratings?
Before using the resistance or continuity function, a circuit must be in this verified electrical condition, with stored energy discharged.
What is deenergized?
A breaker trips only after several extra heaters are added. This load condition is the first suspect.
What is an overload?
Insulation failure lets an ungrounded conductor contact a bonded metal enclosure. This is the type of fault.
What is a ground fault?
A 15-ohm resistance load draws 8 amperes. This is the expected load voltage.
What is 120 volts?
A repaired circuit must be checked against this recorded operating problem to confirm the original issue is gone.
What is the original symptom or complaint?
To measure one branch conductor's load current with a clamp meter, close the jaws around this many conductors.
What is one conductor?
A 120-volt load receives normal voltage unloaded, but its terminal voltage falls sharply when it runs. This type of connection problem is a likely cause.
What is a high-resistance or loose connection in the supply circuit?
An upstream GFCI has tripped and several downstream receptacles are dead. Before replacing all receptacles, identify this upstream protective device condition.
What is loss of supply due to the tripped upstream GFCI? (Investigate the cause before reset.)
An unwanted connection resistance of 0.5 ohm carries 10 amperes. This is the voltage lost at that connection.
What is 5 volts? (V = I x R.)
After correcting a loose connection, inspection finds darkened, brittle conductor insulation. This additional condition requires correction before the circuit returns to service.
What is heat-damaged wiring?
Before a meter is used on distribution equipment, both of these marked safety ratings must suit the measurement location.
What are the measurement category (CAT) and voltage rating?
On a 120/240-volt split-phase system, some 120-volt lights become unusually bright while others dim as loads change. This conductor fault is a serious suspect.
What is a loose or open neutral? (Stop use and have the supply made safe.)
With the circuit isolated and verified deenergized, an ohmmeter reads OL across a removed fuse. This result indicates this fuse condition.
What is an open or blown fuse?
A loose connection has 0.2 ohm resistance and carries 15 amperes. This is the heat power released at that connection.
What is 45 watts? (P = I squared x R.)
Before reenergizing repaired equipment, verify all people are clear and all temporary jumpers, tools, and grounds used for the work have been handled under this plan.
What is the approved return-to-service/lockout removal procedure?
When a high-impedance meter displays suspected ghost voltage, a qualified person can use this appropriately rated meter function to help distinguish capacitive coupling from a firm source.
What is a low-impedance (LoZ) voltage function? (Follow the instrument instructions.)
A running three-phase induction motor suddenly loses one supply phase and overheats. This operating condition has this name.
What is single-phasing?
A high-resistance connection is suspected but reads nearly zero ohms on a handheld meter with power off. Under an approved energized diagnostic procedure, this measurement can reveal the defect while current flows.
What is voltage drop across the suspect connection? (Qualified personnel with required precautions only.)
A 120-volt circuit has 2 ohms total unwanted series resistance and a 10-ohm heater. This is the actual current, assuming constant resistance.
What is 10 amperes? (120 / (2 + 10).)
A nuisance trip seems solved after a reset. To confirm a durable repair, operate the equipment under this representative condition and document the results.
What is its normal expected operating load and cycle? (Confirm the cause was corrected.)