Electrical Systems & Equipment Flashcards

What is the purpose of a Phase Failure Relay in a three-phase motor circuit?

Answer: Protects the motor if one phase of three-phase power is lost

A phase failure (or phase loss) relay detects the loss of any one phase and shuts down the motor to prevent single-phasing damage.

What is the maximum ampere rating for a plug fuse used in a standard Edison-base fuseholder?

Answer: 30A

Standard Edison-base plug fuses are limited to a maximum 30A rating per NEC.

In a 120/240V single-phase three-wire service, what is the voltage between the two ungrounded conductors?

Answer: 240V

In a single-phase three-wire service, the two hot conductors are derived from opposite ends of a center-tapped transformer, providing 240V between them.

Which of the following conductors is NEVER required to be fused or protected by a circuit breaker under NEC rules?

Answer: Grounded neutral conductor

The grounded neutral conductor (white wire) is generally not opened or fused because it carries return current and breaking it could result in overvoltage on connected equipment.

What is the function of a current transformer (CT) in an electrical metering system?

Answer: Steps down high current to a measurable level for metering instruments

Current transformers reduce high primary currents to a standardized 5A secondary output suitable for metering and protection relays.

According to NECA standards, what is the preferred method for terminating aluminum conductors to prevent oxidation?

Answer: Apply antioxidant compound before termination

Antioxidant compound is applied to aluminum conductor terminations to prevent oxide layer formation that increases resistance and causes overheating.

What does the term 'demand factor' refer to in electrical load calculations?

Answer: Ratio of maximum demand to total connected load

Demand factor is the ratio of the maximum demand of a system to the total connected load, acknowledging that not all loads operate simultaneously at full capacity.