309A Transformers and Power Distribution 6

Question 1

What is demand factor as applied to transformer sizing?

Accepted Answer

The ratio of maximum demand to total connected load

Answer

Demand factor is the ratio of the maximum demand of a system to the total connected load. It is always 1.0 or less, as not all connected loads operate simultaneously at full capacity.

Question 2

What is the standard frequency for electrical power distribution in Canada?

Accepted Answer

60 Hz

Answer

Canada, like the rest of North America, uses 60 Hz as the standard power distribution frequency. This affects motor speeds, transformer design, and all frequency-dependent equipment.

Question 3

What protection does a ground fault relay provide in a transformer secondary distribution system?

Accepted Answer

Detection of current flowing to ground indicating insulation failure or contact with grounded surfaces

Answer

Ground fault relays detect current imbalances indicating current flowing to ground through a fault path, providing early warning of insulation deterioration and protection against ground faults.

Question 4

What is the Canadian standard for transformer impedance marking?

Accepted Answer

Marked on the nameplate as percentage impedance at rated kVA

Answer

Canadian standards require the percentage impedance (%Z) at the transformer's rated kVA to be clearly marked on the nameplate. This is essential for calculating available fault current and coordinating protection.

Question 5

What is the purpose of a load-break switch in a medium-voltage distribution system?

Accepted Answer

To safely open and close circuits under load conditions

Answer

Load-break switches can safely interrupt load current (but not fault current) in medium-voltage distribution systems, allowing equipment to be de-energized for maintenance without shutting down the entire system.

Red Seal Electrician 309A Practice Test

Red Seal Electrician 309A Practice Test

309A Transformers and Power Distribution 6