309A Transformers and Power Distribution 4

Question 1

What is the standard voltage for single-phase residential service in Canada?

Accepted Answer

120/240 V single-phase, three-wire

Answer

Canadian residential service is 120/240 V single-phase, three-wire, provided by a centre-tapped transformer. This gives 120 V line-to-neutral for general loads and 240 V line-to-line for large appliances.

Question 2

What is the purpose of a surge arrester on a transformer?

Accepted Answer

To protect the transformer from lightning strikes and switching surges

Answer

Surge arresters divert high-voltage transients (from lightning or switching operations) to ground, protecting the transformer's insulation from damage caused by voltage spikes.

Question 3

What is a K-rated transformer designed to handle?

Accepted Answer

Harmonic loads from non-linear loads such as VFDs and computers

Answer

K-rated transformers are designed with additional capacity in their windings and core to handle the extra heating caused by harmonic currents from non-linear loads like variable frequency drives, computers, and LED lighting.

Question 4

In a three-phase, four-wire wye system at 208/120 V, what is the voltage between any two line conductors?

Accepted Answer

208 V

Answer

In a wye system, the line-to-line voltage is √3 times the phase (line-to-neutral) voltage: 120 × 1.732 = 208 V. This is the voltage available for three-phase loads.

Question 5

What is the primary difference between a dry-type and liquid-filled transformer?

Accepted Answer

Dry-type uses air for cooling while liquid-filled uses oil or other fluids

Answer

Dry-type transformers use air circulation for cooling and are typically used indoors, while liquid-filled transformers use oil or other dielectric fluids for better cooling and insulation in higher-power outdoor applications.

Red Seal Electrician 309A Practice Test

Red Seal Electrician 309A Practice Test

309A Transformers and Power Distribution 4