PC Transformers and Distribution Equipment 3

Question 1

A dry-type transformer with an 80°C rise rating is installed in a room with a 40°C ambient temperature. What is the maximum operating winding temperature?

Accepted Answer

120°C

Answer

80°C

Answer

40°C

Answer

160°C

Question 2

Per NEC 450.21(A), a dry-type transformer rated 112.5 kVA or less may be installed indoors without a transformer vault if it is:

Accepted Answer

Separated from combustible material by at least 12 inches or by a fire-resistant heat-insulating barrier

Answer

Only installed in a locked electrical room

Answer

Placed in a listed enclosure only

Answer

Installed only on the lowest floor of the building

Question 3

The available short-circuit current on the secondary of a transformer is primarily determined by:

Accepted Answer

The transformer's impedance (%Z) and the secondary voltage and current ratings

Answer

The transformer's kVA rating alone

Answer

The primary conductor size feeding the transformer

Answer

The type of overcurrent device protecting the transformer primary

Question 4

A transfer switch in an emergency power system serves what primary function?

Accepted Answer

To automatically or manually switch electrical loads from the normal source to the emergency power source

Answer

To step down voltage for emergency lighting circuits

Answer

To regulate transformer output voltage during brownout conditions

Answer

To provide ground fault protection for emergency generators

Question 5

Per NEC 450.9, indoor transformer ventilation must be adequate to:

Accepted Answer

Limit the temperature rise of the transformer to its nameplate rating under full-load conditions

Answer

Limit ambient temperature to no more than 25°C

Answer

Provide a minimum of 6 air changes per hour in the transformer room

Answer

Keep transformer surface temperature below 40°C at all times

Question 6

What is the purpose of a zig-zag (interconnected-wye) transformer used in a distribution system?

Accepted Answer

To derive a neutral point and suppress third-harmonic currents in ungrounded or delta systems

Answer

To step up voltage for long-distance transmission feeders

Answer

To convert single-phase power to balanced three-phase power

Answer

To increase transformer efficiency by reducing core losses

Question 7

When calculating the available fault current at a panelboard fed from a transformer secondary, which transformer parameter must be obtained from the nameplate?

Accepted Answer

Impedance voltage percentage (%Z)

Answer

Core material type (silicon steel or amorphous)

Answer

No-load (core) losses in watts

Answer

Cooling class designation (ONAN, OFAF, etc.)