309A power distribution and grounding practice questions.

Five free practice questions on power distribution and grounding: block B of the Red Seal 309A Electrician exam, about 28 of its 100 questions. They are in the exam's format: four options, one correct answer. Answer each one before opening the explanation. No account, no email.

01The questions

1. Procedural · Installs ground fault, arc fault and surge protection devices

   A dwelling kitchen has a receptacle on the wall beside the sink, 0.9 m from the edge of the basin. What protection does the CEC require for that receptacle?

   1. AFCI protection only.
   2. Class A GFCI protection.
   3. No special protection, because the sink has plastic drain piping.
   4. A 20 A T-slot receptacle.
   Show the answer

   B. Correct. Receptacles within 1.5 m of a sink in a dwelling must be protected by a Class A GFCI, because a person can be touching the grounded plumbing and the appliance at the same time. Protection may be at the receptacle or by an upstream GFCI device or breaker.

   Why not the others

   A. AFCI addresses arcing faults in wiring; it does nothing for a shock hazard near water. The sink-proximity rule calls for GFCI. A given circuit may need both, but the sink rule is GFCI.

   C. The rule is about proximity to water, not the pipe material; the wet hands and grounded surfaces are the hazard.

   D. T-slot receptacles relate to the circuit's ampacity and appliance loads, not to shock protection near a sink.

2. Recall · Installs power distribution equipment

   A circuit breaker is marked 120/240 V (a slash rating). On which system may it be used?

   1. Only 120 V single-phase circuits.
   2. Any system whose line-to-line voltage is 240 V or less.
   3. One with no more than 120 V to ground and 240 V line-to-line.
   4. Any system up to 240 V, since breakers are rated by their higher number.
   Show the answer

   C. Correct. A slash-rated device is tested to interrupt the lower voltage to ground and the higher voltage line-to-line, so it is limited to solidly grounded systems such as 120/240 V single-phase or 120/208 V wye. On a 240 V corner-grounded delta or an ungrounded 240 V system, a pole may have to interrupt 240 V to ground, which the device is not rated for.

   Why not the others

   A. The device is rated for 240 V line-to-line, so two-pole 240 V loads on a 120/240 V system are fine. The limit is voltage to ground.

   B. Line-to-line voltage is only half the rating; the voltage to ground must also be within the lower figure. A 240 V delta fails that test.

   D. A straight-rated 240 V breaker (marked 240 V, no slash) can be used on any 240 V system; the slash rating is the restricted one.

3. Critical · Performs servicing and maintenance of single-phase consumer/supply services and metering equipment

   A homeowner reports that when the dryer runs, lamps in the kitchen brighten while lamps in the living room dim, and a television has failed. Measurements at the panel show 145 V on one line to neutral and 95 V on the other. What is the cause?

   1. An overloaded main breaker.
   2. An open or high-resistance service neutral.
   3. Low utility voltage on one phase.
   4. A shorted dryer heating element.
   Show the answer

   B. Correct. With the neutral open, the two 120 V halves of the service become a series circuit across 240 V; the voltage divides by load, so the lightly loaded side rises and the heavily loaded side falls. Loads on the high side are damaged. The neutral connection is checked from the utility splice through the meter base to the panel.

   Why not the others

   A. Overload heats and trips the breaker; it does not change the voltage balance between the two lines.

   C. A utility problem would lower one side; it would not raise the other above nominal. Opposite movement of the two voltages is the signature of a lost neutral.

   D. A short would trip the dryer breaker, not shift the line-to-neutral voltages in opposite directions on both sides of the service.

4. Critical · Installs ground fault protection and detection systems

   An ungrounded 480 V delta system has a three-lamp ground detector. During a shift, the lamp for phase B goes dark while the lamps for phases A and C brighten. What does this indicate?

   1. Normal operation: one lamp is always off on an ungrounded system.
   2. A burned-out lamp on phase B.
   3. A ground fault on phase B.
   4. An open phase B conductor.
   Show the answer

   C. Correct. With no fault, each lamp sees phase-to-ground voltage through the system's capacitance and glows at equal, moderate brightness. A solid ground on phase B pulls that phase to ground potential, so its lamp goes out, while the other two phases rise to full line-to-line voltage above ground and their lamps brighten. The fault is located and cleared before a second fault on another phase creates a phase-to-phase short.

   Why not the others

   A. On a healthy ungrounded system all three lamps glow equally; a dark lamp means a fault.

   B. A failed lamp would not cause the other two to brighten; the change in the two healthy lamps is the confirmation that phase B is grounded.

   D. A lost phase would leave the remaining lamps at their normal brightness or dimmer, not brighter; brightening of the other two is the ground-fault signature.

5. Procedural · Installs low-voltage three-phase transformers

   A dry-type delta–wye transformer produces a 120/208 V secondary in a commercial building. Where must the secondary neutral be bonded to ground?

   1. At the transformer or first disconnect, as a separately derived system.
   2. Nowhere, because a transformer secondary is inherently isolated and safe.
   3. Only at the service, through the primary feeder's bonding conductor.
   4. At every downstream panel fed from the transformer.
   Show the answer

   A. Correct. The wye secondary is a separately derived system: its neutral is not electrically continuous with the service neutral, so it needs its own system bonding jumper (at the transformer or the first downstream disconnect, not both) and its own grounding electrode connection, typically to the building's grounding electrode system. Without it, the secondary would float and line-to-ground faults would go undetected.

   Why not the others

   B. Isolation is exactly why the secondary needs its own grounding; an ungrounded 120/208 V system is not permitted for line-to-neutral loads.

   C. The primary bonding conductor bonds the transformer enclosure; it does not ground the secondary neutral, which has no metallic connection to the primary.

   D. Multiple bonding points create parallel neutral paths through the bonding system, the same defect as bonding a sub-panel neutral.

02Answer key

03What block B covers

This block covers the power side of the trade, from the utility connection to the panel. It includes single- and three-phase services and metering, service load calculations, breakers, fuses, GFCI, AFCI and surge protection, panelboards and transfer switches, UPS and power factor correction, grounding and bonding, generators, solar PV and battery storage, high-voltage equipment and transformers.

The Red Seal Occupational Standard names this block “Installs, services and maintains generating, distribution and service systems”. It carries 28% of the exam, about 28 of the 100 questions, split across nine tasks. The share column is each task's weight within the block, as the standard publishes it.

04What the questions turn on

Neutral current and open neutrals

On a 120/240 V service the neutral carries only the difference between the two line currents. If the neutral opens, the two halves become a series circuit: the lightly loaded side rises above 120 V and the heavily loaded side falls.

Bonding is not grounding

Bonding carries fault current back to the source so the breaker opens; grounding connects the system to earth. Bonding conductors are sized from the overcurrent device rating, and the neutral-to-ground bond belongs only at the service.

Breaker loading and interrupting rating

A continuous load is limited to 80 % of the breaker rating unless the device is marked for 100 %. A breaker must also interrupt the fault current available at its terminals, so a 10 kA breaker cannot face 22 kA unless series-rated.

Transformer figures to know

Full-load current is VA divided by voltage, so a 25 kVA transformer with a 600 V primary draws about 42 A. Primary-only protection is capped at 125 % of that. An open-delta bank carries 86.6 % of its two transformers' combined rating.

05The other blocks

The 309A exam has five blocks. Each has its own page of free questions:

The 309A Electrician practice exam page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.