442A power distribution, protection and transformers practice questions.

Five free practice questions on power distribution, protection and transformers: block B of the Red Seal 442A Industrial Electrician exam, about 23 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. Critical · Maintains overcurrent protection devices

   A 100 A thermal-magnetic breaker from a food plant is tested with a high-current test set at 300 % of rating. The manufacturer's band at 300 % is 25 to 80 s. Three tests run back to back give 48 s, then 14 s, then 9 s. What do the results show?

   1. The test set's output is drifting upward as it warms up.
   2. The thermal element is weakening, so the breaker must be replaced.
   3. The bimetal was still hot; only the first result is valid.
   4. The magnetic element is picking up at too low a current.
   Show the answer

   C. Correct. A thermal trip element responds to its own temperature. Tested again before it returns to ambient, it starts partly heated and trips sooner each time. The first test, from cold, is inside the 25 to 80 s band. Let the breaker cool for the manufacturer's stated time between tests, then repeat to confirm.

   Why not the others

   A. Drift in the test set would show as a change in injected current, which the set displays and holds at 300 %. Shrinking trip times at the same current are the breaker heating up, not the test set.

   B. A thermal element that had truly shifted would trip early on the first, cold test as well. The first result is inside the band; the later ones are short only because the bimetal had no time to cool.

   D. The magnetic element trips instantly at a high multiple of rating. At 300 % the breaker is working on its thermal element, and trip times measured in seconds are thermal behaviour.

2. Procedural · Installs low-voltage distribution equipment

   A new MCC lineup arrives at an automotive assembly plant in three shipping splits. Once the splits are set in place and anchored, what joins the main horizontal bus from one split to the next?

   1. The section frames, once the splits are bolted tightly together.
   2. Short cable jumpers lugged between the bus ends at each shipping split.
   3. Splice plates bolted across each split, torqued to the maker's values.
   4. The vertical bus, which runs through from one section into the next.
   Show the answer

   C. Correct. Each shipping split ends with the horizontal bus open, and the maker supplies splice plates (with the ground bus splice) to bridge it. They are bolted on with the specified hardware and torqued to the maker's values, then the joints are checked before the lineup is insulation-tested and energized.

   Why not the others

   A. Bolting the frames together aligns and secures the structure, but the frames carry no load current. The horizontal bus is open at each split until splice plates bridge it.

   B. Field cable jumpers are not part of the certified bus system and have neither the ampacity nor the fault bracing of the bus. The maker's splice plates are the connection the MCC was rated with.

   D. Vertical bus serves the units in its own section and stops there. The horizontal bus carries power along the lineup, and at a shipping split it is joined with the maker's splice plates.

3. Procedural · Maintains bonding systems

   Why are bonding joints at a mine's portable substation tested with a low-resistance ohmmeter that drives several amperes, rather than with a multimeter's ohms range?

   1. A joint held by a few strands reads near 0 Ω on a multimeter.
   2. A multimeter's test voltage can damage the bonding conductor's insulation.
   3. Only a high test current can measure resistance to remote earth.
   4. A multimeter measures only AC resistance, not DC resistance.
   Show the answer

   A. Correct. A multimeter resolves about a tenth of an ohm with milliamperes of test current, so a joint hanging on a few strands or one corroded contact point still reads near zero. A four-wire low-resistance ohmmeter resolves milliohms and pushes amperes through the joint, so a weak joint shows up.

   Why not the others

   B. A multimeter's ohms range uses a few volts at most, far too little to stress any insulation, and many bonding conductors are bare. The problem with a multimeter here is resolution, not damage.

   C. Resistance to remote earth is an electrode test done with an earth tester and probes. A bond test measures the metallic path between two points of the bonding system, which is a different job.

   D. A multimeter's ohms function measures DC resistance with a small DC test current. Its weakness for bond testing is that the current is tiny and the resolution coarse.

4. Recall · Installs low-voltage three-phase transformers

   An old two-phase motor at a pulp mill must be fed from the plant's 600 V three-phase system through two single-phase transformers. Which transformer connection is used?

   1. An open-delta connection.
   2. A Scott (T) connection.
   3. A wye–broken-delta connection.
   4. A zig-zag connection.
   Show the answer

   B. Correct. The Scott connection uses a main transformer with a centre-tapped primary and a teaser transformer connected to that centre tap, which turns three-phase into two voltages 90° apart. It also works the other way, feeding a three-phase load from a two-phase source.

   Why not the others

   A. Open delta also uses two transformers, but it delivers three-phase from three-phase at reduced capacity. Its outputs are 120° apart, not the 90° a two-phase motor needs.

   C. A wye–broken-delta bank uses three transformers to derive a grounding point or detect ground faults. It does not produce a two-phase supply.

   D. A zig-zag connection is used to create a neutral for grounding or to trap triplen harmonics. It does not convert three-phase to two-phase.

5. Procedural · Installs utility and non-utility three-phase supply services and metering equipment

   A mine's new 2 000 A service is fed by six parallel sets of 500 kcmil copper in separate conduits. Because of a column, one conduit route is 7 m longer than the other five. How must the conductors be installed?

   1. Use a larger conductor in the long set to offset its resistance.
   2. Make all six sets the same length as the longest one.
   3. Splice an extra length into the five short sets at a pull box.
   4. Leave the long set as is, since parallel sets share current by ampacity.
   Show the answer

   B. Correct. Rule 12-108 allows conductors No. 1/0 AWG and larger in parallel only when every set is the same length, size, material and insulation type, terminated the same way and free of splices. Current divides by impedance, so a longer set carries less and the others more than their share. The extra length on the shorter runs is taken up in the gutter or a pull box.

   Why not the others

   A. Rule 12-108 requires the same conductor size in every set of a parallel group. A larger conductor changes resistance and reactance differently, and the sets still would not share current as designed; equal lengths are the requirement.

   C. Rule 12-108 requires parallel conductors to be free of splices along their length (a single splice is allowed only at a termination point, under conditions). Equal length is reached by pulling longer conductors and taking up the slack, not by splicing.

   D. Parallel conductors share current by impedance, not by ampacity. The longer set has higher impedance and carries less, overloading the other five, which is why Rule 12-108 requires equal lengths.

02Answer key

03What block B covers

Block B is the power side of the plant: supply services and metering, CTs and VTs, overcurrent, ground fault, surge and voltage protection and how it is tested, switchboards and MCCs, power factor correction, harmonics and UPS systems, bonding and grounding, generators and synchronizing, rectifiers, batteries and solar storage, high-voltage substations, and transformers from control transformers to oil-filled units.

The Red Seal Occupational Standard names this block “Installs and maintains generating, distribution and service systems”. It carries 23% of the exam, about 23 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

DC needs a DC-rated breaker

A DC arc has no current zero to go out at, so it is much harder to interrupt than an AC arc. A breaker on a battery-fed DC panel needs a DC voltage rating and a DC interrupting rating, not an AC-only marking.

Close just before twelve o'clock

When a generator is synchronized by hand, matched voltage is not enough. The synchroscope should turn slowly in the fast direction, so the incoming set picks up a little load, and the breaker closes just before 12 o'clock to allow for its closing time.

Capacitor kvar follows voltage squared

A capacitor bank's reactive output varies with the square of the applied voltage. A 600 V bank running on a 575 V bus delivers about 92 percent of its rating, so judge test readings at the actual bus voltage before condemning a unit.

Zero volts line to line

If each half of a 120/240 V secondary reads 120 V but the two lines read 0 V to each other, the link between the windings joins like-polarity ends and the voltages cancel. Lock out, move the link so the sections add, then recheck.

05The other blocks

The 442A exam has six blocks. Each has its own page of free questions:

The 442A Industrial 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.