442A PLCs, instrumentation and process control practice questions.

Five free practice questions on PLCs, instrumentation and process control: block F of the Red Seal 442A Industrial Electrician exam, about 17 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 · Maintains discrete input/output (I/O) devices

   Before a refinery unit restarts after a turnaround, each tank's high-level switch must be proven from the switch to the operator's alarm. Which test does that?

   1. Ring out each cable pair from switch to card with an ohmmeter.
   2. Jumper each input at the I/O card and confirm the console alarm.
   3. Actuate each switch at the tank and check its alarm at the console.
   4. Force each input on in the controller and confirm the console alarm.
   Show the answer

   C. Correct. Lifting the float or applying test medium at the tank exercises the switch, its contacts, the field cable, the input point, the logic and the alarm in one test: the whole path the operator relies on. Record each result against its loop number.

   Why not the others

   A. Continuity proves only the conductors and terminations. It says nothing about whether the switch actuates at high level or whether the alarm reaches the console.

   B. Jumpering at the card proves the card, logic and alarm but leaves out the switch and its cable, the parts most likely to have been disturbed during the turnaround.

   D. Forcing proves only the logic and the console from the input bit onward. The switch, its cable and the input hardware are never exercised, and a stuck float would pass.

2. Procedural · Programs automated control systems

   A four-digit thumbwheel switch on a sawmill trimmer sets cut length in millimetres and feeds 16 PLC inputs in BCD. With the most significant digit first, the inputs read 0010 0100 0110 1001. What length is set?

   1. 4 269 mm
   2. 9 321 mm
   3. 2 469 mm
   4. 9 642 mm
   Show the answer

   C. Correct. Binary coded decimal (BCD) gives each decimal digit its own four-bit group: 0010 = 2, 0100 = 4, 0110 = 6, 1001 = 9, so 2 469 mm. The program converts BCD to an integer before the value is used in arithmetic or comparisons.

   Why not the others

   A. This reads each group's bits in reverse (0010 as 0100, 0100 as 0010). The right-hand bit of each group is worth 1 and the left-hand bit 8: 0010 = 2, 0100 = 4, 0110 = 6, 1001 = 9, giving 2 469 mm.

   B. 9 321 is the 16 bits read as one straight binary number (hexadecimal 2469). In BCD each four-bit group is a separate decimal digit: 2, 4, 6, 9, so the length is 2 469 mm.

   D. 9 642 reads the digits in reverse order. The most significant digit comes first, so 0010 0100 0110 1001 is 2, 4, 6, 9: 2 469 mm.

3. Procedural · Installs pneumatic control systems

   A double-acting air cylinder drives a log kicker at a sawmill. With air pressure still on, the cylinder must retract whenever the valve's solenoid loses power, from a control power failure or an emergency stop. Which solenoid valve is fitted?

   1. A 3/2 normally closed, single-solenoid valve.
   2. A 5/2 double-solenoid, detented valve.
   3. A 5/2 single-solenoid, spring-return valve.
   4. A 5/3 closed-centre, double-solenoid valve.
   Show the answer

   C. Correct. A 5/2 valve has five ports (supply, two cylinder ports, two exhausts) and two positions, enough to drive a double-acting cylinder both ways. With a spring return, losing the coil supply always puts the spool back to its spring position, which is piped to feed the rod end and retract the cylinder.

   Why not the others

   A. A 3/2 valve has one cylinder port, so it can drive a single-acting, spring-return cylinder but cannot pressurize both ends of a double-acting cylinder. A 5/2 valve is needed.

   B. A double-solenoid 5/2 valve stays in whichever position it was last shifted to when power is lost, so a kicker caught extended would stay out. Retract-on-power-loss needs a spring return.

   D. A 5/3 closed-centre valve springs to its centre position on power loss, blocking both cylinder ports so the rod stops roughly where it is. It does not retract the cylinder.

4. Procedural · Installs discrete input/output (I/O) devices

   A positive-displacement flowmeter on a fuel oil line gives one pulse per 5 L. The panel totalizer counting these pulses must display cubic metres. What scale factor per pulse is entered?

   1. 0.0005 m³
   2. 0.05 m³
   3. 0.005 m³
   4. 0.2 m³
   Show the answer

   C. Correct. 1 m³ = 1 000 L, so each 5 L pulse is 5 ÷ 1 000 = 0.005 m³. Check the setting against a known delivered volume before the totalizer is used for inventory.

   Why not the others

   A. This is 5 ÷ 10 000, one decimal place too far; the totalizer would show one tenth of the true volume.

   B. This is 5 ÷ 100, a decimal slip the other way; the totalizer would show ten times the true volume.

   D. 0.2 is pulses per litre (1 ÷ 5), the inverse of the factor wanted, and it is still in litres, not cubic metres.

5. Recall · Installs automated control systems

   What is the purpose of the keying clips fitted to a PLC chassis slot and to the removable terminal block of the I/O module in that slot?

   1. They let the processor read the module's catalogue number at power-up.
   2. They bond the module's signal common to the chassis and the panel.
   3. They lock the terminal block in place against machine vibration.
   4. They stop a module of another type being fitted to that slot's wiring.
   Show the answer

   D. Correct. Mechanical keying is set to match the module type, so a 120 V AC output module cannot be pushed into a slot, or onto a terminal block, wired for 24 V DC inputs. It prevents damage and wrong connections when modules are swapped in a hurry.

   Why not the others

   A. Identifying a module's type and revision is done electronically, by the processor comparing the module's identity with its configuration (electronic keying). The clips are mechanical keying: they physically block a module of the wrong type.

   B. The chassis is bonded through its mounting and a bonding conductor to the panel's bonding bus, as the maker's instructions direct. Keying clips carry no current; they physically prevent the wrong type of module being fitted.

   C. The terminal block is held by its own latch or screws. Keying clips do not hold anything in place; they are set in a pattern that matches the module type so a different type of module cannot be fitted to that wiring.

02Answer key

03What block F covers

Block F is the automation side of the trade: discrete devices such as limit switches, proximity and photoelectric sensors and float switches, analog transmitters, thermocouples and flowmeters on 4-20 mA loops, PLC, DCS and SCADA systems with their I/O, programming and loop tuning, and the pneumatic and hydraulic controls that move valves and cylinders.

The Red Seal Occupational Standard names this block “Installs and maintains process control systems”. It carries 17% of the exam, about 17 of the 100 questions, split across three tasks. The share column is each task's weight within the block, as the standard publishes it.

04What the questions turn on

Stops and high limits fail safe

A stop or high-level contact goes in series with the starter coil and opens to stop. A broken wire, loose terminal or lost control power then also stops the equipment, because every one of those failures looks like the limit being reached.

Extension wire moves the cold junction

A thermocouple's signal depends on the temperature difference between its measuring junction and the point where its alloys meet copper. Matching extension wire carries that point to the transmitter, which compensates at its own terminals; copper from the head leaves it at the hot sensor.

Retentive timers keep their time

A retentive timer holds its accumulated value when its rung goes false, so running time adds up across interruptions. Because it never clears itself, a reset instruction is needed to ready it for the next cycle.

Meter out a running load

A load that can run ahead of the pump, such as a heavy platen being lowered, is controlled by metering the oil leaving the cylinder. The back-pressure holds the load, and a built-in check valve gives free flow on the return stroke.

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.