Deck 10: PLC Installation Practices, Editing, and Troubleshooting

Question 1

Leakage current can falsely activate a PLC input.

Answer 1

True

Question 2

Where line voltage variations to the PLC are excessive, a constant voltage transformer can be used to maintain a steady voltage.

Answer 2

True

Question 3

When a grounding connection must be made to the metal enclosure of a PLC system, any paint coating must be removed.

Answer 3

True

Question 4

A diode can be connected in reverse-bias across a DC solenoid to suppress voltage spikes.

Question 5

Electromagnetic interference (EMI) may enter a PLC system through radiation.

Question 6

Most solid-state switches will conduct a small amount of leakage current in the off state.

Question 7

An enclosure is used to shield the controller from electrical noise and airborne contaminants.

Question 8

Running signal wiring and power wiring in the same conduit helps to cut down on electrical noise.

Question 9

A properly installed grounding system is designed to provide a high-impedance path to earth ground.

Question 10

A bleeder resistor can be connected to drain off unwanted module leakage current.

Question 11

Ground loop circuits cause problems by adding or subtracting current or voltage from input signal sources.

Question 12

PLC malfunctions due to electrical noise usually produce temporary occurrences of operating errors.

Question 13

A ground loop circuit can develop when each device's ground is tied to a different earth potential.

Question 14

A fiber optic wired control system is most susceptible to electrical noise.

Question 15

When current into a DC resistive load is interrupted or turned off, a very high voltage spike is generated.

Question 16

Common potential noise generating devices include non-inductive resistive loads.

Question 17

Most PLC installations require additional cooling provisions, not included in the original installation.

Question 18

Generally, output modules designed to drive inductive loads include suppression networks as part of the module circuit.

Question 19

A hardwired electromechanical master control relay is not normally included as part of the wiring for a PLC system.

Question 20

PLCs are generally placed within an enclosure.

Question 21

If the scan time is too long, a watchdog error will be declared.

Question 22

The status indicator on an input module will normally be illuminated if the input device is off and examined for an off condition.

Question 23

The cross-reference function allows you to view the transition history of a data table value.

Question 24

Using PLC editing functions, instructions and rungs can be added, deleted, or modified.

Question 25

The temporary end instruction is used when you want to change the amount of logic being scanned.

Question 26

When a PLC is placed in the continuous test mode, the processor operates the user program without energizing any outputs.

Question 27

To make sure that equipment does not operate while PLC maintenance is conducted, lockout and tagout devices must be in use.

Question 28

The programming device monitor normally indicates a true instruction if the addressed input device is off and examined for an off condition.

Question 29

The PLC will only accept multiple programs.

Question 30

The suppression device is normally wired in series with the load device.

Question 31

The offline programming mode permits the user to change the program during the machine operation.

Question 32

A metal oxide varistor (MOV) can be used to suppress AC or DC voltages.

Question 33

One of the diagnostic checks carried out by the processor is the proper operation of all I/O devices.

Question 34

It is rare for sensors and actuators connected to the I/O of the process to fail.

Question 35

An addressing conflict is caused by the same address being used for two or more coil instructions in the same program.

Question 36

The processor of a PLC monitors its battery voltage level.

Question 37

PLC output modules may incorporate fuse or electronic protection for output circuitry.

Question 38

The suspend instruction is used to trap and identify specific conditions.

Question 39

The contact histogram function is used to locate each instance of a selected address.

Question 40

Preparing a PLC control process for initial start-up is known as commissioning.

Question 41

A good location for a PLC enclosure is close to:

A)the machine or processor.
B)large AC motors.
C)high-frequency welders.
D)annealing furnaces.

Question 42

The diode used in the suppression circuit shown:

A)is connected in forward-bias to suppress DC loads.
B)is connected in reverse-bias to suppress DC loads.
C)is connected in forward-bias to suppress AC loads.
D)is connected in reverse-bias to suppress AC loads.

Question 43

PLCs are placed within an enclosure to provide protection against:

A)atmospheric conditions.
B)conductive dust.
C)moisture.
D)all of these.

Question 44

A leakage problem can occur when connecting an output module to a high-impedance load. This problem can be corrected by connecting:

A)a bleeder resistor in series with the load.
B)a bleeder resistor in parallel with the load.
C)an NO contact in series with the load.
D)an NC contact in parallel with the load.

Question 45

I/O leakage problems usually occur with devices that use:

A)solid-state switching circuits.
B)hard contacts.
C)noise-suppression circuits.
D)voltage-suppression circuits.

Question 46

The authoritative source on grounding requirements for a PLC installation is the:

A)plant electrician.
B)plant engineer.
C)equipment manufacturer.
D)National Electrical Code.

Question 47

Which of the following load devices is most likely to require some form of noise or voltage suppression?

A)Lamp
B)Heater
C)Solenoid
D)LED display

Question 48

In addition to being an important safety measure, proper grounding of a PLC system can:

A)lower installation costs.
B)increase the power efficiency of the system.
C)limit the effects of EMI.
D)assist in the operation of the MCR.

Question 49

Excessive line voltage variations to a PLC installation can be corrected by installing a:

A)constant voltage transformer.
B)step-down transformer.
C)step-up transformer.
D)current transformer.

Question 50

Communications protocol is a standardized method for establishing communications between different PLC devices.

Question 51

Which of the following would not normally be located within the PLC enclosure?

A)I/O modules
B)Limit switch
C)Master control relay
D)Isolation transformer

Question 52

Under no circumstances should:

A)fiber optic and power wiring be run in the same conduit.
B)fiber optic and signal wiring be run in the same conduit.
C)signal wiring and power wiring be run in the same conduit.
D)all of these.

Question 53

For most PLC installations, a NEMA ____enclosure is recommended.

A)2
B)4
C)8
D)12

Question 54

Typically, PLC systems installed inside an enclosure can withstand a maximum of:

A)60°C outside the enclosure.
B)50°C outside the enclosure.
C)60°C inside the enclosure.
D)50°C inside the enclosure.

Question 55

A high voltage spike is generated whenever current to:

A)an inductive load is turned off.
B)an inductive load is turned on.
C)a resistive load is turned off.
D)a resistive load is turned on.

Question 56

Certain input field devices may have a small leakage current when they are:

A)in the on state.
B)in the off state.
C)examined for an on condition.
D)examined for an off condition.

Question 57

Proper grounding procedures for a PLC installation specify that:

A)all enclosures, CPU and I/O chassis, and power supplies be connected to a single low-impedance ground.
B)paint or nonconductive materials should be scraped away to provide good ground connections.
C)all ground connections should be made using star washers.
D)all of these.

Question 58

Electrical noise can be coupled into a PLC system:

A)by an electrostatic field.
B)through electromagnetic induction.
C)by a fiber optic system.
D)both by an electrostatic field and through electromagnetic induction.

Question 59

When routing power and I/O signal wiring to and within a PLC enclosure, you should:

A)use the shortest possible wire runs for I/O signals.
B)never run signal and power wiring in the same conduit.
C)run low-level signal conductors as shielded twisted pair.
D)all of these.

Question 60

Malfunctions due to electrical noise interference usually cause:

A)temporary occurrences of operating errors.
B)permanent occurrences of operating errors.
C)temporary loss of memory.
D)permanent loss of memory.

Question 61

A watchdog timer is used to monitor:

A)how long it takes the CPU to complete a scan.
B)battery voltage.
C)memory circuits.
D)all of these.

Question 62

A single output device has failed while the remainder of the PLC system is functioning normally. The indicator light on the output module indicates that a signal is sent to the output point where the device is connected. You would now:

A)trace the circuit back through the logic to locate the inputs.
B)use a programming terminal to call up the rung that controls the output to see if the output coil is on.
C)check the point where the output device's field wiring is connected to the output rack.
D)check the input modules for short-circuit conditions.

Question 63

Which of the following would not normally be included as part of a routine preventive maintenance program?

A)Inspection of I/O field devices
B)Monitoring the program
C)Checking connections to I/O modules
D)Cleaning of the PLC enclosure

Question 64

If an output module fuse blows repeatedly, a probable cause may be:

A)the module's output current is being exceeded.
B)the output device may be shorted.
C)the output field wiring may be shorted.
D)any one of these.

Question 65

Which of the choices of the troubleshooting guide shown indicates a problem with the wiring to the output device or the output device itself?

A)Condition (a)
B)Condition (b)
C)Condition (c)
D)Condition (d)

Question 66

The input wiring or device is suspected as being the source of a PLC problem. If this circuit is not at fault, then the status indicator on the input module should be illuminated when the input device is:

A)closed and programmed for an XIC condition.
B)open and programmed for an XIO condition.
C)closed, regardless of how it is programmed.
D)open, regardless of how it is programmed.

Question 67

Which of the following is not normally included in the array of status indicators found on the processor module?

A)Memory OK
B)Wiring OK
C)Battery OK
D)Power supply OK

Question 68

Assume an open in the field wiring is suspected between the output module and output load device. This condition would be confirmed if:

A)full output voltage was measured at the module and 0 voltage at the load.
B)full output voltage was measured at the load and 0 voltage at the module.
C)the output status indicator on the module is on and the load is not operating.
D)the output status indicator on the module is off and the load is not operating.

Question 69

Which function of a PLC would be used to search each instance of a particular address?

A)Change radix
B)Forcing
C)Cross-reference
D)Contact histogram

Question 70

The data monitor feature of a PLC may allow you to:

A)change the radix or data format.
B)store data for an instruction prior to use.
C)view data within an instruction.
D)all of these.

Question 71

The force instructions:

A)will force data table bits on only.
B)will force data table bits off only.
C)if used indiscriminately could cause haphazard machine operation.
D)can be used only to force inputs.

Question 72

With reference to the condition illustrated, what is the most likely fault?

A)Short circuit in the field device or wiring
B)Input module fault
C)Processor module fault
D)Open in the field device or wiring

Question 73

Editing a PLC program normally refers to:

A)replacement of an existing program.
B)monitoring of an existing program.
C)making changes to an existing program.
D)all of these.

Question 74

In which of the following program modes are modifications executed immediately on entry of the instruction?

A)Continuous test mode
B)Single scan test mode
C)Off-line program mode
D)On-line program mode

Question 75

Which function of a PLC would be used to view the transition history (on or off states) of a bit(s)?

A)Change radix
B)Forcing
C)Cross-reference
D)Contact histogram

Question 76

The majority of PLC system faults are caused by:

A)faulty power supplies.
B)malfunctioning microprocessors.
C)software failure.
D)field wiring and devices.

Question 77

The suppression circuit shown:

A)can be used to suppress DC loads.
B)can be used to suppress AC loads.
C)uses a metal oxide surge suppressor.
D)all of these.

Question 78

With reference to the troubleshooting guide, which condition indicates no fault?

A)Condition (a)
B)Condition (b)
C)Condition (c)
D)Condition (d)

Question 79

With reference to the input module circuit shown, a voltmeter reading of 24 VDC would indicate:

A)normal operation.
B)a possible shorted PB.
C)a possible fault in the field wiring.
D)either a possible shorted PB or a possible fault in the field wiring.

Question 80

Preparing a PLC control process for start-up is called:

A)commissioning.
B)troubleshooting.
C)validating.
D)installing.