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6.10.10 Starting the Operation of a PLC and Fault Detection
The procedure of the first-time operation of a PLC should contain careful steps in order to exclude
destructive damage of the I/O modules, CPU, power supply, or any other equipment involved.
Before putting a PLC into operation for the first time, the entire installation should be examined
and verified in accordance with the manufacturer’s specifications and the national or international
regulations. Additionally, the good grounding of the system should be specifically checked. Before
powering up the PLC, the following actions are needed:
1. Ensure that the supply voltage for the PLC corresponds to the one needed from the PLC.
Many PLCs are capable of receiving more than one electrical supply voltage, such as 230 V
AC and 110 V AC. The selection is made either with a special switch or with a jumper in the
appropriate position. In many cases, it is quite common that the PLC is set for an AC voltage
of 110 V AC and, in the end, supplied with 230 V AC with catastrophic results.
2. Check that all the power and communication cables are correctly positioned in their respec-
tive slots. The communication connectors fitted with locking screws should be carefully
installed in their correct positions.
3. Examine whether all the I/O modules are correctly positioned on the mounting base for the
case of a modular PLC, and that there is a communication “bridge” between each one and
the next one.
4. Put the operational mode switch of the PLC in the STOP position.
5. If the electric installation includes a general emergency switch (e.g., an emergency STOP
button), make sure it is OFF or in the open contact position.
6. Ensure that all the output devices are not powered.
Once the previous steps are completed, supply power to the PLC and check that the status of
all the LEDs on the central processing unit are as the manufacturer of the PLC specifies them. Also
check that the status of all the input devices, ON or OFF, are in conformity with the logic of the
automation program. This simply means that some input devices are normally closed contacts, so
that the corresponding LEDs of the digital inputs are lighted (ON state). Then, the input devices
that are in the normally open contact state should be checked by causing a manual activation of
each of them. For example, in the case of a button, it should be simply pressed, or in case of a
photocell on a conveyor belt, an object should be interfered to check the corresponding activation.
Whenever an input device of this category (e.g., a NO contact) is activated, the corresponding LED
of the digital input of the PLC should also be activated. In this way, it is ensured that the input
devices are functioning properly, if the wiring of the input devices was implemented correctly, and
whether the input device and input address match the one contained in the automation program.
Subsequently, the PLC is set to the run mode (RUN) and the outputs are checked so they behave
262
◾
Introduction to Industrial Automation
according to the logic of the automation program. This test can be based on the indicating LEDs
of the digital outputs, the state of the power relays provided, or the power supply of the loads that
were interrupted according to action 6, above. In order to control the correctness of the automation
program, it does not need to operate large loads (such as large motors) at the stage of the first test
of the PLC.
If the above check reveals an operating problem in either the general operation of the PLC, or
in one or more inputs or outputs, then the debugging process of either the program or the hard-
ware is followed. This process, in order to have a quick and effective outcome, should be systematic
and include the following three basic steps:
◾
The isolation of the operational problem from other possible ones
◾
The identification of the operational symptom or the equipment fault
◾
The careful fault correction and restoration
If the problem concerns the actual operation of the PLC and the CPU, then the indicating
LEDs should be checked in order to diagnose the operating status of the PLC and follow the
manufacturer’s debugging instructions. The CPU and the AC power adapter may have some or all
of the following indication LEDs.
◾
DC or AC POWER ON
◾
TEST MODE (A PLC can be in STOP, RUN, TEST, programming mode, etc.)
◾
PROCESSOR FAULT
◾
MEMORY FAULT
◾
I/O FAULT
◾
LOW BATTERY FAULT
If the problem involves a digital input or output, then some basic actions for the debugging
process are as follows:
1. Suppose a digital input device (e.g., a limit switch) is switched on (the contact of the
switch is closed) and causes a digital output to be activated. If this is not the case then if,
with an activated input device, the indicative LED of the digital input is activated, then
the problem is due to the digital input module or possibly to the logic of the executed auto-
mation program. If, with an activated input device, the indicative LED does not light up,
then the problem is due either to the input device or to its wiring. In this case, measuring
the voltage at the corresponding digital input of the PLC, as shown in Figure 6.55, will
help derive a final conclusion.
2. If the device assumed that a digital output of a PLC is enabled, but the corresponding output
device connected to it is not activated, and the digital output LED does not light up, then
the problem exists in the digital output module. If the digital output indicator LED lights
up but the output device is not activated, then the problem exists either in the wiring of the
output device or on the device itself.
In both cases, measuring the voltage at the digital output of the PLC, as shown in Figure 6.56, will
help to accurately locate the fault.
Basic Operating Principles of PLCs
◾
263
Special attention should be provided when the output of the PLC is utilizing a triac as a switching
element. In this case, the triac has a high internal resistance when not conducting (about 1 M
Ω
), so the
insertion of a voltmeter, which also has a high internal resistance (about 6–10 M
Ω
), results in a volt-
age divider generated when the digital output has no load. In this case, the indication of the voltmeter
may be close to the nominal voltage of the digital output even when the triac is OFF. For the voltmeter
indication to be correct, a 10 K
Ω
resistor should be connected in parallel to the instrument’s terminals,
as presented in Figure 6.57. Then the total resistance of the instrument will be much smaller than that
of the triac, and the indication will be almost zero for a non-conducting triac.
PLC
INPUTS
OUTPUTS
0 V
C
1
C
2
C
3
+24 V DC
0 V
+24 V DC
Q 0.0
I 1.0
I 1.4
S
1
S
3
Q 0.2 Q 0.4
S
2
I 1.2
PLC
INPUTS
OUTPUTS
Q 0.0
I 1.0
I 1.4
Q 0.2 Q 0.4
I 1.2
V
PLC
Inputs
Outputs
Q 0.0
I 1.0
I 1.4
Q 0.2 Q 0.4
I 1.2
V
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