31. Implementation

31.1 Electrical

Electrical Wiring Diagrams

• PLC’s are generally used to control the supply of power to a system. As a result, a brief examination of electrical supply wiring diagrams is worthwhile.

• Generally electrical diagrams contain very basic circuit elements, such as relays, transformers, motors, fuses, lights, etc.

• Within these systems there is often a mix of AC and DC power, but 3 phase AC power is what is delivered universally by electric utilities, so the wiring diagrams focus on AC circuits.

• A relay diagram for a simple motor with a seal in circuit might look like the one shown below,

 

• The circuit designed for the motor controller must be laid out so that it may be installed in an insulated cabinet. In the figure below, each box could be a purchased module(s).

 

• After the Layout for the cabinet is determined, the wire paths must be determined. The figure below lays out the wire paths, and modules to be used.

 

• The last section contained an example of an electrical control system for a motor. It used a number of symbols to represent elements.

• The Joint International Committee (JIC) developed a standard set of symbols for representing electrical circuit elements. These are given below.

 

 

 

 

 

• Discrete inputs: If a group of input voltages are the same, they can be grouped together. An example of this is shown below.

 

• If the input voltages are different and/or come from different sources, the user might use isolated inputs.

 

• There are two problems that occur in these systems,

1. Different power sources in the same system can cause different power supply voltages at opposite ends of a wire. As a result a current will flow, and an unwanted voltage appears. This can destroy components and create false signal levels.

2. Magnetic fields crossing the long conductors or in conductor loops can induce currents, and destroy equipment, give false readings, or add unwanted noise to analog data signals.

• Typical sources of grounding and shielding problems are,

electrostatic

magnetic

electromagnetic

resistance coupled circuits

ground loops

• General shielding design points,

choose a metal cabinet that will shield the control electronics

avoid “noisy” equipment when possible

separate voltage levels, and AC/DC wires from each other when possible.

use shielded cables and twisted pair wires

• Grounding problems include,

Resistance coupled devices can have interference through a common power source, such as power spikes or brownouts caused by other devices in a factory.

Ground loops are caused when too many separate connections to ground are made creating loops of wire that become excellent receivers for magnetic interference that induces differences in voltage between grounds on different machines. The common solution is to use a common ground bar.

 

The case of an object should be tied to ground to give current a path to follow in the case of a fault that energizes the case. (Note: fuses or breakers will cut off the power, but the fault will be on for long enough to be fatal.)

 

• Good grounding rules include,

each PLC component should be grounded back to the main PLC chassis

the ground wire should be separated from power wiring in enclosures

use star washers to ensure good electrical connection

mount ground wires on bare metal, remove paint if needed

use 12AWG stranded copper for PLC equipment grounds and 8AWG stranded copper for enclosure backplate grounds

connect the enclosure to the ground bus

connect the machine ground to the enclosure ground

the ground connection should have little resistance (<0.1 homs is good)

• Electrocution potential must be observed.

Safe current levels are listed below. But be aware that in certain circumstances very low currents can kill, when in doubt, take no chances.

 

Step potential is another problem. Electron waves from a fault travel out in a radial direction through the ground. If a worker has two feet on the ground at different radial distances, there will be a potential difference between the feet that will cause a current to flow through the legs. The gist of this is: if there is a fault, don’t run/walk away/towards.

Always ground systems first before applying power. (The first time a system is activated it will have a higher chance of failure.)

• Fail-safe wiring should be used so that if wires are cut or connections fail, the equipment should turn off. e.g., if a normally closed stop button is used, and the connector is broken off, it will cause the machine to stop, as if the stop button has been pressed, and brake the connection.

NO (Normally open): When wiring switches or sensors that start actions, use normally open switches so that if there is a problem the process will not start.

NC (Normally Closed): When wiring switches that stop processes use normally closed so that if they fail the process will stop. E-Stops must always be NC, and they must cut off the master power, not just be another input to the PLC.

• Other wiring notes,

always use a master power relay that will shut down the process, and connect it to the E-stop system.

use electrical wiring to handle all safety functions

shutdown buttons must be easily accessible from all points around the machine

31.2 Safety

• Some of the basic rules are,

a fail-safe design: Programs should be designed so that they check for problems, and shut down in safe ways. Most PLC’s also have imminent power failure sensors, use these whenever danger is present to shut down the system safely.

proper programming techniques and modular programming will help detect possible problems on paper instead of in operation.

make the program inaccessible to unauthorized persons

use predictable, non-configured programs

use redundancy in hardware

directly connect emergency stops to the PLC, or the main power supply

check for system OK at start-up

provide training for new users and engineers to reduce careless and uninformed mistakes

use PLC built in functions for error and failure detection

use well controlled startup procedures that check for problems

provide clear and current documentation for maintenance and operators

modular well designed programs

• A reasonable troubleshooting guide (note: not debugging),

1. Look at the process and see if it is in a normal state. i.e. no jammed actuators, broken parts, etc. If there are visible problems, fix them and restart the process.

2. Look at the PLC to see which error lights are on. Each PLC vendor will provide documents that indicate which problems correspond to the error lights. Common error lights are given below. If any off the warning lights are on, look for electrical supply problems to the PLC.

HALT: something has stopped the CPU

RUN: the PLC thinks it is OK (and probably is)

ERROR: a physical problem has occurred with the PLC

3. Check indicator lights on I/O cards, see if they match the system. i.e., look at sensors that are on/off, and actuators on/off, check to see that the lights on the PLC I/O cards agree. If any of the light disagree with the physical reality, then interface electronics/mechanics need inspection.

4. Consult the manuals, or use software if available. If no obvious problems exist the problem is not simple, and requires a technically skilled approach.

5. If all else fails call the vendor (or the contractor) for help.

• Don’t do it.

• But if you really have to, remember that the machine can be unsafe while doing this.

• While a program is running in a PLC, you can specifically force inputs or outputs to turn on/off. This is best use for,

testing outputs: if you are doing this you will be checking wiring and the output card. This can be done directly.

force inputs to determine how the program will respond: if you are doing this you don’t understand your program, and you should sit down and figure it out.

• Possible bad outcome of forcing outputs,

damaged or jammed machine components

injuries to workers

• Care must be taken to avoid certain environmental factors.

Dirt: Dust and grime can enter the PLC through air ventilation ducts. As dirt clogs internal circuitry, and external circuitry, it can effect operation. A storage cabinet such as Nema 4 or 12 can help protect the PLC.

Humidity: Humidity is not a problem with many modern materials. But, if the humidity condenses, the water can cause corrosion, conduct current, etc. Condensation should be avoided at all costs.

Temperature: The semiconductor chips in the PLC have operating ranges where they are operational. As the temperature is moved out of this range, they will not operate properly, and the PLC will shut down. Ambient heat generated in the PLC will help keep the PLC operational at lower temperatures (generally to 0°C). The upper range for the devices is about 60°C, which is generally sufficient for sealed cabinets, but warm temperatures, or other heat sources (e.g. direct irradiation from the sun) can raise the temperature above acceptable limits. In extreme conditions heating, or cooling units may be required. (This includes “cold-starts” for PLCs before their semiconductors heat up).

Shock and Vibration: The nature of most industrial equipment is to apply energy to change workpieces. As this energy is applied, shocks and vibrations are often produced. Both will travel through solid materials with ease. While PLCs are designed to withstand a great deal of shock and vibration, special elastomer/spring or other mounting equipment may be required. Also note that careful consideration of vibration is also required when wiring.

Interference: Electromagnetic fields from other sources can induce currents.

Power: Power will fluctuate in the factory as large equipment is turned on and off. To avoid this, various options are available. Use an isolation transformer. A UPS (Uninterruptable Power Supply) is also becoming an inexpensive option, and are widely available for personal computers.

31.2.3.1 - Enclosures

• NEMA has provided a set of ratings for cabinets housing voltages less than 1000V AC. The basic classifications are outlined below,

Type 1: General purpose: indoors

Type 2: Dirt and water resistant: indoors

Type 3: Dust-tight, rain-tight and sleet(ice) resistant: outdoors

Type 3R- Rainproof and sleet(ice) resistant: outdoors

Type 3S- Rainproof and sleet(ice) resistant: outdoors

Type 4: Water-tight and dust-tight: indoors and outdoors

Type 4X: Water-tight and Dust-tight: indoors and outdoors

Type 5: Dust-tight and dirt resistant: indoors

Type 6: Waterproof: indoors and outdoors

Type 6P: Waterproof submersible: indoors and outdoors

Type 7: Hazardous locations: class I

Type 8: Hazardous locations: class I

Type 9: Hazardous locations: class II

Type 10: Hazardous locations: class II

Type 11: Gas-tight, water-tight, oiltight: indoors

Type 12: Dust-tight and drip-tight: indoors

Type 13: Oil-tight and dust-tight: indoors

 

• Most factory floor applications are well suited to type 12 enclosures.

31.3 Process Modeling

• Instrumentation Symbols,

 

Table 1: ANSI/ISA-S5.1-1984 Instrumentation Symbols and Identification

LETTER	FIRST LETTER	SECOND LETTER
A	Analysis	Alarm
B	Burner, Combustion	User’s Choice
C	User’s Choice	Control
D	User’s Choice
E	Voltage	Sensor (Primary Element)
F	Flow Rate
G	User’s Choice	Glass (Sight Tube)
H	Hand (Manually Initiated)
I	Current (Electric)	Indicate
J	Power
K	Time or Time Schedule	Control Station
L	Level	Light (pilot)
M	User’s Choice
N	User’s Choice	User’s Choice
O	User’s Choice	Orifice, Restriction
P	Pressure, Vacuum	Point (Test Connection)
Q	Quantity
R	Radiation	Record or Print
S	Speed or Frequency	Switch
T	Temperature	Transmit
U	Multivariable	Multifunction
V	Vibration, Mechanical Analysis	Valve, Damper, Louver
W	Weight, Force	Well
X	Unclassified	Unclassified
Y	Event, State or Presence	Relay, Compute
Z	Position, Dimension	Driver, Actuator, Unclassified

• Line symbols include,

 

• Sensor and actuator symbols include,

 

31.4 References

31.1 Paques, Joseph-Jean, “Basic Safety Rules for Using Programmable Controllers”, ISA Transactions, Vol. 29, No. 2, 1990.

31.5 Problems

Problem 31.1 The best location for the PLC enclosure is:

a) close to the incoming power

b) in the control room

c) close to the machine or process

d) far away from the machine or process