Panel components and sensors

Opening a control panel and knowing what each device is, what it is for and how it fails is the foundation of any technician. A good PLC programmer needs it too: you cannot program a machine well without understanding what the contactor behind each output does or why a sensor does not see a part. This topic covers the components you will find in almost every panel and machine: protection, switching, sensors and pneumatics. It follows IEC practice, with notes where North American practice differs.

Protection: the miniature circuit breaker

The miniature circuit breaker (MCB) protects the cables of the installation against two different problems:

• Overload: a little more current than intended flows for some time. The thermal part (a bimetallic strip that bends as it heats up) trips after a delay, shorter the bigger the overload.
• Short circuit: the current shoots up at once. The magnetic part (a coil that pulls in a plunger) trips in milliseconds.

Its main ratings are the rated current (for example 10 A or 16 A), the breaking capacity (the highest short-circuit current it can safely interrupt, for example 6 kA or 10 kA) and the curve, which says at how many times the rated current the magnetic part trips.

• Curve B: for circuits with resistive loads and long cables.
• Curve C: the most common in general installations and control circuits.
• Curve D: for loads with large inrush currents, such as transformers or some motors.

The residual current device

The residual current device (RCD, also RCCB, or GFCI in North America) compares the current going out with the current coming back. If some is missing, it is leaking to earth, and it trips. It protects people (30 mA in IEC installations; 5 mA GFCIs in North America) and against fires caused by leakage (less sensitive ones, such as 300 mA). It does not protect against overload or short circuit: that is the breaker's job.

There are several types depending on the leakage currents they detect. Type AC only detects sinusoidal AC leakage; type A also detects pulsating DC; type B also detects smooth DC, such as a three-phase variable frequency drive can produce. That is why drive manufacturers state which type to use.

The test button (T) should be pressed periodically: it checks that the mechanism trips.

The motor protection circuit breaker

A motor protection circuit breaker (MPCB, or manual motor starter) combines magnetic short-circuit protection and adjustable thermal overload protection in one device designed for motors. It is set to the rated current on the motor nameplate. Many have auxiliary contacts so the PLC knows if it has tripped.

The contactor

A contactor is an electrically operated switch: when its coil is energised, it closes its contacts. It lets a small PLC output (24 V, a few milliamps) switch a motor drawing many amps.

The numbering follows a logic that applies to many devices:

• Power: 1-2, 3-4 and 5-6 (line at the top, load at the bottom). Some North American devices mark them L1-T1, L2-T2 and L3-T3.
• Auxiliaries: the units digit gives the function. Ending in 3-4, normally open (13-14, 43-44); ending in 1-2, normally closed (21-22, 31-32). The tens digit is the sequence number.
• Coil: A1-A2. Check its voltage: a 24 V DC coil burns out on 230 V AC, and a 230 V coil will not pull in on 24 V.
• Overload relay: its normally closed trip contact is 95-96, and the normally open one is 97-98.

Contactors are chosen by utilization category: AC-3 is for squirrel-cage motors started and stopped in normal running. A 9 A AC-3 contactor can switch a motor of about 4 kW at 400 V.

You can watch a contactor working in its two most common circuits, start-stop with seal-in and forward-reverse with interlocks, in the motor control circuit simulator.

Interface relays

They separate the PLC output from the load: the PLC output only feeds the relay coil, and the relay contact feeds the load. They protect the output card and let you switch other voltages.

Sensors

Besides the type, the output matters: PNP (sourcing, gives +24 V when it detects) or NPN (sinking, connects to 0 V when it detects), and normally open or closed. Three-wire sensors are powered separately (brown +24 V, blue 0 V, black output); two-wire sensors are in series with the load and let a small leakage current through even when not detecting, which is sometimes enough to turn on a sensitive input.

You can wire a PNP or NPN sensor to a PLC input, wire by wire, and see where the current flows in the PNP and NPN sensor simulator.

Pneumatics

Compressed air moves cylinders and grippers. It reaches the machine, usually at about 6 bar (90 psi), through an air preparation unit (FRL): filter (removes water and particles), pressure regulator (sets the working pressure and holds it) and, sometimes, lubricator.

Solenoid valves and their names

Valves are named with two numbers: ports / positions.

• 3/2: three ports and two positions. For single-acting cylinders, which return on a spring.
• 5/2: five ports and two positions. For double-acting cylinders, which extend and retract on air.
• 5/3: five ports and three positions. The centre position can leave the cylinder locked, with both chambers exhausted or with both under pressure, depending on the model.

By how they return to position:

• Monostable (single solenoid): one coil and a spring. If the signal is removed (or the power fails), it returns to its rest position.
• Bistable (double solenoid): two coils. It stays in the last commanded position even when the signal is removed: it has «memory». This matters for the program and for safety: after a power failure, the cylinder does not return by itself.

Speed and pressure

• A cylinder's speed is set with flow controls, usually by restricting the exhaust air, which gives a steadier movement.
• The pressure regulator reduces the supply pressure to the working pressure and holds it. The safety (relief) valve, on the other hand, opens to release air if the pressure exceeds a maximum value.

Pilot lights and push buttons

Pilot light colours have a standard meaning on machines (IEC 60204-1): red, emergency or danger; yellow, abnormal condition; green, normal condition; blue, mandatory action; white, neutral information. The emergency stop is red on a yellow background.

How components fail

• Breaker tripping: overload, short circuit or a curve too low for the inrush.
• RCD tripping: a real leak, moisture in a box, damaged insulation or the sum of small leakages from many devices on the same RCD.
• Contactor not pulling in: no voltage at A1-A2, open coil or wrong coil voltage.
• Contactor humming: low coil voltage or dirt between the core faces.
• Sensor failing now and then: working at its range limit, dirt, a loose bracket or a damaged cable in a moving area.
• Cylinder slow or weak: low pressure, leaks or a flow control closed too far.

Summary

• Circuit breaker: protects the cable. Rated current, breaking capacity and curve (B, C or D).
• RCD: protects people against earth leakage. Type AC, A or B depending on the load.
• Motor protection circuit breaker: magnetic plus adjustable thermal protection for one motor.
• Contactor: power 1-6, NO auxiliaries ending in 3-4 and NC ending in 1-2, coil A1-A2.
• Sensors: inductive (metal), capacitive (almost anything), photoelectric and magnetic; PNP or NPN output.
• Valves: 3/2 for single-acting, 5/2 for double-acting; monostable returns by itself, bistable remembers.