Johdota device guide

Schematic symbols

The symbols of a wiring diagram explained: contacts, coils, push-buttons, protective devices and indicator lights. Also device letters, terminal numbers and how to read a diagram.

How to read a diagram

Self-holding circuit at rest. Nothing is pressed, so S0 is closed and S1 is open.

A diagram is drawn at rest: no voltage, nothing pressed and no relay pulled in. That is why an NC contact is shown closed and an NO contact open.

In IEC diagrams the control circuit is drawn between two rails, L at the top and N at the bottom, and each current path is read from top to bottom. The load, such as a coil or a light, is at the bottom of the path. In the US and Canada the same circuit is drawn as a ladder: the rails are on the left and right, and each rung is read from left to right.

A coil and its contacts share the same tag. When the coil of K1 gets voltage, every contact marked K1 changes over, even if it is drawn on the other side of the diagram.

In the picture S0 is the NC stop button and S1 the NO start button. The K1 contact 13–14 across S1 keeps the coil on when S1 is released. Contact 23–24 lights H1. Self-holding step by step →

The motor supply L1, L2 and L3 is usually drawn as a separate power circuit with the protective devices, the contactor's main contacts and the motor.

Showing symbols for

Symbols by device (IEC)

Electrical symbols follow IEC 60617 and valve symbols ISO 1219. Pick a device to see how it works and how it is wired.

SPush-buttonsNO contact 13–14 · NC contact 11–12, 21–22 SEmergency stop buttonContacts NC, e.g. 11–12 · Colour red, yellow background SRotary and selector switchesPositions 0-I or H-0-A · Contacts as in the switching table SLimit switchNC contact e.g. 21–22 · NO contact e.g. 13–14 · Actuator roller, lever or plunger HIndicator lightsTerminals X1–X2 · Connection between L and N KAuxiliary relayCoil A1–A2 · Contacts e.g. 11–12–14 KContactorMain contacts 1/L1–2/T1, 3/L2–4/T2, 5/L3–6/T3 · Coil A1–A2 · Auxiliary contacts 13–14, 21–22 …KTTimer relaySupply A1–A2 · Control input B1 · Contact 15 common, 16 NC, 18 NO KImpulse relayControl pulse to coil A1–A2 · Push-buttons NO, in parallel QSwitch-fuseSupply from the top · Output to the contactor · Protects against short circuit · Overload protection thermal overload relay QMotor protection circuit breakerPower circuit 1/L1–6/T3 · Auxiliary contacts 13–14, 21–22 · Setting motor rated current FThermal overload relayPower circuit 2/T1, 4/T2, 6/T3 · NC contact 95–96 · NO contact 97–98 QLocal isolatorLocation next to the motor · Locking in position 0 · Auxiliary contact early-break APhase failure and monitoring relaysMonitors L1, L2, L3 · Output 11 common, 12, 14 · Healthy 11–14 closed YSolenoid valveCoil 24 V DC · Supply port 1 · Working ports 2 and 4 · Exhaust 3 and 5 · Coils 14 and 12

Device letters (IEC)

A device tag is a letter and a number, for example K1 or S2. The letter tells what kind of device it is. Practice varies a little between standard editions and companies. The guide and the game use these:

`S`	Push-buttons, Emergency stop button, Rotary and selector switches, Limit switch
`H`	Indicator lights
`K`	Auxiliary relay, Contactor, Impulse relay, Counter relay
`KT`	Timer relay
`Q`	Switch-fuse, Motor protection circuit breaker, Local isolator
`F`	Thermal overload relay
`M`	Induction motor
`B`	Sensors, Transmitter 4–20 mA
`A`	Phase failure and monitoring relays, Safety relay, Pneumatic cylinder
`Y`	Solenoid valve
`U`	Variable speed drive
`PLC`	Small PLC

Symbols in the US and Canada (NEMA)

North American ladder diagrams use NEMA-style symbols (IEEE 315). A contact is drawn as two short parallel lines, an NC contact has a slash through it, and a coil is a circle. These are the symbols used in the US and Canada version of the game.

Pushbutton, NO (momentary)

Pushbutton, NC (momentary)

Emergency stop, NC, mushroom head

Selector switch contact (closed in H)

Relay contact, NO

Relay contact, NC

Control relay coil

Starter / contactor coil

Timing relay coil

Counter coil

Timed contact NOTC (on-delay)

Timed contact NCTO (on-delay)

Timed contact NOTO (off-delay)

Timed contact NCTC (off-delay)

Limit switch, NO

Limit switch, NC

Proximity switch, NO

Photoelectric sensor, NO

Pressure switch, NO

Float switch, NO

Temperature switch, NC

Pilot light, green

Pilot light, red

Buzzer (ABU)

Solenoid (valve)

PLC input

PLC relay output contact

Drive (VFD) input

Transmitter 4–20 mA (ISA bubble, field mounted)

Starter main contacts (3-pole)

Overload relay heaters

Disconnect switch (3-pole)

Circuit breaker with thermal trip (motor protection)

Device letters (NFPA 79)

Tags are a number and letters, for example 1CR or 2PB. The game uses the device designations of NFPA 79 Annex E:

`PB`	Push-button
`SS`	Selector switch
`LS`	Limit switch
`CR`	Control relay
`M`	Motor starter (contactor)
`TR`	Timing relay
`OL`	Overload relay
`CB`	Circuit breaker, also a motor protection circuit breaker
`DISC`	Disconnect switch
`LT`	Pilot light
`SOL`	Solenoid
`MTR`	Motor

Terminal numbers

The most common numbers you see on devices and in diagrams. Always check the device's own instructions.

`13–14, 23–24 …`	NO contact. The first digit is the contact's number, the second (3–4) says it is normally open. Contactor →
`11–12, 21–22 …`	NC contact (1–2). Contactor →
`A1–A2`	Coil of a relay or contactor. Contactor →
`1/L1–2/T1, 3/L2–4/T2, 5/L3–6/T3`	Main contacts. The supply goes to the L side, the motor to the T side. Contactor →
`95–96`	NC contact of an overload relay. Opens when the relay trips. Thermal overload relay →
`97–98`	NO contact of an overload relay, for example for a fault light. Thermal overload relay →
`15–16–18`	Changeover contact of a timer relay: 15 common, 16 NC, 18 NO. Timer relay →
`X1–X2`	Terminals of an indicator light. Indicator lights →
`U1, V1, W1, U2, V2, W2`	Winding ends of a motor in its terminal box. Induction motor →

Frequently asked questions

Why is an NC contact drawn closed?

Diagrams are drawn at rest. An NC contact is then closed and only opens when the device is operated or the relay pulls in.

What do A1 and A2 mean?

They are the coil terminals of a relay or contactor. With voltage across them the relay pulls in and its contacts change over.

How do I know which contact belongs to which relay?

From the tag. A coil and its contacts share the same tag, for example K1, even when they are drawn in different places.

What is the difference between the power circuit and the control circuit?

The power circuit feeds the motor or other load. The control circuit switches it with buttons, relays and the contactor coil.