Electrical Symbols

To be able to read a machine plan, one needs to know what all those symbols stand for. The following is a short explanation of the most frequently used.

This is a normally-open contact. If you push it, the circuit closes and it passes potential and current. When not pushed, nothing passes.

Contrary to the one above, this is a normally-closed contact. The circuit opens when pushed, otherwise it is closed.

Switching a relay can cause voltage peaks in the circuit. To avoid the latter having a detrimental influence on the circuit, a relay with a diode is used, this to channel superfluous current.

Often a relay with R/C circuit is used to avoid arcing when switching the relay. The voltage peak that could causes the arc is handles by the R/C circuit.

This is a push-button, it lets current pass when pushed and bounces back to its open position when force is no longer applied.

This illuminated push-button lights up when pushed. On some of the less well-designed types your finger sadly covers the light.

The circuit of a key selector can only be closed or opened by use of the correct key. It is for example used to put a machine in programming mode.

A flow sensor closes when a flow of some liquid or gas is detected. It is used when flow needs to be detected but not measured.

This inductive sensor closes its circuit when a metal object comes close enough.

A pressure switch closes depending on pressure. The part on the left of the symbol represents the membrane typically used in these devices.

When coming in a strong enough magnetic field, this magnetic sensor closes its circuit.

A limit switch closes a mechanical contact when approaching the end of a trajectory.

An emergency button. When pushed it opens the circuit and keeps it open. Mostly these buttons need to be turned to close the circuit again.

This is a differential circuit breaker. The circuit opens when the current going to the lines is different, this as a safety measure.

Another safety measure is the thermal magnetic circuit breaker. This device quickly opens when too much current is flowing through the circuit.

A three phase AC motor. The three phases are connected to the left, the neutral conductor is the one leaving at the right side of the symbol.

This is a 3-phase electrical AC motor like the previous, but it contains two windings instead of one. This allows it to run at two different speeds.

A three phase motor like the ones above sometimes come with a mechanical break. It mechanically holds the motor shaft in position.

This is an AC/DC converter, as the name says, is converts alternating current into direct current.

This symbol represents a transformer, which derives its name from the fact that it transforms between potentials.

An electrically controlled, or actuated valve.

This is a connector, which is like a plug. A wall socket could for example be represented like this, as could a microphone connector.

An grounded outlet, like the ones at home.

An electric lock, typically seen on the doors of CNC machines to avoid the operator from opening them while the program is still running.

A horn makes noise when actuated. A horn of a car is a good example of this, several production machines however have horns to.

A light bulb.

Signalization lights, which indicate the current status of the machine. Green might refer to normal operation, orange might signal an operator intervention to be necessary, and red usually signals an error.