Electrical Circuit Protection and Control Components

Circuit Abnormalities

Overcurrents (Sobreintensitats)

• Overload: Higher consumption than the nominal current ($I_{normal}$) sustained over time.
• Short Circuits (Shorts): Current $I$ is typically much higher (e.g., $I \approx 3 \times I_{normal}$).

Electrical Contacts

Contacts can be direct or indirect.

Overvoltages

• Permanent: Caused by neutral point displacement due to breakage or imbalance. This reduces the lifespan of equipment.
• Transitional: A sudden, large increase in voltage (e.g., lightning strikes or switching operations). They originate from internal or external sources.

Protective Devices

Fuse

Interrupts the circuit when the current flow exceeds a certain limit for a specific time. Protects against overload and short circuits.

Thermal Relay

Protects against overloads using a thermal device that controls a switching mechanism. It uses a bimetallic strip whose differential dilation causes the circuit to open.

Circuit Breakers (Interruptors Automàtics / Disjuntors)

Detect and eliminate overloads and short circuits. Overload control is managed by a thermal relay, while short circuit protection uses an electromagnetic device (a coil attracts an iron armature to trip the circuit).

Differential Interrupter (RCD/GFCI)

Detects and eliminates any anomaly related to leakage current in the installation. It consists of an interrupter, a trip unit, and a toroidal core. Normally, the input current ($I_{in}$) and output current ($I_{out}$) cancel each other out (opposite directions). If there is a small difference (leakage), this causes a variation in magnetic flux, inducing a voltage in the sensing coil, which triggers the trip mechanism and opens the circuit.

Command Devices (Dispositius de Comandament)

These devices govern the circuit and execute its functions.

Selector Switch (Seccionador)

Opens and closes the circuit under no-load conditions.

Interrupter (Interruptor)

Manual operation. Allows the closing of the circuit under load.

Machine Maneuvering and Control Devices

These elements supply power to receptors (like contactors) and govern the control circuit (like relays and timers).

Contactor

A device that can operate remotely to open or close the power circuit of a receiver. It does not have a stable position (it requires continuous power to remain closed).

Parts of an Electromagnetic Contactor:

• Electromagnet: Formed by a coil and a magnetic core. When current flows, the armature is attracted, displacing the different contacts.
• Power (Main) Contacts: Connect or disconnect the current in the main power circuit.
• Auxiliary Contacts: Used to execute control sequences.

Command Relays

Receive electrical signals from auxiliary elements (e.g., push buttons) and execute an action.

• Electromagnetic Relays: Function similarly to a contactor (coil, armature, etc.).
• Solid State Relays (SSR): Do not have mobile contacts and operate using a control circuit and a trigger mechanism.

Timers

Delay and/or maintain the opening or closing of a current circuit for a set time, both upon connection and disconnection.

Auxiliary Control Devices

Responsible for ensuring dialogue between the operator and the machine.

• Push Button and Selector Switch: Manual operation devices (e.g., rotary manual switches used to divert supply).
• Mechanical Detectors (Limit Switches): Control the displacement of moving parts. The signals obtained are used to start or sequence operations.
• Signaling Devices: Report the status of the machine or circuit. They call attention to abnormalities using optical (lights) or acoustic phenomena.

Specific Circuits

Star-Delta Starter (Arrancador Estrella-Triángulo)

This process limits the current consumption of the motor during connection to the network. The starting current is reduced to one-third of the current drawn during a direct line connection. However, the starting torque is also reduced.