Control Devices and Protective Relays: A Comprehensive Guide
Control Devices
Definition
A control device is a key component of a charge control circuit, facilitating communication between the user’s control circuit and the machine.
Types of Control Devices
- Switches: Multi-position switches used to intervene in the control of a circuit.
- Pilot Lights: Indicate a command action.
Buttons and Switches: Symbols
Buttons, used for maneuvering in control circuits, consist of two parts:
- Control Device: The part of the button that the operator interacts with.
- Contacts: Responsible for performing the control action. Can be normally open (NO), normally closed (NC), or a combination.
Switches or knobs have various fixed positions and are used for selections within different circuits.
Troubleshooting Control Circuits
Before troubleshooting a control circuit, perform these checks:
- Verify voltage presence.
- Inspect protection elements.
- Check the wiring diagram (control power and control circuit) for electrical continuity.
Contactors are a major cause of control circuit failures.
Protective Devices
Purpose
Protective devices detect and/or remove breakdowns or incidents that may occur in electrical receptors, facilities, and circuits.
Types of Damage
- Overcurrent and Overload: An increase in current consumption beyond the service current. Permanent overcurrent is considered a fault and requires removal. Short-duration overcurrent (e.g., motor starting) is acceptable.
- Short Circuit: A significant increase in power consumption beyond the operating current, often caused by the union of several active conductors. Requires elimination within 5 seconds.
- Insulation Defect: A connection between active conductive parts (e.g., electrical boxes) and other active parts (e.g., wires) under nominal voltage.
Breakdown or Incident: Caused by a malfunction of electrical facilities or receptors.
Service Current: The current a receiver consumes continuously.
Thermal Relays
Definition
A thermal relay detects unacceptable overcurrent in recipients without intervention during acceptable overcurrent.
Types
- Standard Thermal Relays: Used in single-phase, two-phase, or three-phase receivers, commonly for motor protection.
- Differential Thermal Relays: Used in three-phase receivers; detect phase loss, useful for three-phase motor protection.
- Compensated Thermal Relays: Provide overcurrent protection and are insensitive to temperature variations.
Components
- Bimetallic or Main Contacts: Formed by two metals with different expansion coefficients.
- Auxiliary Contacts: Open or close control circuits, typically one NO and one NC in the rest position.
- Test Button: Activates the relay to verify operation.
- Heating Coils: Heat the bimetal.
- Spring: Returns the auxiliary contacts to the rest position.
Operation
During unacceptable overcurrent, the heating coils (sized according to the control current) heat the bimetal, causing deformation. This deforms the bimetal, which displaces the NC auxiliary contacts to stop feeding the contactor coil, disconnecting the receiver. The NO auxiliary contacts activate a signaling element. The relay resets manually or automatically after the bimetal cools and the spring returns the contacts to rest.
Selection Criteria
- Maximum overcurrent time without tripping.
- Operating current in amperes.
Circuit Breakers (Magnetic)
A protective device that automatically interrupts unacceptable overcurrent and short circuits without opening during acceptable overcurrent. Not to be used as a maneuvering element; contacts remain closed at rest.
Fuses
A protective device that automatically interrupts unacceptable overcurrent and short circuits. Does not act during acceptable overcurrent. One of the oldest electrical protection devices.
Types
- Threaded Fuse: Requires threading into a fuse holder (D and D0 systems).
- Blade Fuse: NH system (low-voltage, high-capacity). Includes a fuse base, interchangeable cartridge, and cartridge replacement accessory.
Classes
- Category G (General Use): Protects against overcurrent and short circuits.
- Category A (Accompanying): Protects against short circuits; requires additional overcurrent protection (thermal relay). Service types: L (cables/conductors), M (switchgear), R (semiconductors), Tr (transformers).
Switches (Differential)
A protective device that detects and eliminates insulation defects in receptors.