Electrical Safety: Protection Devices and Grounding Schemes

Electrical Protection Devices

Protection devices safeguard electrical installations and connected components from potential overloads. They also protect people from electric shock, preventing current leakage to the ground and the consequent economic damage to the user.

Fuses

A fuse is connected in series with the circuit so that all the current flows through it. It consists of a base with two fixed contacts, a plug-piece cartridge or blade, and fusible material surrounded by a medium that acts as a means of extinction.

• Overload: Occurs when the current value in an electrical installation exceeds the fuse's rating.
• Short circuit: Occurs when a fault in an installation element or an accident causes a sudden surge in current, causing the fuse to blow.

Differential Protection

A differential element protects people and animals from insulation defects in electrical installations.

Operation: The inflow current should be equal to the outflow current. If there is a leakage current or a fault, the two intensities will not be equal. The torus is designed so that when the intensities are unequal and the gap reaches a certain value, it induces a current in the coil. This current then feeds the relay, causing it to open and act on the differential button. When the test button (T) is pressed, it simulates a leakage current. The test resistance absorbs some of the current, causing it to flow outside the toroid.

Circuit Breakers

A circuit breaker is an element that protects the installation against overloads and short circuits.

Operation: When the current flowing through the circuit breaker exceeds its calibration due to an overload, the bimetal heats up faster as the current value increases (thermal effect). If the current flowing through the breaker is very high, the coil attracts the core, causing the mobile contact to open (magnetic effect).

Grounding Schemes

TN Scheme

In a TN scheme, a common point of the power source is directly connected to the ground, usually the neutral or compensating conductor. The exposed conductive parts of the installation are directly connected to this conductor.

IT Scheme

In an IT scheme, the power source has no point directly connected to the ground. The exposed conductive parts of the installation are directly connected to the ground.

TT Scheme

In a TT scheme, a common point of the installation is directly connected to the ground. The exposed conductive parts are connected to a separate ground electrode, independent of the power supply's ground.

Electrical Contacts

Direct Contact

Direct contact refers to the contact of humans or animals with live parts of electrical materials and equipment.

Indirect Contact

Indirect contact refers to the contact of humans or animals with parts that have become live due to an insulation fault.

Grounding

Grounding is a direct electrical connection, without fuses or other protection elements, between a part of the installation and an electrode buried in the ground.