Electrical Safety: Preventing Contact and Protection Measures

Questionnaire 5

1. Preventing Grid Contact

To prevent contact between a person and a grid, ensure there is no galvanic connection between them. Natural elements like land or soil can act as galvanic connections.

2. Default Voltage (UD) and Contact Voltage (UC)

• Default Voltage (UD): The voltage between a source or ground masses and a reference point in case of a fault.
• Contact Voltage (UC): The portion of the default voltage, or voltage from the ground, that could be experienced by a person.

3. UNE 20,460 Direct Contact Protection

The UNE 20,460 standard defines the following protection measures against direct contacts:

• Protection by insulation of active parts.
• Protection by barriers or enclosures.
• Protection by obstacles.
• Protection by distance.
• Additional protection by residual current devices (RCDs).

4. Indirect Contact Protection Measures

Protective measures against indirect contacts include:

• Automatic power cut-off.
• Use of Class II equipment or equivalent insulation.
• Non-conductive locations.
• Equipotential bonding connections not connected to local ground.
• Electrical separation.

5. Grounding Requirements in Electrical Installations

The following items need to be connected to the ground in an electrical installation:

• Centralization counters.
• Metal guides of lifting gear.
• Metal safety general protection devices.
• Lightning protection systems.
• TV and FM antennas.
• Plumbing, heating, and gas installations.
• Structural steel and reinforcement of concrete walls, supports, and significant metallic elements.

6. Voltage Protection Relay

A voltage protection relay is designed to prevent the persistence of excessively high contact voltage in areas of the installation not part of the active circuit.

Features:

• Initiates a power outage in all active conductors upon detecting a dangerous voltage.
• Responds to a maximum default voltage of 50V in dry locations or 24V in wet locations.
• Disconnects active conductors in the shortest possible time.

7. Conductor Protection Resistance Calculation

To calculate the resistance of a protective conductor for a device behind a magnetothermic switch with a lens factor (n) of 4.5, a nominal intensity of 20A, in a dry location:

I_n = n * I = 4.5 * 20A = 90A

R_cp = U_L / I_n = 50V / 90A ≈ 0.55 ohms

Therefore, the resistance of the protective conductor (R_cp) should be approximately 0.55 ohms.