Overview

Protection functions are provided by relays or multifunctional devices like the Schneider Sepam.

Protective relays (or multifunctional devices) are devices that permanently compare the electrical variables of networks (such as current, voltage, frequency, power, and impedances) with predetermined values, and then automatically emit orders for action (usually the opening of a circuit-breaker) or give off an alarm when the monitored value goes above the threshold.

The role of protective relays is to detect any kind of abnormal phenomena that may arise in an electrical circuit, such as short-circuits, variation in voltage, machine faults, etc.

The relay may be:

• without auxiliary power (autonomous) when the energy required for it to operate is supplied directly by the monitored circuit (see Figure 7-1). The actuator must be sensitive because the energy supplied by the circuit is limited;

  
  Figure 7-1: connection of an overcurrent relay without auxiliary power

• with auxiliary power supply when the energy required for it to operate is supplied by an auxiliary voltage source (AC or DC) independent of the circuit monitored (see Figure 7-2).

Figure 7-2: connection of an overcurrent relay with auxiliary power

7.1 Phase overcurrent protection (ANSI code 50 or 51)

The function of this protection is to detect single-phase, two-phase or three- phase overcurrents.

Protection is activated when one, two or three of the currents concerned rise above the specified setting threshold.

This protection can be time delayed and in this case will only be activated if the current monitored rises above the setting threshold for...