Coordination of LV Protection Devices

The requirements of electrical power distribution

The design of LV installations leads to basic protection devices being fitted for three types of faults:

• Overloads
• Short-circuits
• Insulation faults

Safety and availability of energy

Implementation of these protection devices must allow for:

• The statutory aspects, particularly relating to safety of people
• Technical and economic requirements.

The chosen switchgear must:

• Withstand and eliminate faults at optimised cost with respect to the necessary performance
• Limit the effect of a fault to the smallest part possible of the installation in order to ensure continuity of supply.

Achievement of these objectives requires coordination of protection device performance, necessary for:

• Managing safety and increasing durability of the installation by limiting stresses
• Managing availability by eliminating the fault by means of the circuit-breaker immediately upstream.

The circuit-breaker coordination means are:

1. Cascading
2. Discrimination.

If the insulation fault is specifically dealt with by earth leakage protection devices, discrimination of the residual current devices (RCDs) must also be guaranteed.

AUTHOR: Schneider Electric experts