Short-Time Overcurrent Protection

Presentation

Short-time overcurrent protection helps to protect equipment against phase-to-phase, phase-to-neutral and phase-to-ground short circuits with total selectivity. It includes two characteristics, definite time and inverse time, which depend on the status of the I²t setting.

Availability

Short-time overcurrent protection is available on:

• MicroLogic 5.0 and 6.0 trip units

• 3-pole and 4-pole circuit breakers

Short-time overcurrent protection is powered by the current flowing through the internal current transformers of the circuit breaker and it does not require additional external power supply.

Operating Principle

@writer: the diagram is shared in MTZ MicroLogic B and NS MicroLogic Basic user guides.

The short-time overcurrent pickup Isd sets the level of short-circuit current at which the circuit breaker trips when reaching the short-time overcurrent time delay.

The short-time overcurrent time delay tsd sets the length of time during which the circuit breaker carries a short circuit within the short-time overcurrent pickup range.

The short-time overcurrent time delay can be adjusted to:

• Four setting values with I²t ON.

  • Up to 10 Ir, the tripping curve is an inverse time curve. The time delay decreases as the current increases.

  • Above 10 Ir, the tripping curve is a definite time curve with a constant tripping time.

• Five setting values with I²t OFF. The tripping curve is a definite time curve with a constant tripping time.

Short-time overcurrent protection is based on the true RMS current of phases and neutral.

In order to trip on an intermittent fault, the trip unit accumulates the intermittent currents in the short-time tripping range that do not last long enough to trigger a trip. This accumulation may lead to shorter tripping times than those set.

Setting the Protection

The short-time overcurrent protection settings are:

• Isd: short-time overcurrent protection pickup

• tsd: short-time overcurrent protection time delay

• I²t (tsd): short-time overcurrent protection curve (I²t ON or I²t OFF)

They can be set by using the Isd and tsd multi-position dials on the front face of the MicroLogic trip unit.

Protection Settings

Short-time pickup Isd.