Short-Time Overcurrent Protection (S or ANSI 50TD/51)

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:

• TS 5.0 and 5.0S, and 6.0S trip system

• 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

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:

• Five 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.

• Six 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 system 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 rotary switches on the front face of the TS trip system.

Setting the Isd Pickup

The short-time protection pickup Isd is set by using the Ir and Isd rotary switches on the front face of the TS trip system.

The setting value is expressed in multiples of Ir.

Follow the procedure below to set the short-time protection pickup:

1. Set the Ir long-time protection pickup.

2. Turn the Isd rotary switch to the value required.

3. Isd pickup = Isd setting x Ir pickup.

   NOTE: When Ir is set to off, Isd pickup = Isd setting x In rated current.

The Isd setting values are: 1.5, 2, 2.5, 3, 4, 5, 6, 8, 9, 10. Accuracy: +/-10 %.

Setting the tsd Time Delay

Predefined Events

Predefined events cannot be modified by the user. For general information about events, refer to Event Management.

Recommended Actions

Resetting a Trip Event

For information about resetting the circuit breaker after a trip due to an electrical fault, refer to the relevant document:

• EasyPact MVS1 Circuit Breakers with TS Trip System - User Guide
• EasyPact MVS2 Circuit Breakers with TS Trip System - User Guide