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


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.


Short-time overcurrent protection is available on:

oMicroLogic 5.0 X, 6.0 X, and 7.0 X control units for IEC standard

oMicroLogic 5.0 X and 6.0 X control units for UL standard

o3-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 an additional external power supply.

Operating Principle




The short-time overcurrent threshold 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 threshold range.

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

oFour setting values with I²t ON.

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

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

oFive 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, up to harmonic 15.

In order to trip on an intermittent fault, the control 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:

oIsd: short-time overcurrent protection threshold

otsd: short-time overcurrent protection time delay

oI²t (tsd): short-time overcurrent protection curve (I²t ON or I²t OFF)

They can be set as follows:

oOn the MicroLogic X display screen, at Home > Protection > I Short time

oWith EcoStruxure Power Commission software (password-protected)

oWith EcoStruxure Power Device app (password-protected)

oBy sending a setting command using the communication network (password-protected).

Short-time overcurrent protection can be duplicated when dual settings are activated.

Protection Settings





Factory setting




1.5–10 x Ir

0.5 x Ir(1)

1.5 x Ir

+/- 10%

tsd with I²t ON




tsd with I²t OFF





(1) Finer resolution settings are possible with EcoStruxure Power Commission software and EcoStruxure Power Device app

For more information, refer to the setting guidelines.

Operating Times at 10 x Ir

The operating times of short-time protection depend on the tsd time delay. They are valid for I²t ON or OFF.

tsd time delay.

0 s

0.1 s

0.2 s

0.3 s

0.4 s

Non-tripping time

> 0.02 s

> 0.08 s

> 0.14 s

> 0.23 s

> 0.35 s

Maximum breaking time

< 0.08 s

< 0.14 s

< 0.20 s

< 0.32 s

< 0.50 s

Zone Selective Interlocking (ZSI)

The ZSI characteristics and external wiring of the zone selective interlocking function, are described specifically.

If ZSI IN is not set to 1 (open circuit between Z3 and Z4 terminals), the maximum breaking time is 0.08 s regardless of the tsd setting value.

When ZSI IN is set to 1 and connected to the ZSI OUT of a downstream device (or when the ZSI function is not used and there is a jumper between the Z3 and Z4 terminals), the tsd time delay is used.

The Isd threshold activates ZSI OUT (Z1 and Z2 terminals).

NOTE: MasterPact MTZ circuit breakers are delivered with a jumper installed between Z3 and Z4.

Predefined Events

The function generates the following predefined events:





0x6401 (25601)

Isd trip



0x6301 (25345)

Isd operate



0x6201 (25089)

Isd start (I > Isd)



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

Protection events are generated as follows:

oThe start event is generated when the protection picks up.

oThe operate event is generated when the protection time delay elapses.

oThe trip event is generated when the circuit breaker tripping voltage release (MITOP) activates.

Recommended Actions



Recommended actions

0x6401 (25601)

Isd trip

Reset the device or use the Power restoration assistant within the EcoStruxure Power Device app.

Resetting a Trip Event

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

oMasterPact MTZ1 - Circuit Breakers and Switch-Disconnectors - User Guide

oMasterPact MTZ2/MTZ3 - Circuit Breakers and Switch-Disconnectors - User Guide