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:

MicroLogic Active 5.0 and 6.0 control units for IEC standard
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 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:

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, 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:

Isd: short-time overcurrent protection threshold
tsd: short-time overcurrent protection time delay
I²t (tsd): short-time overcurrent protection curve (I²t ON or I²t OFF)

They can be set as follows:

On the MicroLogic Active pin code-protected display screen, by pressing the Protection Settings button, and selecting the S (short-time overcurrent protection) area. See Protection Setting Procedure.
With EcoStruxure Power Commission software (password-protected)
With EcoStruxure Power Device app (password-protected)

Two modes of configuring the protection settings are available:

Quick setting mode is used to configure quick protection settings
Fine adjustment mode is used to configure fine protection settings

Quick Protection Settings

Quick protection settings are available:

On the MicroLogic Active HMI
With EcoStruxure Power Device app

Setting	Unit	Values	Factory setting	Accuracy
Isd	A	(1.5 / 2 / 2.5 / 3 / 4 / 5 / 6 / 8 / 10) x Ir	1.5 x Ir	+/- 10%
tsd with I²t ON	s	0.1 / 0.2 / 0.3 / 0.4 / 0.6 / 0.8	I²t OFF/0
tsd with I²t OFF	s	0 / 0.1 / 0.2 / 0.3 / 0.4 / 0.6 / 0.8	I²t OFF/0

For more information, refer to the setting guidelines.

Fine Protection Settings

Fine protection settings are available:

On the MicroLogic Active HMI
With EcoStruxure Power Commission software
With EcoStruxure Power Device app

Setting	Unit	Values	Factory setting	Accuracy
Isd	A	1.5–10 x Ir (step 0.1 x Ir)	1.5 x Ir	+/- 10%
tsd with I²t ON	s	0.1 / 0.2 / 0.3 / 0.4 / 0.6 / 0.8	I²t OFF/0	+/- 10%
tsd with I²t OFF	s	0 / 0.1 / 0.2 / 0.3 / 0.4 / 0.6 / 0.8	I²t OFF/0	+/- 10%

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	0.6 s	0.8 s
Non-tripping time	> 0.02 s	> 0.08 s	> 0.14 s	> 0.23 s	> 0.35 s	> 0.55 s	> 0.71 s
Maximum breaking time	< 0.08 s	< 0.14 s	< 0.20 s	< 0.32 s	< 0.50 s	< 0.63 s	< 0.80 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:

Code	Event	History	Severity
0x6401 (25601)	Short Time (Isd) trip	Trip	High
0x6201 (25089)	Isd start	Protection	Low

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

Protection events are generated as follows:

The start event is generated when the protection picks up.
The trip event is generated when the circuit breaker tripping voltage release (MITOP) activates.

Recommended Actions

Code	Event	Recommended actions
0x6401 (25601)	Short Time (Isd) trip	Reset the circuit breaker. See Resetting a Trip Event.
0x6201 (25089)	Isd start	No action required. For information only.

Resetting a Trip Event

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

MasterPacT MTZ1 IEC Circuit Breakers with MicroLogic Active Control Unit - User Guide
MasterPacT MTZ2/MTZ3 IEC Circuit Breakers with MicroLogic Active Control Unit - User Guide