Electrical Distribution Protection

Presentation

MicroLogic 5, 6, and 7 trip units provide protection against overcurrents, ground-fault currents, and earth-leakage currents for commercial or industrial applications.

MicroLogic 5, 6, and 7 trip units offer protection characteristics that comply with the requirements of standard IEC/EN 60947-2. For more information, refer to DOCA0140EN ComPact NSX - Circuit Breakers and Switch-Disconnectors 100–630 A - User Guide.

Description

When choosing the MicroLogic trip unit to use, take account of:

• Overcurrents (overloads and short-circuits)

• Ground-fault current or earth-leakage current

• Conductors that need protection

• The presence of harmonic currents

• Coordination between the devices

Selectivity Between Devices

Coordination between the upstream and downstream devices, especially selectivity, is essential to optimize continuity of service. The large number of options for setting the protection functions on MicroLogic 5, 6, and 7 trip units improves the natural coordination between ComPact NSX circuit breakers. For more information, refer to LVPED217032EN ComPact NSX & NSXm Catalogue.

Three selectivity techniques can be used:

1. Current selectivity, which corresponds to staging of the long-time protection pickup

2. Time selectivity, which corresponds to staging of the short-time protection pickup

3. Energy selectivity, which corresponds to staging of the circuit breaker energy levels: this applies for very high intensity short-circuit currents.

Selectivity Rules

The selectivity rules depend on:

• The type of trip unit on the circuit breakers installed upstream and downstream: electronic or thermal-magnetic.

• The accuracy of the settings.

Selectivity of Overload Protection

For overload protection, the selectivity rules between electronic trip units are as follows:

1. Current selectivity:

   • A ratio of 1.3 between the Ir pickup for long-time protection of the trip unit on the upstream circuit breaker Q1 and that of the trip unit on the downstream circuit breaker Q2 is usually sufficient.

   • The tr time delay for long-time protection of the trip unit on the upstream circuit breaker Q1 is identical or higher than that of the trip unit on the downstream circuit breaker Q2.

2. Time selectivity:

   • A ratio of 1.5 between the Isd pickup for short-time protection of the trip unit on the upstream circuit breaker Q1 and that of the trip unit on the downstream circuit breaker Q2 is usually sufficient.

   • The tsd time delay for short-time protection of the trip unit on the upstream circuit breaker Q1 is higher than that of the trip unit on the downstream circuit breaker Q2.

   • If the upstream circuit breaker is in position I²t OFF, the downstream circuit breakers must not be in position I²t ON.

3. Energy selectivity:

   • Energy selectivity is provided by the circuit breaker design and build characteristics. The selectivity limit can only be guaranteed by the manufacturer.

   • For circuit breakers in the ComPact NSX circuit breakers range, a ratio of 2.5 between the upstream circuit breaker Q1 and that of the downstream circuit breaker Q2 guarantees total selectivity.

Ground-Fault Protection Selectivity

For ground-fault protection, only the rules for time selectivity should be applied to the protection Ig pickup and tg time delay:

• A ratio of 1.3 between the Ig pickup for ground-fault protection of the trip unit on the upstream circuit breaker Q1 and that of the trip unit on the downstream circuit breaker Q2 is usually sufficient.

• The tg time delay for ground-fault protection of the trip unit on the upstream circuit breaker Q1 is higher than that of the trip unit on the downstream circuit breaker Q2.

• If the upstream circuit breaker is in position I²t OFF, the downstream circuit breakers must not be in position I²t ON.

Earth-Leakage Protection Selectivity

For earth-leakage protection, only the rules for time selectivity should be applied to the protection IΔn pickup and Δt time delay:

• A ratio of 3 between the IΔn pickup for earth-leakage protection of the trip unit on the upstream circuit breaker Q1 and that of the trip unit on the downstream circuit breaker Q2 should be applied.

• The Δt time delay for earth-leakage protection of the trip unit on the upstream circuit breaker Q1 is higher than that of the trip unit on the downstream circuit breaker Q2.

Selectivity Limit

Depending on the staging of circuit breaker ratings and protection function settings, selectivity can be:

• Limited (partial selectivity) up to a value Is of the short-circuit current

• Total (total selectivity), performed irrespective of the value of the short-circuit current

Selectivity Table

Schneider Electric provides selectivity tables showing the type of selectivity (partial or total) between each circuit breaker for its entire range of circuit breakers. These selectivity tables are tested in accordance with the recommendations of standard IEC/EN 60947-2. For more information, refer to LVPED217032EN ComPact NSX & NSXm Catalogue.

MicroLogic 5 Trip Unit

MicroLogic 5 trip units provide the following protection functions: Long-time overcurrent protection (Ir, tr) Short-time overcurrent protection (Isd, tsd) Instantaneous overcurrent protection (Ii)

MicroLogic 6 Trip Unit

MicroLogic 6 trip units provide the following protection functions: Long-time overcurrent protection (Ir, tr) Short-time overcurrent protection (Isd, tsd) Instantaneous overcurrent protection (Ii) Ground-fault protection (Ig, tg)

MicroLogic 7 Trip Unit with Integrated Earth-Leakage Protection

MicroLogic 7 trip units provide the following protection functions: Long-time overcurrent protection (Ir, tr) Short-time overcurrent protection (Isd, tsd) Instantaneous overcurrent protection (Ii) Earth-leakage protection (IΔn, Δt)

Setting the Protection

Set the protection functions:

• On the MicroLogic trip unit, by using the adjustment dials and the keypad (depending on the protection function and the MicroLogic type)

• With EcoStruxure Power Commission software (password-protected)

• By sending a setting command using the communication network (password-protected)

Integrated Instantaneous Protection

In addition to the adjustable instantaneous protection, MicroLogic trip units for electrical distribution protection feature a SELLIM non-adjustable integrated instantaneous protection which can improve selectivity.

Reflex Tripping

In addition to the protection functions integrated in the MicroLogic trip units, ComPact NSX circuit breakers have reflex protection. This system breaks very high fault currents by mechanically tripping the device with a “piston” actuated directly by the pressure produced in the circuit breaker from a short-circuit. This piston operates the opening mechanism, resulting in ultra-fast circuit breaker tripping.