Understanding and interpreting the Trip Curve MTZ Active MicroLogic 5.0 / 6.0

1. What does this curve represent?

This graph shows when the circuit breaker will trip based on:

• Current (X-axis) → How much current is flowing. It shows current as a multiple of:
• Ir (long-time setting) on left graph
• In (nominal current) on right graph
• Time (Y-axis) → How long the breaker takes to trip. It shows trip time in seconds (log scale)

Top = slow trip (seconds to hours)

Bottom = fast trip (milliseconds)

2. What are the different protection zones?

The curve has three main protection regions:

A. Long-Time Protection (Overload)

Purpose: Protects against overload conditions

Settings:

• Ir = 0.4 to 1 × In
• tr = 0.5 to 30 seconds (0.5 for bottom curve and 30 for top side curve)

Behaviour:

• Small overload → trips slowly
• Bigger overload → trips faster

B. Short-Time Protection

• Range, Isd: Medium faults (~1.5 to 10 × Ir)

Purpose:

• Handles short-circuit currents with delay
• Allows coordination with downstream breakers

Also, Short-Time Protection Can be:

• I²t ON → curve follows energy (I²t) as inverse time (Trip time decreases as current increases, i.e., faster tripping for higher current. Better for thermal protection and coordination
• I²t OFF → fixed delay regardless of fault level

C. Instantaneous Protection

• Range, Ii = 2 to 15 × In (optional OFF)

Purpose:

• Trips immediately for severe faults

Behaviour:

• Almost no intentional delay
• Protects against high short-circuit currents

3. Why are there multiple overlapping curves?

Each band represents:

• Tolerance of breaker operation
• Different adjustable settings

Real breaker trips within this band, not exactly one line.

4.Why the curve “breaks” in between transition between different protection functions

The curve is not a single continuous function — it is a combination of multiple protections:

“The curve is intentionally segmented because each part represents a different protection function (overload, short-time, instantaneous). The ‘breaks’ occur where control shifts from one protection logic to another, each having a different operating principle.”

5. How to use this curve practically?

In reference to the below graph,

Overload: If current = 5×Ir and tr set at 30s, refer yellow lines:

1. Move vertically from 5 on X-axis
2. Intersect long-time curve of tr=30s
3. Move left → read time

Result: Breaker trips in around 35-40 s

Short circuit: If current = 10×Ir and tr set at .8s with I²t ON, refer magenta lines:

1. Move vertically from 10 on X-axis
2. Intersect short-time curve tr=.8s
3. Move left → read time

Result: Breaker trips in around .6-.8s

Instantaneous: If current = 10×In, refer orange lines:

1. Move vertically from 10 on X-axis
2. Intersect inst. curve
3. Move left → read time

Result: Breaker trips in around .02-.05 s

Released for: Schneider Electric India