Protective Relay Coordination Tutorial: From Fuse-MCCB-Relay Chain to a Full Coordination Chart

Protection coordination is one of those skills where the theory is simple and the practice is unforgiving. The principle is straightforward: when a fault occurs, only the protective device immediately upstream of the fault should operate. Every other device should remain closed, keeping supply to healthy circuits. In practice, achieving this across three or four levels of protection — from the final subcircuit fuse up to the main switchboard relay — requires systematic analysis that most engineers skip.

I have investigated dozens of unplanned outages at mine sites that traced back to protection mis-coordination. The 400A ACB trips instead of the 50A fuse. The entire process floor goes dark because nobody checked whether the devices could discriminate at the actual fault level. This tutorial walks through the coordination process for a simple three-level system, step by step, so you can build a coordination chart from scratch.

The System We Are Coordinating

Our system has three protection levels:

The prospective fault current at the sub-distribution board is 15 kA. At the final subcircuit, it is 8 kA. The system is 415 V, three-phase, 50 Hz.

The objective: for any fault at any point, only the nearest upstream device operates. The others remain closed.

What a Time-Current Curve (TCC) Shows

A TCC plot — also called a time-current coordination diagram — plots every protective device on a single chart:

• X-axis: Current in amperes, logarithmic scale (typically 1 A to 100 kA)
• Y-axis: Operating time in seconds, logarithmic scale (typically 0.01 s to 1000 s)

Each device appears as a curve (or band) showing how long it takes to operate at any given fault current. The key rule is:

For discrimination, the downstream device curve must lie entirely below and to the left of the upstream device curve across the full range of fault currents.

If the curves cross or overlap at any current, discrimination is lost at that current level. The wrong device trips, and a larger portion of the network is unnecessarily de-energised.

IEC 60947-2, Annex A — Coordination between circuit-breakers — selectivity

Step 1: Plot the Downstream Device (50 A HRC Fuse)

HRC (High Rupture Capacity) fuses have a fixed, non-adjustable characteristic. The manufacturer publishes the time-current curve, and that is what you get. This is actually an advantage for coordination — the fuse characteristic is predictable and repeatable.

A typical 50 A BS 88 HRC fuse has these approximate operating times:

The fuse curve drops steeply — at high fault currents, fuses are extremely fast. This current-limiting behaviour is one of their most valuable properties for coordination, because it means the downstream fuse clears before slower upstream devices can respond.

Step 2: Plot the Middle Device (250 A MCCB)

MCCBs have two distinct operating regions:

Thermal Region (Overload)

The thermal trip provides inverse-time protection for overloads. At 1.1 times rated current, it may take 3600 seconds to trip. At 2 times rated current, it trips in approximately 60-120 seconds. At 6 times rated current, it trips in approximately 2-5 seconds. This curve is usually adjustable via the thermal trip setting (Ir), typically adjustable from 0.7x to 1.0x of the MCCB's nominal rating.

Magnetic Region (Short Circuit)

The magnetic (instantaneous) trip operates for fault currents above a threshold, typically within 10-30 milliseconds. For a 250 A MCCB, the magnetic trip is usually adjustable from 5x to 10x the thermal setting (Im = 5-10 x Ir).

For our 250 A MCCB, we set:

• Ir = 250 A (thermal trip at 1.0x nominal)
• Im = 2500 A (magnetic trip at 10x Ir)

The magnetic trip at 10x gives us the widest possible gap between the fuse's operating range and the MCCB's instantaneous trip, maximising the discrimination range.

Step 3: Plot the Upstream Device (800 A Relay)

The overcurrent relay on the main switchboard provides the most flexibility. Numerical relays allow precise adjustment of multiple parameters:

Pick-Up Current (Is)

The pick-up current is the minimum current at which the relay starts timing. It is set as a multiple of the CT rated secondary current.

With an 800/1 CT, setting Is = 0.9 means the relay picks up at 0.9 x 800 = 720 A primary.

Time-Current Characteristic

IEC 60255-151 defines several standard inverse-time characteristics:

IEC 60255-151, Clause 5.5 — Standard inverse-time characteristics

Where I is the multiple of pick-up current and TMS is the time multiplier setting.

Selecting Settings for Coordination

For our relay to coordinate with the downstream 250 A MCCB, it must not operate before the MCCB at any fault level up to 15 kA. Let us select:

• Characteristic: Standard Inverse (SI)
• Pick-up (Is): 1.0 x 800 A = 800 A
• Time Multiplier Setting (TMS): To be determined by coordination

Instantaneous Setting (I>>)

The relay also has an instantaneous element for high-level faults. We set this above the maximum fault level that the downstream MCCB can see, typically:

I>> = 12 kA (above the 8 kA maximum at the sub-distribution board, with margin)

This ensures the instantaneous element only operates for faults between the relay and the MCCB, where the MCCB is not expected to clear the fault.

Step 4: Determine the Time Multiplier Setting (TMS)

This is the critical coordination step. The relay must have a grading margin above the MCCB at the maximum fault current where both devices see the fault.

Grading Margin Rules

IEC 60255-151, Clause 5.7 — Time grading considerations

The minimum grading margin between devices depends on their technology:

For our numerical relay coordinating with the MCCB, we use a 0.20 second grading margin.

Calculating TMS

At the maximum fault current at the sub-distribution board (15 kA), the MCCB operates in its magnetic region — approximately 0.02 seconds (20 ms). The relay must not operate faster than 0.02 + 0.20 = 0.22 seconds at this current.

The current as a multiple of relay pick-up: 15,000 / 800 = 18.75

Using the Standard Inverse formula:

For t = 0.22 s:

We would set TMS = 0.10 (rounding up to the nearest available setting for safety margin).

Let us verify: at 15 kA, the relay operating time is 0.10 x 2.317 = 0.23 seconds. The MCCB magnetic trip is 0.02 seconds. The margin is 0.23 - 0.02 = 0.21 seconds. This exceeds our 0.20 second requirement. Coordination is achieved.

Step 5: Check for Crossover Points

Plotting all three devices together, we check for any current at which curves cross.

Fuse vs MCCB

At low currents (50-250 A), only the fuse is in its operating range. The MCCB thermal element starts at approximately 275 A (1.1 x 250 A). Below this, only the fuse responds. Good.

At high currents (above 2500 A), the MCCB magnetic trip operates at approximately 0.02 seconds. The 50 A fuse at 2500 A operates in approximately 0.01 seconds. The fuse is faster. Good — the downstream device clears first.

Between 250-2500 A, both the fuse and MCCB thermal elements are active. The fuse at 250 A operates in approximately 0.8 seconds. The MCCB thermal at 250 A (1.0x rated) takes longer than 3600 seconds. Clear separation. As current increases, both curves drop, but the fuse curve drops faster. Discrimination is maintained.

MCCB vs Relay

At the MCCB's magnetic threshold (2500 A), the MCCB operates at 0.02 seconds. The relay at 2500 A (3.125 x pickup): t = 0.10 x 0.14 / (3.125^0.02 - 1) = 0.10 x 0.14 / 0.0230 = 0.61 seconds. The relay is much slower than the MCCB. Discrimination maintained.

At 15 kA (maximum fault), we already verified: MCCB at 0.02 s, relay at 0.23 s. Margin 0.21 s. Discrimination maintained.

Step 6: Document the Coordination Settings

The final coordination chart should be documented with all settings and the verified discrimination ranges:

Common Coordination Failures

1. Not Checking at Actual Fault Levels

Engineers coordinate devices at rated current and assume it holds everywhere. It often does not. The MCCB magnetic trip band is where most discrimination failures occur. Always check at the actual prospective fault current at each point in the network.

2. Ignoring Manufacturing Tolerances

A Type C MCB trips magnetically at 5-10 times rated current. That is a 2:1 range. A 50 A Type C MCB might trip magnetically at 250 A or 500 A. If the upstream MCCB's magnetic trip is set to 500 A, there is an overlap band where either device might trip first.

3. Relay Settings Never Updated After System Changes

A new transformer is installed, increasing fault levels. The relay settings that coordinated at 10 kA may not coordinate at 25 kA. Protection settings must be reviewed whenever the network topology or fault levels change.

IEC 60947-2, Annex A — Selectivity between circuit-breakers

4. Fuse Ageing

Fuses degrade over time, particularly if they have been subjected to repeated high-current events (motor starts, transformer inrush) that partially melt the element without blowing it. An aged fuse operates more slowly than its published curve suggests, which can improve or worsen coordination depending on its position in the chain.

The Advantage of Fuse-Breaker Coordination

One pattern I consistently recommend for industrial installations is a fuse at the final subcircuit with an MCCB or relay upstream. Fuses have inherently fast, current-limiting operation at high fault currents. Their I2t let-through is well-defined and repeatable. This makes them easy to coordinate with — the upstream device simply needs to be slower than the fuse's total clearing time at every fault level.

MCB-MCB or MCCB-MCCB combinations are harder to coordinate because both devices have similar response speeds in the magnetic region. At high fault currents, two circuit breakers of different ratings may operate at the same speed, making discrimination impossible without zone-selective interlocking or cascade (backup) coordination.

Protection coordination is a paper exercise that prevents real outages. The time invested in building and verifying a TCC chart for every distribution level pays for itself the first time a fault occurs and only the correct device operates. The rest of the plant keeps running.