Understanding Electrical Selectivity
- MTS DNC ENERGY CONSULTANTS LIMITED
- Dec 30, 2025
- 3 min read
🎛️ Why Only the Right Breaker Should Trip
In any electrical installation, faults are unavoidable. What matters is how the system reacts when a fault occurs. This is where selectivity becomes critical.
In this post, we explain what selectivity is, why it matters, and how it is achieved in a simple, practical way.
🎛️ What Is Selectivity?
Selectivity (also known as discrimination) means that only the protective device closest to a fault operates, while all upstream devices remain closed.
In simple terms:
A socket fault should trip the socket circuit — not shut down the entire building.
🎛️ Why Selectivity Is Important
Good selectivity ensures:
Minimal disruption during faults
Improved safety
Easier fault finding
Better continuity of supply
Protection of critical systems
Without selectivity, a minor fault can result in unnecessary power loss to large areas of a building.
🎛️ A Simple Example
Imagine a building with:
A main incomer breaker
A distribution board breaker
A final circuit breaker
If a fault occurs on a socket circuit:
✅ The socket circuit breaker should trip
❌ The distribution board should stay on
❌ The main incomer should stay on
That is selective protection.
🎛️ How Selectivity Is Achieved
Selectivity is achieved by coordinating protective devices so they do not trip at the same time.
This coordination is based on current, time, or energy.
🎛️ Current Selectivity
Downstream devices are rated to trip at lower fault currents than upstream devices.
Smaller breakers trip first
Larger breakers remain closed
This works well for overload protection.
🎛️ Time Selectivity
Upstream devices are intentionally delayed, allowing downstream devices to clear the fault first.
Downstream breaker trips instantly
Upstream breaker waits briefly
This method is common with MCCBs and main incomers.
🎛️ Energy (I²t) Selectivity
Some breakers limit the energy they allow through during a fault.
The downstream device clears the fault quickly
The upstream device does not “see” enough energy to trip
This is often achieved using manufacturer-tested breaker combinations.
🎛️ RCD Selectivity (Very Important)
When multiple RCDs are installed:
The RCD closest to the fault should trip
Upstream RCDs should remain closed
This is typically achieved by:
Using time-delayed (Type S) RCDs upstream
Using higher residual current ratings upstream
Without RCD selectivity, a single earth fault can trip multiple boards at once.
🎛️ Total vs Partial Selectivity
✅ Total Selectivity
The downstream device trips for all fault currents, and the upstream device never trips.
⚠ Partial Selectivity
Selectivity exists only up to a certain fault level. Beyond that, more than one device may trip.
Partial selectivity is common and acceptable in many installations when justified by design.
🎛️ Selectivity and IS 10101
IS 10101 requires that:
Protective devices are properly coordinated
Faults are cleared as close to the source as practicable
Unnecessary loss of supply is avoided
This is especially important in:
Commercial buildings
Healthcare facilities
Cleanrooms
Data centres
Industrial installations
🎛️ Final Thoughts
Selectivity is not about adding more protection — it’s about using protection intelligently.
A well-designed system ensures:
Faults are contained
Critical areas stay powered
The installation operates safely and reliably
Disclaimer: The content shared in these posts is intended for informational purposes only and should not be interpreted as design advice, specifications, or a calculation template. For professional guidance or design services, please contact us through our contact form.
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