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Discrimination / Selectivity Checker

Check whether upstream and downstream protective devices will discriminate correctly

Reference Info & Formulas
What is Discrimination?

Only the device nearest the fault should trip, leaving all other circuits live. This is known as discrimination or selectivity.

Without it, a fault on one circuit can take out the entire board or a large group of circuits.

BS 7671 Regulation 536.4

Quick Ratios

MCB/MCB (same type): 1.6:1 thermal only

BS 88 Fuse/MCB: 2:1 usually works

BS 1361 Fuse/MCB: 1.6:1

BS 88/BS 88: 2:1 full discrimination

These are simplified rules of thumb. For critical installations, verify with manufacturer time-current curves.

Upstream Device
The device furthest from the fault (e.g. main switch or sub-main)
Downstream Device
The device nearest the fault (e.g. final circuit MCB)
Fault Level
Prospective fault current at the downstream device location
kA

Typically 1-6 kA for domestic, up to 16 kA commercial

Data: BS 7671:2018+A2:2022 Regulation 536.4, BS EN 60898-1

For guidance only. The responsibility for any electrical installation lies with the qualified person carrying out the work. Always verify calculations independently and apply professional judgement.

How This Calculator Works

The calculator assesses whether two protective devices will discriminate correctly based on their type, rating, and the prospective fault current.

Discrimination depends on the time-current characteristics of both devices. For two devices to discriminate, the downstream device must always operate faster than the upstream device at every possible fault current up to the prospective fault level. The calculator checks rating ratios, instantaneous trip overlaps, and energy let-through relationships.

Ratio = Upstream Rating / Downstream Rating
Upstream Rating
= Rated current of the device furthest from the fault (A)
Downstream Rating
= Rated current of the device nearest the fault (A)

BS 7671 Regulation 536.4

For MCBs of the same type, the instantaneous trip ranges overlap when the fault current exceeds the downstream device's magnetic trip threshold. Above this point, both devices trip in under 10ms and discrimination cannot be guaranteed regardless of the rating ratio.

Quick Reference -- Discrimination Ratios

Minimum rating ratios for discrimination

BS 7671 Appendix 3, manufacturer data
Device CombinationMinimum RatioDiscrimination Range
MCB / MCB (same type)1.6:1Thermal region only (below instantaneous trip)
Type C upstream / Type B downstream1:1 (C > B rating)Up to Type C instantaneous trip
Type D upstream / Type B or C downstream1:1 (D > B/C rating)Up to Type D instantaneous trip
BS 88 Fuse / MCB2:1Full range (fuse I²t exceeds MCB)
BS 1361 Fuse / MCB1.6:1Full range
BS 88 Fuse / BS 88 Fuse2:1Full range (pre-arc vs total I²t)
BS 3036 Fuse / MCB2:1Full range (verify with curves)

These are simplified rules of thumb. For critical applications, always verify with the actual time-current curves and I\u00B2t characteristics from the device manufacturer.

Practical Notes

MCB Instantaneous Trip Overlap

When two MCBs of the same type have overlapping instantaneous trip ranges, discrimination cannot be achieved regardless of the rating ratio. For example, a 63A Type B MCB trips instantaneously at 189-315A, while a 32A Type B trips at 96-160A. At fault currents above 189A, both devices trip within milliseconds and the outcome is random. A fuse upstream eliminates this problem.

Use Fuses for Reliable Discrimination

BS 88 (HRC) fuses upstream of MCBs provide the most reliable discrimination. The fuse's continuously variable time-current characteristic means the larger fuse is always slower than the MCB at any fault current. This is why main switches in many commercial boards use fused isolators rather than MCBs.

Verify With Manufacturer Data

This calculator uses simplified ratio-based rules. For safety-critical circuits (fire alarms, emergency lighting, medical equipment), always obtain the actual time-current curves and I²t data from the device manufacturer and verify discrimination at the specific prospective fault current of the installation.

Frequently Asked Questions

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