7.4 Discrimination & Selectivity

Discrimination & Selectivity

Quick Answer: Discrimination (also called selectivity) means that for a fault on a downstream circuit, only the nearest protective device opens. Upstream breakers or fuses remain closed so the rest of the installation stays on. Achieved by coordinating ratings and time–current curves (and manufacturer selectivity tables). Backup protection that deliberately involves the upstream device can defeat selectivity.

Why Selectivity Matters on Real Jobs

Imagine a Queensland commercial board: main MCCB feeding several subcircuits, each with an MCB. A nail through a cable on one lighting circuit should trip that MCB. If the main MCCB also opens, every tenant loses power, refrigerated stock warms, and fault-finding becomes theatre. That outcome is a loss of discrimination.

AS/NZS 3000 and good practice encourage arrangements where protective devices are selective so that faults are cleared as close to the defect as practicable. The capstone unit uses this language in scenario questions: identify which device should operate, and why both operating is undesirable.

Current Discrimination

Current discrimination uses different continuous ratings and curve shapes so that, for fault currents in a given range, the downstream device’s operating characteristic lies to the left of (operates before) the upstream device.

Simple fuse example taught in trade courses: a minor downstream overload melts a small fuse while a much larger upstream fuse’s element barely heats. For high fault currents, both fuse I²t values must still be checked — at extreme levels, discrimination can fail if the upstream fuse’s pre-arcing energy is exceeded.

Breaker example: a 20 A Type C final-circuit MCB downstream of a 63 A MCCB with a suitably set short-time delay may discriminate for many fault levels if manufacturer tables say so. Equal ratings stacked without study often do not discriminate on heavy faults — both magnetic trips may race.

Time Discrimination

Time discrimination introduces intentional delay on the upstream device’s short-circuit response (common on MCCBs with adjustable short-time delay). The downstream MCB’s instantaneous trip clears first; the upstream device only opens if the fault persists (for example if the downstream device fails or the fault is on the bus between them).

Rules of thumb for assessment reasoning:

• Delay improves selectivity but increases the I²t the installation must withstand while waiting.
• Cable short-circuit withstand (Chapter 4) and device ratings must still survive that delay.
• Instantaneous trips on both devices with no delay and similar magnetic thresholds often collide on bolted faults.

Total Versus Partial Discrimination

Manufacturer literature and exam wording distinguish:

• Total discrimination (full selectivity): for all fault currents up to a stated maximum (often up to the downstream breaking capacity or a published limit), only the downstream device operates.
• Partial discrimination: selectivity holds only up to a certain fault current; above that level, both devices may operate.

A design that is “selective for overloads but not for bolted faults at the board” is partial. That may still be acceptable if documented and risk-assessed, but candidates must not claim total discrimination without evidence.

Discrimination Versus Backup Protection

Section 7.3 introduced backup / cascade protection: an upstream device helps the downstream device clear a fault beyond the downstream Icu. That help often means the upstream device also operates or shares energy limitation. Result:

• Breaking capacity problem may be solved.
• Selectivity for that high-level fault may be sacrificed.

Competent design states which goal was prioritised. Waving “the main fuse backs it up” without admitting the selectivity cost is incomplete thinking.

Series Device Themes on Final Circuits

Domestic and small commercial boards often have:

• Service protection / main switch arrangement upstream.
• Submain protection midstream (if any).
• Final-circuit MCBs or fuses downstream.

For a fault on a final circuit, ideal sequence: final-circuit device opens only. If the fault is on the submain bus, the submain device should open while the service protection remains closed if discrimination is achieved. Map the fault location to the device zone before answering which breaker “should” trip.

Reading Selectivity Questions Under Exam Pressure

1. Locate the fault (final circuit, submain, main tails).
2. Name the device that protects that zone.
3. Ask whether upstream devices are meant to stay closed.
4. If both tripped in a scenario stem, call it loss of discrimination and suggest causes: identical instantaneous settings, inadequate rating ratio, missing time delay, or cascade behaviour.

Common Exam Traps

• Believing two MCBs of the same In and same curve will always discriminate because one is “downstream.” On a bolted fault both may trip.
• Confusing RCD discrimination (S-type time-delayed RCDs upstream of general 30 mA devices) with overcurrent discrimination — related idea, different technology; do not mix residual-current and overcurrent answers blindly.
• Claiming total discrimination from instinct without curve or manufacturer support.
• Fixing nuisance main-breaker trips by upsizing the main In without checking whether downstream devices and Iz still coordinate — you may mask a selectivity or earth-fault problem.

Pulling Chapter 7 Together

A capstone-ready answer names the check you are performing. “Put in a bigger breaker” without walking this table is how candidates fail coordination items.

Closing Study Habits for Queensland Licence Work

• Sketch the board hierarchy on every scenario question.
• Write Ib, In, Iz beside every cable decision.
• State curve type when motors or transformers appear.
• Compare fault level to Icu before celebrating a neat CCC result.
• Say “selectivity” aloud when describing which device should trip.

Master those habits and Chapter 7 stops being a list of symbols — it becomes the way you defend a safe Australian installation under AS/NZS 3000.