Circuit Breaker Selectivity, Discrimination & Cascading

Selectivity (also called discrimination) means that when a fault occurs, only the breaker immediately upstream of the fault opens, and every breaker above it stays closed. Cascading (also called back-up protection) is a different idea altogether: it uses the current-limiting action of an upstream breaker so that a specifically tested downstream breaker can be applied where the prospective fault current exceeds its own standalone breaking capacity.

The consequence for design is that neither can be established from ampere ratings or kA figures on a bill of materials. Selectivity has to be proved against a stated fault current — "selective up to 15 kA" is a completely different claim from "selective" — and cascading is only valid for the exact upstream/downstream device combination published by the manufacturer. Get either wrong and a 20 A branch fault can take out a 3200 A incomer.

The Situation Every Plant Recognises

A short circuit occurs on one small outgoing feeder.

What should happen?

Ideally:

Only that feeder trips.

What should NOT happen?

Outgoing MCCB trips

plus:

Main incomer ACB trips

plus:

Complete factory shuts down.

The ability of the protection system to isolate only the part of the installation affected by the fault is called:

Selectivity

or:

Discrimination

But selectivity is only one part of circuit-breaker coordination.

Another concept is:

Cascading / Back-Up Protection

Cascading uses the current-limiting performance of an upstream breaker to allow an appropriately tested downstream breaker combination to withstand fault conditions beyond the downstream breaker's standalone breaking capacity.

These two concepts solve different problems.

Quick Difference

Concept	Primary Purpose
Selectivity / Discrimination	Keep healthy sections energized
Cascading / Back-Up	Use upstream protection to support downstream breaking capability
Coordination	Ensure devices work together correctly
Protection Grading	Arrange settings/time-current characteristics
Current Limitation	Reduce peak current and let-through energy

A good LV protection design may use several of these simultaneously.

Selectivity, cascading and coordination are normally addressed together for ACB, MCCB and MCB systems, because in a real switchboard they are decided together.

What Is Circuit Breaker Selectivity?

Consider:

Main ACB

↓

Distribution MCCB

↓

Final MCCB

↓

Load

A fault occurs downstream of the final MCCB.

The desired result is:

Final MCCB trips

while:

distribution MCCB remains closed
main ACB remains closed

That is selective protection.

The purpose is:

maximum continuity of supply with minimum fault isolation area.

Why Selectivity Matters

Selectivity is especially valuable in:

continuous-process plants
data centers
hospitals
infrastructure
utilities
manufacturing
critical pumping systems

If a 20 A branch fault causes a:

3200 A main incomer

to trip, protection may technically have cleared the fault—but system availability is poor.

Selectivity vs Discrimination

In LV protection terminology:

Selectivity

and:

Discrimination

are often used to describe substantially the same coordination objective:

the protective device closest to the fault should operate without unnecessarily tripping upstream devices.

Therefore this blog uses the terms interchangeably where appropriate.

Total Selectivity

Suppose downstream prospective fault current can reach:

30 kA

and manufacturer coordination data confirms the upstream/downstream combination is selective for faults throughout that entire range.

This can be considered:

total selectivity

for the relevant application/current range.

Partial Selectivity

Suppose the breaker combination is selective only up to:

15 kA

but the prospective fault current is:

30 kA

For faults below 15 kA:

the downstream breaker operates selectively.

For larger faults:

both devices may operate depending on their characteristics.

This is:

partial selectivity.

Therefore never accept:

“These breakers are selective.”

Ask:

Selective up to what fault current?

Why Breaker Ampere Rating Alone Does Not Prove Selectivity

Suppose:

Main MCCB = 630 A

Outgoing MCCB = 250 A

It seems logical that the smaller breaker should trip first.

But during a high short circuit, both breakers may enter their instantaneous operating zones.

The upstream breaker may trip at nearly the same time.

Therefore selectivity depends on:

breaker characteristics
trip-unit settings
fault current
device combination
current limiting
time delay

not simply:

630 A > 250 A

Time-Current Selectivity

One method is to separate trip curves.

For overload and lower fault-current regions:

Downstream device operates faster

while:

Upstream device has a higher setting/time delay

This is traditional time-current grading.

The protection study should ensure:

enough grading margin
equipment withstand is not exceeded
fault clearing remains sufficiently fast

Current Selectivity

Breakers may also use different pickup thresholds.

For example:

Downstream breaker instantaneous pickup:

lower

Upstream breaker:

higher

This can provide selective operation over a defined fault-current range.

But once fault current becomes high enough to exceed both instantaneous thresholds, selectivity may be lost unless additional device behavior supports it.

Energy Selectivity

Modern current-limiting breakers can coordinate based on:

current limiting
let-through energy
dynamic interaction

This can extend selectivity beyond what simple time-current curves appear to show.

manufacturer selectivity tables can account for energy-selectivity behavior in coordinated breaker combinations.

This is why:

TCC curves alone are not always enough to determine high-current selectivity between modern current-limiting breakers.

Manufacturer coordination tables matter.

Zone Selective Interlocking

In larger ACB systems, another approach is:

Zone Selective Interlocking — ZSI

Conceptually:

Downstream breaker detects fault.
It signals upstream breaker.
Upstream breaker maintains intentional delay.
Downstream breaker clears fault.

If upstream sees a fault but receives no restraint signal from downstream:

it can trip faster.

This approach attempts to combine:

selectivity

with:

faster fault clearance.

Availability depends on the selected breaker/trip-unit system.

Why Short-Time Withstand Icw Matters

Suppose main ACB is intentionally delayed:

0.3 seconds

to allow a downstream breaker to trip.

During that period, the ACB and associated bus system continue carrying fault current.

Therefore:

Icw — short-time withstand current

becomes important.

The desire for selectivity must be coordinated with:

ACB Icw
busbar withstand
cable withstand
transformer/equipment withstand

Do not keep increasing upstream delay just to improve discrimination.

A worked feel for the numbers helps. On a 1000 kVA, 433 V transformer at approximately 5% impedance, the transformer-limited fault current works out to roughly 27 kA before source and cable impedance are added. If the incomer ACB is given a 0.3 s short-time delay to grade with a downstream MCCB, the main busbar and the ACB must both hold that current for 0.3 s — so an ACB with Icw of 50 kA for 1 s is comfortable, while one rated Icw = 36 kA for 0.5 s needs checking against the actual delay and the busbar withstand. Our recommendation: fix the grading delays only after the busbar Icw of the assembly is known, not before. The exact fault current and withstand figures must come from the project short-circuit study and the assembly's verified ratings.

What Is Cascading?

Cascading is fundamentally different.

Consider:

Prospective fault current at a downstream board:

35 kA

Downstream breaker standalone Icu:

25 kA

Normally:

25 kA breaker appears inadequate.

But an upstream current-limiting breaker may dramatically limit:

peak current
let-through energy

during the fault.

A manufacturer-tested combination may then permit that particular downstream breaker to be used in that system.

Cascading Is Not a Calculation to Guess

You must not say:

“Main breaker is 65 kA, therefore all outgoing 25 kA breakers are safe.”

That is incorrect.

Cascading requires:

specific manufacturer coordination data

for:

upstream device
downstream device
voltage
configuration

Use the manufacturer's published:

cascading tables
back-up tables
series-rating tables

for the exact combination.

Why Does Cascading Work?

A strong current-limiting upstream breaker begins restricting the fault before the full prospective short-circuit current develops downstream.

As a result, the downstream breaker experiences lower:

peak current
thermal energy

than it would experience if installed by itself.

But this performance depends on the interaction of the actual devices.

Hence:

tested/published combination required.

Selectivity and Cascading Can Appear Opposite

Selectivity wants:

downstream breaker only trips.

Cascading may depend on the upstream breaker participating in limiting the fault.

That sounds contradictory.

And traditionally, cascading can reduce selectivity.

However, modern coordinated device combinations can sometimes provide both enhanced cascading and significant selectivity.

Again:

Use manufacturer tables—not assumptions.

Example 1 — No Selectivity

Main MCCB:

400 A

Outgoing MCCB:

160 A

Fault:

20 kA

Both breakers have instantaneous operating regions active at that fault.

Result:

Both may trip.

Factory loses complete section.

Protection cleared fault—but selectively poor.

Example 2 — Selective Coordination

Main breaker:

ACB with adjustable short-time delay

Outgoing breaker:

MCCB

Protection settings and manufacturer data confirm outgoing breaker clears the fault while the main ACB remains closed.

Result:

Only faulty feeder disconnects.

Example 3 — Cascading

Available fault level:

50 kA

Downstream breaker standalone rating:

25 kA

Upstream current-limiting device plus exact downstream breaker combination has a manufacturer-published back-up rating suitable for:

50 kA

That combination may be used according to the published data and applicable project requirements.

But replacing the downstream breaker with another brand/model invalidates the assumption unless separately verified.

Why Mixing Breaker Brands Needs Care

Suppose:

Upstream = Brand A

Downstream = Brand B

Can they be selective?

Possibly.

But the strongest evidence often comes from manufacturer coordination studies/tables for their tested systems.

Do not invent selectivity by visually comparing two catalog curves if the manufacturer does not support the combination—especially in the high short-circuit region.

Selectivity Between ACB and MCCB

This is a common industrial arrangement:

Main ACB

↓

Outgoing MCCB

The ACB's adjustable:

long-time
short-time
instantaneous
ground-fault

functions can provide strong coordination flexibility.

However, settings must still consider:

downstream MCCB characteristics
cable
load
fault current
ACB Icw
bus withstand

Selectivity Between MCCBs

MCCB-MCCB coordination can be more challenging because both may have fast instantaneous action at high currents.

Manufacturer selectivity tables are particularly useful here.

Selectivity Between MCCB and MCB

Typical:

Distribution MCCB

↓

Final MCB

At lower overload/fault currents, curve separation may provide good discrimination.

At high fault current, actual selectivity should again be checked using manufacturer data.

Do not assume:

MCB always trips before MCCB.

Motor Protection Coordination

Motor feeders may contain:

MCCB/MPCB
contactor
overload relay
motor

Now coordination involves more than breaker selectivity.

The system should also ensure the contactor/overload combination behaves appropriately under short-circuit conditions.

The breaker manufacturer and motor-control manufacturer commonly publish tested coordination data.

Protection Selectivity vs Process Selectivity

Sometimes electrical selectivity alone is not enough.

Suppose a large motor trips.

Should process interlocks also stop upstream equipment?

Maybe.

Protection system objective:

isolate electrical fault only.

Process-control objective may intentionally stop additional equipment for safety/process reasons.

Do not confuse:

process shutdown logic

with:

electrical breaker discrimination.

Ground-Fault Selectivity

Earth-fault protection also needs grading.

Possible layers:

final feeder
distribution board
main incomer

If every ground-fault element has the same:

pickup
time delay

a small downstream earth fault may trip the complete switchboard.

Settings should be coordinated with:

earthing arrangement
minimum/maximum earth-fault current
personnel/equipment safety
required disconnection time

In our FAT experience this is the layer most often left ungraded. On MCC and PMCC boards for water-treatment and chemical plants we regularly see earth-fault pickup left at the same percentage and the same time delay on the incomer, the bus section and the outgoing feeders — three levels, one setting. A damp cable gland on one 15 kW dosing pump then trips the whole board. Grading earth fault costs nothing at the settings stage and is close to impossible to retrofit once the plant is running. See panel earthing and earth busbar design for the earthing side of the same decision.

Why Fault-Level Study Comes First

You cannot properly evaluate selectivity without knowing fault current.

At each bus determine:

maximum short-circuit current
minimum fault current where required

Maximum current influences:

breaking capacity
high-current selectivity

Minimum current influences:

whether protective devices will reliably detect/clear the fault

Therefore:

short-circuit study + protection study belong together.

Breaker Breaking Capacity Still Matters

Circuit breakers must be suitable for their prospective fault conditions unless an applicable back-up/cascading arrangement is deliberately used.

The current international circuit-breaker standard is:

IEC 60947-2:2024 for breakers within its scope.

For India, BIS currently lists:

IS/IEC 60947 Part 2:2016, reviewed in 2022.

The project should define which edition is contractually applicable.

Selectivity Is Not the Same as Breaking Capacity

Breaker A:

Icu = 65 kA

Breaker B:

Icu = 50 kA

That tells you their breaking-capacity characteristics.

It does NOT tell you whether:

A and B are selective together.

Breaking capacity asks:

Can the breaker interrupt the fault?

Selectivity asks:

Which breaker will interrupt the fault?

Different questions.

Cascading Is Not the Same as Increasing Breaker Icu

If a 25 kA breaker is used in a valid 50 kA cascading combination, the breaker itself has not magically become a standalone:

50 kA breaker.

The combination is suitable according to the published back-up data.

If you later remove/change the upstream device:

the downstream application must be reassessed.

Why Procurement Often Gets This Wrong

Quotation A:

all downstream MCCB = 50 kA

Quotation B:

selected downstream MCCB = 25 kA
manufacturer cascading used

Quotation B may appear technically weaker.

But if cascading has been correctly engineered, it may be a valid optimized solution.

Conversely, quotation B is unacceptable if the supplier simply reduced kA ratings without documented back-up coordination.

The buyer should ask for evidence.

This is the item we push back on at clarification stage, because the difference between an engineered cascading scheme and a cost-cut is a single document. If a bidder offers 25 kA outgoing breakers on a board with a 50 kA prospective fault, the correct response is to ask for the published back-up table page showing that upstream model against that downstream model at 415 V. If the page exists, the offer is sound and often cheaper. If the answer is "the main breaker will take care of it", the offer is not comparable at all — and in our experience that answer arrives more often than the table does.

What Should a Consultant Specify?

Instead of:

“All breakers shall be fully discriminated.”

state:

required selectivity level
prospective short-circuit current
affected circuit hierarchy
total/partial selectivity acceptance
protection study requirement
cascading allowed/not allowed
manufacturer coordination evidence

For critical systems, clearly define where total selectivity is required.

Practical Protection Study Workflow

Step 1 — Build SLD

Identify:

sources
transformers
generators
bus couplers
breakers
cables
major loads

Step 2 — Calculate Fault Levels

At each major bus.

Step 3 — Check Breaking Capacity

Icu/Ics as required.

Step 4 — Select Protection Settings

long time
short time
instantaneous
earth fault

Step 5 — Plot TCCs

Check overload and time-current grading.

Step 6 — Check Manufacturer Selectivity Tables

Especially for high fault-current behavior.

Step 7 — Check Cascading Tables

If back-up protection is being used.

Step 8 — Check Equipment Withstand

Icw
busbar
cables
transformers

Step 9 — Document Final Settings

Do not leave settings at factory default.

Common Selectivity Mistakes

Mistake 1: Bigger Breaker Means Selectivity

Ampere rating alone does not prove discrimination.

Mistake 2: Checking Only TCC Curves

Modern current-limiting breaker interaction may require manufacturer tables.

Mistake 3: Assuming All Faults Are the Same

Fault current changes throughout the system.

Mistake 4: Increasing Upstream Delay Without Checking Icw

Equipment must withstand the fault during the delay.

Mistake 5: Using Cascading Without Exact Manufacturer Combination

Never guess back-up capability.

Mistake 6: Treating Cascaded Breaker as Having Higher Standalone Icu

The coordinated combination provides the benefit.

Mistake 7: Ignoring Ground-Fault Selectivity

Earth-fault protection needs grading too.

Mistake 8: Changing Breaker Brand After Study

Coordination must be rechecked.

Mistake 9: Leaving Trip Settings at Default

Protection settings should follow the approved study.

We have opened panels two years into operation where every electronic trip unit was still on its factory default — long-time at maximum, short-time delay at minimum, instantaneous enabled. The protection study existed, approved and filed. Nobody had transferred the numbers into the trip units at commissioning. That plant had a genuine selectivity design and no selectivity in practice.

Diagnosing a Selectivity Failure in the Field

Symptom	Likely cause	Action
Downstream MCCB and upstream ACB both trip on a feeder fault	Both devices in their instantaneous zone at that fault current	Check high-current selectivity limit in the manufacturer table; consider short-time delay on the upstream device or ZSI
Only the upstream incomer trips, downstream breaker stays closed	Downstream pickup set too high, or wrong CT/sensor rating fitted	Verify sensor rating against the drawing and reset pickup per the approved study
Small earth fault on one feeder trips the whole board	Identical earth-fault pickup and delay at every level	Grade earth-fault pickup and time delay downward from incomer to feeder
Selectivity works on test but fails on a real fault	Study done at maximum fault current only, or settings never transferred to the trip unit	Recheck at both maximum and minimum fault current and record actual as-set values
Nuisance trip of a healthy feeder during motor starting	Short-time or instantaneous pickup below the motor inrush	Recheck pickup against locked-rotor and inrush from the motor data sheet
Coordination stopped working after a breaker replacement	Different model or trip-unit family fitted than the one studied	Reissue the coordination check for the device actually installed

FAT and Protection Verification

Panel FAT should verify:

correct breaker model
trip-unit type
CT/sensor rating
protection settings
breaker communication
trip operation
interlocks

But FAT does not recreate every system fault condition.

The protection study/documentation should provide the engineering basis.

During commissioning:

final settings
field configuration
communication
trip signals

should be checked against the approved study.

How Wisdom Techno Solutions Approaches Protection Coordination

For a technically optimized PCC/MCC/PMCC, breaker selection should not be performed only by:

Ampere + kA + price.

Relevant engineering inputs include:

calculated fault level
breaker hierarchy
selectivity requirement
Icu/Ics/Icw
cable ratings
motor/load characteristics
customer operating philosophy

Depending on project scope, we coordinate panel breaker selection and settings with:

approved protection studies
manufacturer selectivity tables
manufacturer cascading tables
client specifications

Wisdom Techno Solutions is a licensed partner for three design-verified switchgear platforms: Rittal Ri4Power (Rittal Solutions Partner since 2021 — the Ri4Power system carries Intertek ASTA design verification to IEC 61439-1 and -2 Edition 3, with copper and aluminium busbar systems up to 6300 A), C&S CX (Licensed Partner since 2023, up to Form 4B Type II) and Siemens SIEPAN Elite/8PU (IEC 61439 Solutions Partner since 2024). We build to each system's verified configuration and routine-verify every assembly in-house at our Vadodara, Gujarat facility, for industrial, EPC and infrastructure projects across India. That places the assembly-level withstand behind the coordination study inside a verified design envelope.

The objective should be:

Clear the smallest possible part of the electrical system when a fault occurs—without exceeding the capability of any protective device or assembly.

Conclusion

Three terms should not be confused.

Selectivity

Ensures the breaker nearest the fault operates while healthy upstream sections remain energized.

Cascading

Uses the current-limiting ability of an upstream device to support a specifically coordinated downstream breaker with lower standalone breaking capacity.

Coordination

Ensures all protective devices work together correctly across:

overload
short circuit
ground fault
equipment withstand

The best breaker is not simply:

the breaker with the highest kA rating.

A well-engineered protection system asks:

What is the fault current?
Which breaker should trip?
How quickly?
Which breaker must remain closed?
Can upstream equipment withstand the delay?
Is cascading being used?
Is the combination manufacturer-verified?

That is how protection engineering improves both:

safety

and:

availability.

Planning a PCC, MCC or PMCC with multiple protection levels?

Share the SLD, transformer/source data, fault levels and required selectivity philosophy with Wisdom Techno Solutions for project-specific panel and breaker coordination.

Related Guides

Frequently Asked Questions

What is circuit breaker selectivity?

Circuit breaker selectivity is coordination arranged so that the protective device immediately upstream of a fault operates, while every device above it stays closed. It is achieved through a combination of time grading, different pickup thresholds, current-limiting behaviour and — on larger ACB systems — zone selective interlocking. The purpose is availability: isolate the smallest possible part of the installation rather than shutting down a whole switchboard for a fault on one 20 A feeder. Selectivity is always stated against a fault current, because a combination that is selective at 8 kA may not be selective at 30 kA.

What is discrimination?

Discrimination is another term for the same objective as selectivity — the device nearest the fault clears it without unnecessarily tripping devices upstream. British and Indian practice tends to use "discrimination", while IEC-aligned documents and manufacturer literature increasingly use "selectivity". The two words are interchangeable in LV switchboard work, so a specification that demands "full discrimination" and one that demands "total selectivity" are asking for the same thing. Neither phrase means anything, however, until the fault current up to which it must hold is stated.

What is total selectivity?

Total selectivity means the upstream and downstream device combination remains selective for every fault current up to the prospective short-circuit current at that point in the system. For example, if the prospective fault at a distribution board is 30 kA and the manufacturer's coordination data confirms selectivity across that whole range, the pair can be described as totally selective for that application. It is a claim about a device combination at a stated voltage and fault level — not a property of a single breaker. Critical circuits such as data centre distribution, hospital supplies and continuous-process plants are where total selectivity is normally worth specifying.

What is partial selectivity?

Partial selectivity means the combination is selective only up to a defined current, below the prospective fault current at that location. If a pair is selective to 15 kA but the available fault is 30 kA, faults under 15 kA will be cleared by the downstream device alone, and larger faults may trip both devices. That is acceptable on non-critical feeders, since severe faults are rarer and the fault is still cleared safely. It is not acceptable where a wider trip would stop a batch process or a safety-related load, so the RFQ should say which feeders may be partially selective and which may not.

What is cascading?

Cascading, also called back-up protection or series rating, uses the current-limiting action of an upstream breaker to allow a downstream breaker to be applied where the prospective fault current exceeds its own standalone breaking capacity. The upstream device starts restricting the fault before the full prospective current develops, so the downstream device sees a lower peak current and lower let-through energy than it would if installed alone. This only works for combinations the manufacturer has actually tested and published, because the benefit comes from how those two specific devices interact. Cascading does not raise the downstream breaker's own Icu — it makes the pair suitable, not the part.

Can a 25 kA breaker be used where fault current is 50 kA?

Yes, but only if the manufacturer publishes a back-up or cascading rating for that exact upstream and downstream combination at the system voltage, or another engineered method (such as a current-limiting fuse arrangement) is applied and documented. What is never acceptable is reasoning from ratings alone — "the incomer is 65 kA, so the 25 kA outgoing breakers are covered" is not engineering. If the upstream device is later changed, or a different downstream model is fitted during maintenance, the back-up basis is lost and the application must be reassessed. Keep the relevant table page in the panel documentation so the next engineer can see why a 25 kA device is there.

Does a higher ampere breaker guarantee selectivity?

No. A 630 A upstream MCCB and a 250 A downstream MCCB look like an obvious hierarchy, but at a high short circuit both devices can enter their instantaneous zones and open at almost the same instant. Selectivity depends on trip characteristics, pickup and delay settings, current-limiting behaviour and the tested device combination — not on the ratio of ampere ratings. This is the single most common wrong assumption we see in breaker schedules.

Is selectivity visible from TCC curves alone?

Not always. High-current/energy behavior of modern breakers may require manufacturer selectivity tables.

Why is Icw important for selectivity?

Because time-based selectivity works by deliberately holding the upstream breaker closed for a short interval — commonly 0.1 to 0.4 s — while the downstream device clears the fault, and during that interval the upstream breaker, the busbar and the connections all carry the full fault current. Icw, the rated short-time withstand current, is the figure that says whether they can do so without damage; it is always quoted with a duration, such as 50 kA for 1 s. If the grading delay is increased without checking Icw and the assembly's busbar withstand, the protection scheme becomes selective on paper and destructive in practice. The applicable Icw must come from the assembly's verified ratings, not from the breaker catalogue alone.

Which standard applies to industrial LV circuit breakers?

IEC 60947-2 is the product standard for low-voltage circuit breakers, and it is the source of the Icu, Ics and Icw definitions used throughout coordination work. It governs the device only — the switchboard the breakers are mounted in is covered by IEC 61439-1 and IEC 61439-2, which is why a breaker's Icu is not the assembly's fault rating. The project specification should state which edition is contractually applicable, since editions differ in detail.

What Indian standard applies?

BIS currently lists IS/IEC 60947 Part 2:2016, reviewed in 2022, as the Indian adoption for low-voltage circuit breakers. For Indian tenders it is usually cleanest to cite the IS/IEC designation, because that is what inspection agencies and document reviewers look for. Where a client's specification cites a newer IEC edition instead, agree at the kick-off meeting which one governs rather than discovering the mismatch during document approval.

Should cascading use breakers from the same manufacturer?

Use only combinations for which valid manufacturer coordination/back-up data exists. Mixing devices without evidence should not be assumed suitable.

What information is needed for breaker coordination?

At minimum: the single line diagram, source and transformer data (kVA, voltage, percentage impedance, vector group), calculated fault levels at each bus, the breaker models and trip-unit types actually being offered, CT or sensor ratings, cable sizes and lengths, motor data for the larger drives, and the client's continuity philosophy — which loads must not be interrupted for a fault elsewhere. Missing fault levels is the usual hold-up, because without them no selectivity claim can be evaluated. If a fault-level study does not yet exist, get one done before the breaker schedule is frozen; retrofitting selectivity after the panel is built is far more expensive than engineering it at the drawing stage.

Breaker Selectivity