An apprentice engineer asks, “My utility supply is 100A. Should my factory’s main breaker also be 100A?”
First, let’s gently correct the terminology, as it reveals the “Master-level” secret. A factory main isn’t an “MCB” (a Miniature Circuit Breaker, the “household” kind). It’s an “MCCB“ (a Moulded Case Circuit Breaker) or an “ACB“ (an Повітря Circuit Breaker). These are “industrial beasts.”
But the реальний problem is his question. He’s proposing a “Fair Fight” (100A vs. 100A).
In electrical protection, what happens in a “fair fight”?
A disaster.
You never want a “fair fight.” You want a “hierarchy.” The professional term for this is “Selectivity,” and here’s why it’s the most important concept in power design.
The “Water Pipe” Analogy: What is Selectivity?
Селективність (also called “Discrimination”) means the smallest breaker (the one closest to the fault) trips first.
The “Aha!” moment is “The ‘Water Pipe’ Analogy.”
- The Utility Fuse (100A): This is the “City Water Main” under the street.
- Your Main Breaker (80A): This is the “Angle Valve” under your kitchen sink.
- The Fault: Your kitchen faucet starts leaking (a “fault”).
The Goal: You want to selectively shut off the “Angle Valve.” You never want a simple faucet leak to shut down the “City Water Main,” plunging your entire factory into darkness.
This reveals the core philosophy: “You protect yours, I protect mine.” The utility’s 100A fuse is there to protect their transformer from your fire. Your 80A breaker is there to protect your panel from your machine’s fault. They are not on the same team.
The Horror Story: “The ‘Shameful Call'” (aka Nuisance Tripping)
So, what happens if you ignore this? What if you create that “Fair Fight” (100A vs. 100A)?
You get this “nightmare” scenario: “Nuisance Tripping.”
It’s 3 AM. A single machine on your line has a serious (but manageable) short circuit.
The Race: Both your 100A main breaker and the utility’s 100A main fuse see the massive fault. They “race” to see who can trip first.
The “Nightmare”: The utility’s upstream fuse wins (or they both trip).
The Consequence:
- Your entire factory goes dark.
- You check your main breaker. It’s… fine.
- You realize the utility’s fuse is blown. It’s on a pole 100 meters away.
- You must make “The ‘Shameful Call'” to the power company.
- You wait 4 hours (in the dark) for a utility truck, who then hands you a $5,000 “emergency call-out” bill for replacing their fuse that your bad design blew up.
PRO-TIP: You never want the utility to be your “first line of defense.” That’s your job. This “nuisance trip” was a “cascading failure” caused by a complete lack of Selectivity.
The “Master’s Tool”: Why 80A vs. 100A is the Wrong Question
“Okay, I get it!” the apprentice says. “I’ll just install an 80A breaker to ‘beat’ their 100A!”
This is the “amateur’s” trap. It’s a guess.
That 80A breaker might be selective for a small overload (like 90A). But in a massive short circuit (e.g., 2,000A), both the 80A and 100A breakers see a “9-alarm fire” and will try to trip миттєво. You still might have a “race condition.”
“Amateurs” guess with Amps. “Pros” prove with Time.
The “Master’s Tool” is the Time-Current Curve (TCC).
- This is the “DNA” or “reaction-time chart” for every single breaker and fuse.
- A “Pro” (like a VIOX engineer) performs a “Coordination Study” або “Selectivity Study.”
- We overlay the TCC curve of your downstream breaker (the “Soldier”) with the TCC curve of the upstream fuse (the “General”).
The Goal: We must ensure that for будь-який given fault (from 100A to 10,000A), the “Soldier’s” curve is always “below and to the left” of the “General’s” curve. There should be no white space where they overlap.
This is a guarantee that our “Soldier” (your VIOX MCCB) will always win the “race” and contain the “battle” before* the “General” (the utility fuse) even hears the gunshot.
Conclusion: In Protection, “Hierarchy” is “Safety”
You never want a “Fair Fight.” You want a “Hierarchy.”
“Selectivity” is the art of ensuring your “Soldiers” (your individual breakers) solve the problem long before the “Generals” (your main breaker or the utility) are forced to act.
This is what separates a “professional” panel from an “amateur” one.
Amateurs guess (80A vs. 100A). Pros calculate.
VIOX provides full TCC curve data for all our MCCBs and ACBs, enabling you to design a truly selective і safe system. Don’t cause a “Shameful Call.”
Design with certainty. Browse our VIOX MCCBs and get the TCC data you need.
Примітка про Технічної точності
**Standards & Sources Referenced**
- This article is based on the core electrical engineering principle of "Selectivity" (also "Discrimination") as defined by IEC 60364-5-53 and "Coordination" as defined by NEC (National Electrical Code) Article 100.
- The use of Time-Current Curves (TCCs) is the standard industry method for performing a coordination study.
- The distinction between MCBs (Miniature Circuit Breakers, IEC 60898), MCCBs (Moulded Case Circuit Breakers, IEC 60947-2), and ACBs (Air Circuit Breakers, IEC 60947-2) and their applications is correct.
**Timeliness Statement**
All technical principles and standards referenced are accurate as of November 2025.
