25/04/2026
Don’t Buy That Breaker Until You Understand Icu, Ics & Icw
Many technicians see 50kA on a breaker and assume they’re fully protected.
But that number means different things depending on whether it’s Icu, Ics, or Icw.
Let’s make it simple:
1️⃣ Icu: Ultimate Breaking Capacity
This is the maximum fault current the breaker can interrupt.
If a breaker is rated Icu = 50kA, it can clear a fault up to 50kA without failing.
But after such a fault, the breaker may no longer be reliable. It survived, yes. but it might need replacement.
So Icu is basically the breaker’s survival limit.
2️⃣ Ics: Service Breaking Capacity
This is the fault level the breaker can interrupt and still remain usable afterwards.
It is usually given as a percentage of Icu.
Example:
Icu = 50kA
Ics = 75 percent of Ics = 37.5kA
If a 35kA fault occurs, the breaker trips and can be reset safely.
If the full 50kA fault occurs, it may still trip, but internal damage is possible.
In critical applications, engineers prefer Ics = 100% of Icu.
3️⃣ Icw: Short-Time Withstand Current
This one is about endurance, not breaking.
Icw shows how much fault current the breaker can withstand for a short time (usually 1s or 3s) without damage.
Example:
Icw = 25kA for 1 second
That means the breaker can carry 25kA briefly while waiting for a downstream breaker to trip.
This is important for selective coordination, especially in generator boards and large distribution panels.
