Understanding The Three-Stage Protection of MCCB

A Practical Guide to Setting L / S / I Protection Correctly

Q1. What Is “Three-Stage Protection”?

Three-stage protection refers to the commonly used electronic trip functions in MCCBs (Molded Case Circuit Breakers):

1. Long-Time Delay (L)

• Purpose: Overload protection and temperature-rise control

• Trip curve: Inverse-time only

2. Short-Time Delay (S)

• Purpose: Moderate short-circuits, selective coordination

• Trip curve: Fixed or I⊃2;t delay

3. Instantaneous (I)

• Purpose: High-level short-circuits, arc energy reduction

• Trip curve: No delay or extremely short delay

Q2. Why Must All Three Stages Be Set? Why Not Rely on Instantaneous Only?

Each stage has its essential function:

• Long-time delay: Prevents cables and loads from long-term overheating

• Short-time delay: Enables proper upstream/downstream coordination (nearest device trips first)

• Instantaneous: Clears severe faults within 1–2 cycles and reduces thermal/mechanical stress

If you set only instantaneous protection:

1. Small overloads are undetected → long-term cable damage

2. Upstream breaker trips instantly → full-system blackout instead of a localized fault

3. Motor starting or inrush currents cause nuisance tripping

Q3. What Data Should Be Prepared Before Setting the Protection?

Q4. How to Set Long-Time Delay Current Ir?

Must satisfy: Ib ≤ Ir ≤ Iz

Recommended practice:

• Cable-protection priority: 0.8 In – 0.9 In

• Load with large fluctuation: 0.9 In – 1.0 In

• When cable capacity allows, prefer the higher end of the range—short-circuit protection is handled by S and I stages.

For DOL motors (starting current ≥ 6 Ir), increase Ir appropriately or extend the time delay.

Q5. How to Set Long-Time Delay Time Tr?

• IEC recommendation: trip at 6 × Ir within 3–10 s

• For motor starting: Tr ≥ 1.2 × maximum start duration

Q6. How to Set Short-Time Delay Current Isd?

IEC/GB range: 1.5 Ir – 10 Ir

• Branch circuits: 2–4 Ir (avoid inrush)

• Main feeders: 5–8 Ir (allow upstream to wait for downstream)

Check TCC curves: Ensure at least 0.2 s time margin at the 0.1 s point between upstream and downstream breakers.

Q7. How to Choose a Safe Short-Time Delay Tsd?

• With fuses or small-frame MCCBs: 0.1–0.2 s

• With ATS(Automatic Transfer Switch) / bus-tie transfer: 0.05 s (or 0.04 s)

Q8. How to Set Instantaneous Pickup Ii for Fast Tripping Without Miscoordination?

1. Lower bound: Ii ≤ 0.8 × Isc_min (ensures minimum fault triggers trip)

2. Upper bound: Ii > downstream short-time delay peak (avoid upstream tripping first)

3. Thermal-magnetic MCCBs: fixed 10–15 In Electronic MCCBs: adjustable 2–15 Ir

4. Since instantaneous has no delay, set ≥ 1.5 × short-time setting to maintain coordination.

I am Frank, Electrical Engineer in AISIKAI Team. I will share technical articles on Switches, Circuit Breakers and other electrical devices. With 10 years of electric project experience, I am commited to provide professional electrical solutions.