ACB Trips Before 250 A MCCB on a Submain Arcing Fault
A 630 kVA, 11 kV/415 V distribution transformer (6% impedance) feeds a main 1000 A air circuit breaker. The ACB's long-time setting is 0.9 In, short-time pickup 2.0 In with a 0.2 s time delay, and instantaneous is switched off. A 250 A thermal-magnetic moulded-case circuit breaker feeds a submain to a 75 kW DOL motor and a small distribution board. The MCCB has a fixed instantaneous trip at 10 In (2500 A) and a breaking capacity of 25 kA. A recurring fault in the submain causes the main ACB to trip and shut down the plant, while the 250 A MCCB shows no trip. The calculated three-phase fault current at the submain origin is 8.2 kA, but the fault is an arcing earth fault estimated at 2.2 kA. The maintenance team proposes increasing the ACB short-time delay to 0.5 s to “ride through” the event.
1. Explain, with settings and current values, why the ACB trips before the MCCB for a 2.2 kA arcing fault. Calculate the discrimination margin.
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ACB short-time element
ACB short-time pickup = 2.0 × 1000 A = 2000 A, with a 0.2 s delay. The 2.2 kA fault is above 2000 A, so the ACB short-time element starts its 0.2 s timer and trips the main breaker.
MCCB response
MCCB instantaneous pickup = 10 × 250 A = 2500 A. The 2.2 kA fault is below 2500 A, so the MCCB does not trip instantaneously. It must be cleared by its thermal element. At 2200/250 = 8.8 times rated current, a thermal-magnetic MCCB may take several seconds, possibly tens of seconds, depending on its curve and pre-loading. The ACB at 0.2 s is therefore faster and trips first.
Discrimination margin
[\text{Margin}=\frac{2200}{2000}=1.10]
Only a 10% margin exists between the fault current and the upstream short-time pickup. Normal current grading requires a margin of at least 1.5–2.0, plus time grading. This is a classic loss of discrimination: the downstream device has no instantaneous or short-time element that responds below the upstream pickup, and its thermal element is too slow.
The 8.2 kA three-phase fault current is not relevant to this event; the arcing earth fault is current-limited to about 2.2 kA. The fault level is high enough to be dangerous but too low to operate the MCCB instantaneous element.
2. Evaluate the proposal to increase the ACB short-time delay to 0.5 s. Give a better coordination solution and state the risks of the proposal.
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Evaluation of the 0.5 s proposal
Increasing the ACB delay may allow the MCCB thermal element to clear the 2.2 kA fault, but it is not a reliable fix. The MCCB thermal time at 8.8 times rated current is not guaranteed to be less than 0.5 s, especially if the breaker is warm or the fault current varies. If the fault persists beyond 0.5 s, the ACB still trips and the plant still shuts down. Meanwhile, the longer delay increases the arc-flash energy at the main board and exposes cables and busbars to more thermal stress. It treats the symptom, not the missing downstream protection.
Better solution
- Replace the fixed thermal-magnetic 250 A MCCB with an adjustable MCCB that has a short-time or adjustable instantaneous element. Set its instantaneous pickup at about 1.8 kA: above the 75 kW motor starting current (approximately 6 × 132 A ≈ 790 A) but below the 2.2 kA arcing fault. The MCCB will then clear the fault instantaneously, before the ACB short-time element.
- Raise the ACB short-time pickup to about 3.0 kA and keep a 0.3 s delay, so there is current and time margin between the devices. Verify that the 3.0 kA setting still protects the transformer and main busbars.
- Alternatively, add a dedicated earth-fault relay on the 250 A submain, with a pickup of about 100 A and a 0.2 s delay, and set the upstream ACB earth-fault element to 0.5 s. This gives time grading for earth faults without delaying short-circuit protection.
- Carry out a proper coordination study using the actual trip curves and the fault current at each point. Check cable thermal withstand for the chosen clearing time.
The key principle is that the downstream device must operate first for a fault in its zone. Increasing upstream time is acceptable only when the downstream device is guaranteed to clear the fault within that time.
3. What tests and records would you require before re-energizing the submain after the coordination change?
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Before re-energizing:
- Settings record: a signed schedule of the ACB and MCCB long-time, short-time, instantaneous and earth-fault settings, with the adjustable ranges actually available on the devices.
- Primary injection test on the MCCB and, where practicable, on the ACB, to confirm the trip pickups and times at the intended settings. Secondary injection is acceptable for the ACB if primary injection is not feasible, but the CT ratio and wiring must be verified.
- Trip test and mechanical check: confirm the breakers open freely, indicators are correct, and auxiliaries/alarms work.
- Insulation and earth-fault path checks on the submain, motor and distribution board. Repair the original arcing fault; coordination changes alone do not remove the damaged insulation.
- Fault-level verification: record the calculated 8.2 kA three-phase fault level and the estimated arcing fault level, and confirm that the MCCB breaking capacity exceeds the prospective fault current at its terminals.
- Coordination study with the final settings, retained for future maintenance.
After energizing, monitor the first motor starts and record any trips, currents and breaker operations. The recurring fault itself must be located and repaired; the protection change only limits the shutdown zone.