Incomer ACB tripping during chiller starts
A 1500 kVA, 11 kV/400 V, Dyn11 transformer with 5.75% impedance supplies a glove factory. The calculated LV fault level at the main switchboard terminals is 36 kA. The incomer is a 2500 A air circuit breaker; a 630 A moulded-case breaker feeds a 315 kW chiller through a soft starter with a full-load current of 545 A and a start current limited to 3.5 times FLC for 14 s. Since commissioning the incomer ACB has tripped four times during chiller starts, each time blacking out the plant; the 630 A MCCB has never tripped. ACB settings are L 1.0 (2500 A), S 0.6 × Ir (1500 A) with 0.4 s delay, I 8 × In (20 kA), and G 0.2 × In at 0.4 s. The MCCB is set L 1.0 (630 A) and Ii 10 × In (6.3 kA), with no adjustable short-time delay. The maintenance contractor proposes to raise the ACB S pickup to 1.0 and the delay to 1.0 s, and to switch off the earth-fault element because 'the transformer is already earthed'. Production wants the plant running tonight.
1. Diagnose the trips quantitatively. Which protective element is operating, and why does the MCCB never trip?
Read the worked answer
- Chiller start current: 3.5 × 545 ≈ 1908 A, sustained for 14 s.
- ACB short-time element: pickup 0.6 × 2500 = 1500 A, delay 0.4 s. The start current exceeds the pickup and persists far beyond 0.4 s, so the S element times out and trips the incomer. This matches the observation that trips occur only on chiller starts.
- ACB long-time element: 1908 A is below the 2500 A pickup, so L is not the initiating cause, although thermal memory will shorten later starts.
- MCCB: instantaneous setting 10 × 630 = 6300 A. The 1908 A start current is far below it, and at about 3 × In the thermal element will ride through a 14 s start. Hence the MCCB holds while the upstream device trips.
- Earth-fault element: the start current is balanced, so residual current is near zero and G is not involved.
The conclusion is a settings/discrimination problem, not an equipment fault: the upstream device is set more sensitively than the downstream device's characteristic permits for this load.
2. Evaluate the contractor's proposed setting changes and state what you would approve.
Read the worked answer
S pickup raised to 1.0 (2500 A): this gives 2500/1908 ≈ 1.31, about 31% margin above the start current, which would stop the nuisance trips. Acceptable in principle.
Delay raised to 1.0 s: unnecessary. With the pickup at 2500 A the 0.4 s delay already covers the start. Increasing it to 1.0 s lengthens clearing for faults between 2500 A and the 20 kA instantaneous threshold and therefore raises arc-flash incident energy at the board. Reject the increase.
Consequences to check before approval:
- With S at 2500 A, the S and L pickups coincide, so S no longer backs up L. Verify that the L element still protects the transformer and busbar for overloads.
- The I element must stay at 20 kA, below the 36 kA switchboard withstand, providing fast clearing of high faults.
- Discrimination with the MCCB: below 6.3 kA only the ACB sees the fault (S, 0.4 s); above 6.3 kA the MCCB clears instantaneously while the ACB S element holds. Check the MCCB I²t and the ACB short-time withstand at 6.3 kA for 0.4 s to confirm the time margin is real, not assumed.
- Check the 11 kV overcurrent relay curve: the ACB S at 2500 A/0.4 s must remain below the HV relay operating time for a 400 V fault referred to the HV side, and the transformer must withstand the fault for the resulting time.
Approved: S = 1.0 × Ir, tS = 0.4 s, I = 20 kA and G unchanged, subject to a stamped coordination study, primary injection testing of the ACB and an arc-flash label. Rejected: disabling the earth-fault element and the 1.0 s delay.
3. The contractor wants to disable the earth-fault element. How do you respond, and what evidence do you require before the plant is re-energised?
Read the worked answer
I refuse. The G element (0.2 × In = 500 A, 0.4 s) protects against earth faults whose current is below the phase-overcurrent pickups, for example arcing faults near the far end of a feeder or a deteriorating neutral-to-earth connection. The earthed transformer star point is what provides the fault current path; it is not a substitute for detection. Disabling G leaves the installation without sensitive earth-fault protection and is contrary to the design intent and the Electricity Regulations 1994.
Evidence required before re-energisation:
- A stamped coordination and settings study covering the LV ACB, the MCCBs and the 11 kV relay.
- ACB primary injection test proving the S, I and G elements at the final settings, witnessed.
- 11 kV relay secondary injection and trip test, including the intertripping scheme.
- Transformer and busbar short-circuit withstand check for the revised clearing times.
- Arc-flash study and equipment labels.
- A documented start sequence with the soft-starter current and start time recorded on the first live start.
- As-built setting sheets posted in the switchroom under revision control.
I would then witness one full chiller start and record the actual starting current against the approved settings before handover.