Generator, UPS and a 3-pole ATS on a life-safety standby system
A 500 kVA, 400 V standby diesel generator with X″d = 10.5% backs up a 10-storey office. Through a 3-pole automatic transfer switch it supplies a fire pump (90 kW, direct-on-line), two smoke extract fans (30 kW each, DOL), the lifts, and a 160 kVA UPS serving a level-3 server room. The generator star point is solidly earthed at the generator, and the transformer star point is also earthed at the substation; the ATS switches three poles only. On test, starting the fire pump from the generator drags the busbar to about 78% of nominal for roughly 0.4 s and the UPS transfers to battery every time; on one occasion the frequency dipped to 46 Hz and the extract fans dropped out. The UPS input specification allows ±10% voltage. The fire consultant also asks whether the battery can carry the server room for the 30 minutes required by the client's business continuity policy.
1. Estimate the voltage dip caused by the fire pump start, and recommend what should be changed.
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Estimate
- Fire pump full-load current: 90,000 / (√3 × 400 × 0.93 × 0.88) ≈ 159 A.
- DOL starting current at 6.5 × FLC ≈ 1,030 A, giving a starting apparent power of √3 × 400 × 1,030 ≈ 715 kVA.
- Approximate transient dip ≈ (starting kVA / generator kVA) × X″d = (715 / 500) × 10.5% ≈ 15%, i.e. about 85% residual, before adding cable drop and AVR response. The observed 78% is in the right region once those are included.
Why it matters
A 15–22% dip exceeds the UPS input window of ±10%, so the UPS transfers to battery on every fire pump test. That consumes battery life and, more importantly, means the UPS is not available as a ride-through device for the event it was bought for. The frequency dip to 46 Hz is a separate issue: the motor's starting kW (starting kVA × starting power factor, roughly 200–250 kW) is a step of about half the set's rated kW, and a slow governor will undershoot. Underspeed trips on contactor coils are what dropped the extract fans.
Options, in order of preference
- Reduce starting kVA: soft starter or VFD, subject to the fire strategy and the pump manufacturer's acceptance of reduced starting torque.
- Confirm the generator's transient performance from its datasheet (most manufacturers publish a maximum step load and dip), and upsize if the step exceeds it - typically a 500 kVA set should accept a 50% block load within the stated dip.
- Stagger starting in the cause-and-effect sequence so the UPS is not on the busbar during the pump start, or hold the UPS on battery through the dip by widening its input window if the manufacturer permits.
Do not simply raise the UPS transfer threshold without confirming the UPS can actually ride through; the load will still see the dip.
2. Evaluate the earthing and neutral arrangement created by the 3-pole ATS and both star points being earthed.
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The problem
With a 3-pole ATS the neutral is solid through from the transformer to the generator. If the generator star point is also bonded to earth, there are now two neutral-to-earth bonds on one electrically continuous neutral. That gives:
- Parallel neutral paths. Neutral current divides between the neutral conductor and the earth path (PEN, earthing system, structural steel), producing circulating currents, unpredictable magnetic fields and possible overheating of bonding conductors.
- Split earth fault current. Fault current divides between the two earth points, so the earth fault loop impedance seen by protection is no longer the design value and disconnection times may not be met.
- RCD problems. Any residual current device upstream will see the diverted current as an imbalance and may trip nuisance or fail to trip correctly - both are safety issues.
- PEN hazard. If the utility supply is TN-C-S (PME), the generator's independent earth creates a parallel return that the supply authority's assumptions do not cover.
Remedies
Either:
- 4-pole ATS, switching the neutral, so the generator becomes a separately derived system; the generator neutral is bonded to earth at one point only, with its own electrode and an earth fault loop verified for disconnection.
- Or keep one common earthed neutral and remove the generator's separate neutral-to-earth bond, accepting that the neutral is earthed only at the transformer - only valid if the neutral is never switched and the earthing arrangement is genuinely common.
The choice must be made deliberately, drawn, labelled, and reflected in the earth fault loop calculation for the generator mode, not left to site practice.
3. Comment on the UPS autonomy claim, the load step on the generator, and what you would require before signing off the standby system.
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Autonomy
A 160 kVA UPS is often supplied with a battery giving 10 minutes at full load, not 30. The client's business continuity policy is a design input; it must be converted into a battery specification at the actual measured load, not the nominal rating, and at end of life (typically 80% of rated capacity). Ask for the battery calculation: load kW, battery voltage, end-of-discharge voltage, discharge rate and the resulting minutes, plus the recharge time to 80% after a full discharge.
Load step
On transfer, the generator sees the UPS load plus its recharge current, plus the fire pump start if the sequence overlaps. Check the set's published block-load capability against that total. The UPS recharge can be a significant extra load for the first hour after a transfer.
Before sign-off
- A witnessed load test on the generator at 100% of the connected essential load, with the fire pump started from cold and the UPS on line, recording voltage and frequency dip, recovery time and any contactor drop-out.
- A UPS battery discharge test at the actual load, with results recorded against the calculated autonomy.
- A written cause-and-effect for the transfer sequence, including start staggering and any non-essential load shedding to keep the fire pump start within the generator's step capability.
- Correction of the neutral/earthing arrangement, with the earth fault loop impedance measured in generator mode.
If the 30-minute autonomy cannot be demonstrated at the actual load, that is a client decision about risk and budget - but it must be a decision recorded in writing, not an assumption.