In short. Power system design support helps the responsible designer integrate a generator into an existing or new LV system: the operating modes, the transfer architecture, the much lower fault current a generator delivers, the neutral and earthing arrangement, protection behaviour on each source and the commissioning logic. It works from a clear brief and an agreed boundary, and it is not a substitute for the statutory design responsibility of the licensed consultant or contractor.
Start with a brief and a boundary
Design support goes wrong when nobody has written down what question is being answered. The brief should say whether the work concerns load assessment, source selection, transfer, distribution, protection or control philosophy, and who holds responsibility for the complete installation design. In India the electrical installation must comply with the CEA Regulations on measures relating to safety and electric supply, and approvals rest with the licensed electrical contractor and, where applicable, the electrical inspectorate. Our calculations and drawings are inputs to that responsible design, coordinated with it, and never presented as statutory approval.
Review every operating mode
An LV system with a generator has at least four states, and the loads, fault levels, neutral path and protection behaviour differ in each. The single-line diagram and the operating philosophy must describe the same system in every state.
| Mode | Source feeding the load | Fault current at the main board | Design questions |
|---|---|---|---|
| Normal | DISCOM transformer | High, set by transformer impedance | Protection coordination, cable withstand |
| Outage | Generator alone | Low, a few multiples of generator rated current and decaying | Will breakers and fuses still clear faults, is the neutral earthed |
| Transfer | Open or closed transition between the two | Momentarily both, if closed transition | Interlocks, synchronising, neutral switching, inrush on retransfer |
| Maintenance or test | Generator on test load, utility on building, or bypass | Depends on arrangement | Isolation, bypass-isolation ATS, test without load loss |
Generator fault current is not utility fault current
A distribution transformer with 5 percent impedance can deliver twenty times its rated current into a bolted fault. A generator cannot. Its initial fault current is set by the alternator’s sub-transient reactance and falls within cycles, often settling at two to three times rated current if the excitation system can sustain it at all.
If ≈ Irated / X″d
Here If is the approximate initial symmetrical fault current, Irated is the alternator rated current, and X″d is the sub-transient reactance in per unit on the alternator base. Worked example (illustrative): a 500 kVA alternator at 415 V has a rated current of about 696 A. With X″d of 0.14 per unit the initial fault current is roughly 5 kA, compared with perhaps 25 to 30 kA from a 1,000 kVA utility transformer on the same board, and within a few hundred milliseconds the generator contribution may fall below 2 kA. An MCCB with an instantaneous setting at 10 times its 630 A rating will never see 6.3 kA from this set, so a downstream fault is cleared slowly by the thermal element or not at all, and the generator’s own protection trips the whole board. The usual responses are lower instantaneous settings on generator-fed feeders, alternators with permanent magnet excitation that sustains around three times rated current for several seconds, residual-current earth-fault protection where loop impedance is inadequate, and in larger systems a coordination study with both source contributions. The method is in fault current, the settings consequences in protection coordination and the study process in the coordination study workflow.
Neutral and earthing on two sources
The generator neutral must be earthed in the generator mode and must not create a parallel neutral path in the normal mode. Where the generator is close to the main board and the utility neutral is solidly earthed at the transformer, a three-pole transfer with the generator neutral bonded to the common neutral-earth point is a frequent arrangement. Where the generator is remote, where earth-fault protection uses residual sensing on both sources, or where the inspector requires a separately derived source, a four-pole transfer that switches the neutral is used and the generator neutral is earthed at the set. Choosing wrongly produces nuisance earth-fault trips, circulating neutral currents or an unearthed neutral during an outage. The decision logic is in earthing and neutral architecture, and Indian practice under IS 3043 of separate body and neutral earth electrodes at the set is part of the review.
Transfer architecture
The transfer scheme is chosen from the loads’ tolerance of interruption and the need to test and maintain. An open-transition ATS or AMF panel gives a break of a few hundred milliseconds to a few seconds and is the standard for most sites. Delayed transition adds a pause so that motor residual voltage decays. Closed transition briefly parallels generator and utility and needs DISCOM consent, synchronising control and reverse-power protection. Bypass-isolation ATS allows the switch to be maintained without dropping the load. Design support compares these against the load classification and specifies the interlocks that prevent two unsynchronised sources meeting. Design support also checks that the generator breaker, the ATS and the cables are rated for the utility fault level in normal mode, and for voltage drop at starting load in generator mode, because neither mode is the only state. The overall architecture is covered in the integrated power system design guide.
Commissioning logic is a design output
A design that cannot be proven was not finished. For each mode the support package states what will be tested, under what condition and with what acceptance criterion: utility failure simulation, start time, transfer time, retransfer delay, neutral state in each mode, earth-fault operation on generator supply, interlock proof, and alarm and shutdown behaviour. A set may start automatically while the transfer path is still wrong, and only a written test proves otherwise.
Retain the basis of the design
The design record connects requirements to assumptions, manufacturer data, calculations and verification. Preliminary values are marked preliminary. Interfaces that remain unresolved, such as a transformer impedance not yet confirmed, are listed with an owner. Where the assessment shows that a detailed coordination, transient or harmonic study is needed, that requirement stays visible rather than disappearing into a general note. The record allows the responsible designer, the inspector and the commissioning engineer to see what was assessed and what remains.
Common mistakes
- Applying utility-mode protection settings in generator mode; a downstream fault is not cleared by the feeder breaker and the generator trips the whole building instead.
- Choosing a three-pole or four-pole transfer by habit rather than by the earthing design; the result is nuisance earth-fault trips or an unearthed neutral on generator supply.
- Treating design support as statutory approval; the licensed designer and the inspectorate are bypassed and the installation cannot be energised legally.
- Leaving commissioning criteria undefined; the test day produces readings without decisions and the system is accepted on a start-and-stop.
What to send us
Send the single-line diagram, the transformer rating and impedance, the main switchboard details with breaker types and settings, the generator or candidate generator data sheet including alternator reactances, the proposed transfer scheme, the earthing arrangement and any earth-pit test results, the load classification and the site survey report. Name the responsible designer so that the work can be coordinated. Use the application engineering enquiry; the findings feed directly into installation planning.
Questions engineers ask
Why is generator fault current so much lower than utility fault current?
Because the alternator’s internal reactance is large compared with a transformer’s impedance, and because the field cannot sustain high current once the sub-transient period passes. Initial current is typically five to ten times rated and it decays within cycles, so protection designed around utility fault levels may not operate on generator supply.
Do I need a four-pole ATS?
You need one when the generator must be a separately derived source with its own neutral earth, when residual earth-fault protection is used on both sources, or when the inspector or the DISCOM requires it. If the generator neutral can be bonded at the same neutral-earth point as the utility without creating parallel paths, three-pole transfer is acceptable. The earthing design decides, not the price list.
Can design support replace an electrical consultant?
No. The licensed designer or contractor holds the statutory responsibility for the installation under the CEA regulations. Design support provides generator-specific calculations, interface definitions and commissioning logic that the responsible designer incorporates and approves.
Does a generator change my earth-fault loop impedance?
Yes. The source impedance of the generator is much higher than the transformer’s, so the loop impedance rises and the prospective earth-fault current falls. A loop that disconnects within the required time on utility supply may not on generator supply, which is why residual-current protection is often specified for generator-fed circuits.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.