In short. Protection coordination arranges the trip characteristics of every device between generator and load so that only the device nearest a fault operates, and before the cable or equipment is damaged. On generator supply this is harder than on utility supply because the fault current is low and decays quickly. A coordination study must therefore be run for the generator mode using the alternator’s decrement curve, and voltage-restrained overcurrent is usually needed at the generator breaker.
What coordination is trying to achieve
Every protective device has a time-current characteristic: the time it takes to operate against the current flowing through it. Coordination, also called selectivity or discrimination, arranges those characteristics so that for any fault location the device immediately upstream operates first and all others remain closed. When it works, a short circuit on a machine feeder trips that feeder’s MCCB and nothing else. When it fails, the same fault trips the generator breaker, the whole plant goes dark, and the set may shut down on over-current with the fault still present. The second outcome is common on sites where protection was set for utility supply and never revisited after the generator was added.
The generator decrement curve
The starting point of a generator coordination study is the decrement curve: the current the alternator delivers into a terminal short circuit plotted against time. It starts high in the sub-transient period, falls through the transient period and settles at a sustained value set by the excitation system, as described in the fault current guide. This curve is overlaid on the time-current plot with every downstream device. Two checks follow. The generator’s own protection must sit above and to the right of the largest downstream device, so a feeder fault does not trip the generator breaker. And the decrement curve must stay above the operating characteristic of every device that must clear a fault for the whole time that device needs; if it drops below a device’s pick-up first, that device will never trip on generator supply.
The time-current plot in practice
A time-current plot is drawn on log-log axes with current horizontal and time vertical. The decrement curve, cable damage curves, any transformer inrush point and the trip curves of each breaker or fuse are plotted on one sheet referred to a common voltage. Where two device curves overlap there is a band of fault currents for which both could operate, and that is a miscoordination. The time-current coordination page covers the plotting method, and the protection coordination study workflow page describes a full study.
Generator mode and utility mode are two different studies
| Aspect | Utility supply | Generator supply |
|---|---|---|
| Available fault current at main board | High, often 25 to 50 kA | Low, often 3 to 10 kA per set, decaying |
| Fault current at end of long feeder | Still several kA | May approach the feeder’s full-load current |
| Instantaneous element at incomer | Can be set high and stays selective | May never see enough current to pick up |
| Time-delayed elements | Operate quickly because current is far above pick-up | Operate slowly or not at all as current decays towards pick-up |
| Earth-fault current | Set by transformer neutral earthing | Set by generator neutral earthing, which may differ |
| Risk | Nuisance tripping, arc-flash energy | Fault not cleared, generator trips on over-current with fault still present |
Because of these differences, many relays offer two setting groups so the incomer switches to a generator-mode group when the ATS transfers. Where downstream MCCBs are fixed thermal-magnetic devices, the only freedom is choosing devices whose magnetic pick-up falls within the generator’s fault current at that location, which is a design-time selection problem rather than a commissioning setting problem.
Protection at the generator breaker
A plain overcurrent element at the generator breaker cannot be set correctly. Set above the sustained fault current so it does not trip on load, it will not operate once the fault current has decayed; set low enough to catch the decayed current, it trips on motor starting. The standard answer is voltage-restrained or voltage-controlled overcurrent, ANSI device 51V, which lowers the pick-up when terminal voltage is depressed. During load and motor starting the voltage stays reasonably high and the element is insensitive; during a close-up fault the voltage collapses and the element becomes sensitive enough to catch the decaying current. The generator breaker protection should also include over and under voltage, over and under frequency, reverse power where sets parallel, negative-sequence or phase unbalance, and earth fault appropriate to the neutral earthing method. The generator protection relay architecture page lists these functions and the reverse power and generator protection page explains the paralleling functions.
Downstream devices on generator supply
Feeder and sub-distribution devices need three things confirmed for generator mode. The minimum fault current at the far end of each circuit, calculated with one generator at its sustained value and the full cable impedance, must exceed the device’s magnetic or short-time pick-up with margin. The device must clear within the cable’s adiabatic withstand at that current, using the k²S² ≥ I²t relationship from IS 732 and IEC 60364. And each device must leave a grading margin of roughly 0.2 to 0.4 seconds below the next device upstream. Where the first check fails, the options are a larger cable, a device with a lower magnetic pick-up, an earth-fault element that does not depend on phase current, or a definite-time element set below the sustained fault level. The protection device selection page compares the device types.
Keeping the study alive
A coordination study is only correct for the system it describes. Adding a feeder, replacing a breaker with a different make, changing the generator or converting to parallel operation can each invalidate it. We advise sites to keep the study, setting sheets and as-left relay settings in one controlled file, record every change against it, and repeat secondary injection on the generator breaker relay during the annual maintenance visit. A rental set replacing the normal source during an outage should be checked against the same logic, because its alternator will have different reactances and sustained fault capability.
Related photographs and videos
More related media (1)
Common mistakes
- Copying the utility-mode settings into the generator mode, so feeder faults at the far end of long cables are never cleared on generator supply.
- Fitting a plain 50/51 overcurrent relay at the generator breaker, which either trips on motor starting or fails to clear a sustained fault once the current has decayed.
- Choosing MCCBs on frame size alone without checking that the magnetic pick-up is below the generator’s minimum fault current at that board.
- Changing relay settings on site to stop a nuisance trip without recording the change, so the coordination study no longer describes the plant.
What to send us
Send the single-line diagram with breaker makes, frame sizes and existing settings, cable sizes and lengths, the alternator nameplate and datasheet with reactances and excitation type, and the DISCOM fault level if you have it. Photographs of the generator controller and the incomer relay setting screens are useful. Tell us whether sets parallel and whether there is an ATS or a manual changeover. Use the application engineering enquiry and we will scope a coordination review for both operating modes.
Questions engineers ask
Why does my feeder breaker trip the whole generator instead of just the feeder?
Most often because the feeder device’s magnetic pick-up is above the fault current the generator can supply at that point, so the feeder device sits in its slow thermal region while the generator’s own over-current protection, or its under-voltage protection, operates first. A coordination check in generator mode will show the overlap and the remedy.
What is 51V protection and do I need it?
51V is voltage-restrained or voltage-controlled overcurrent. It reduces the pick-up current when terminal voltage is low, which lets it detect a decaying generator fault without tripping on motor starting at normal voltage. Any generator that feeds a distribution system with downstream protection should have it, or an equivalent function in the controller.
What grading margin should I use between breakers?
For electronic trip units a margin of about 0.2 seconds between operating times at the same fault current is usually adequate. For electromechanical relays or thermal-magnetic MCCBs allow 0.3 to 0.4 seconds to cover tolerance and breaker opening time. Manufacturer selectivity tables can justify tighter margins for specific device pairs.
Can MCBs downstream of a small generator coordinate properly?
Often not fully. Sets under about 100 kVA deliver fault currents that may fall inside the magnetic band of 32 A to 63 A MCBs, so both the MCB and the generator protection may operate. The practical approach is to accept limited selectivity and make sure the generator protection at least clears the fault safely.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.