In short. Inverter-based sources such as UPS, BESS and solar limit their output current electronically, typically to between one and two times rated current for a short time. They do not supply the large currents that trip standard breakers, so protection on inverter-fed circuits must be designed for that.
Why current is limited
Power semiconductors can carry high current for only milliseconds. The inverter’s control therefore clamps current at a small multiple of rated current, and may disconnect if the fault persists. The result is a fault current that depends on the controller, not on machine reactance.
Typical behaviour
| Source | Fault current behaviour |
|---|---|
| Online UPS (inverter mode) | Limited to about one to two times rated for a short time, then may shut down or transfer to bypass |
| UPS bypass mode | Fault current comes from the upstream source, so ordinary breakers can trip |
| Grid-forming BESS | Can deliver a bounded overcurrent for a short period, device specific |
| Grid-following BESS or solar | Follows the grid and typically injects only about rated current or slightly more, and disconnects on severe voltage loss |
Exact limits and durations are manufacturer-specific. Always use the data sheet.
Protection implications
- Fuses and thermal-magnetic breakers may not clear a fault on the inverter output; the fault may be cleared by the inverter shutting down, collapsing the voltage on healthy loads.
- Selective tripping downstream of a UPS needs small, fast branch devices. See critical-power protection architecture.
- Where the UPS transfers to bypass during a fault, the bypass path and static switch must withstand the fault current until the upstream device clears. Check the static switch’s rating.
- Earth-fault detection needs a defined return path and sensitivity.
Modelling
- Obtain the inverter’s overcurrent capability and duration from the manufacturer.
- Represent it as a current-limited source in the study software, not as a machine with reactance.
- Run cases for inverter mode and bypass mode.
- Check selectivity and clearing times in each.
- Check arc-flash energy, which may be higher on inverter supply if clearing is slow.
Mix with generators
Where a generator and inverter share a bus, the generator supplies most of the fault current. In islanded microgrids with only inverters, fault detection is harder and needs dedicated schemes. See utility, DG, UPS and BESS studies.
Illustrative example
A UPS output board is protected by 16 A branch breakers. In inverter mode, the UPS can deliver about 150 percent of its rated current for a short time. A short circuit at a rack is only a few multiples of the branch breaker rating, so the breaker may take longer than expected to trip, dipping the voltage for other racks. The study recommends smaller, faster branch devices and a UPS selected with sufficient overload capability.
Why electronic sources behave differently
Synchronous machines are voltage sources behind a reactance and can supply several times rated current. Inverters are current-limited by their semiconductors, so their fault contribution is typically limited to about 1.1 to 2 times rated current for the short time before they trip or limit.
| Source | Typical fault contribution | Duration |
|---|---|---|
| Double-conversion UPS inverter | About 1.5 to 2 times rated | Milliseconds to a few cycles, then limited |
| UPS via static bypass | Mains-fed, high | Until bypass device or protection operates |
| Grid-forming BESS inverter | Up to about 1.2 to 2 times rated, vendor specific | Controlled |
| Solar PV inverter | About 1.1 to 1.2 times rated | Short |
Obtain the figures from the manufacturer, since controls differ.
Consequences for protection
- Low fault current may not pick up overcurrent protection, so earth fault and under-voltage functions or fast fuses may be needed.
- UPS bypass can transfer a fault to the mains-fed path; check bypass rating and downstream coordination.
- In islanded operation on a BESS, selectivity may fail.
Study treatment
Model the inverter as a current source at its limit in the fault study. Run separate cases for mains, generator and battery operation. Do not assume the generator fault level applies when only inverters supply the load.
See UPS and nonlinear loads and hybrid system studies.
Common mistakes
- Treating UPS output like a utility source.
- Not studying bypass mode.
- Using device ratings far above branch needs.
What to send us
Send UPS or BESS data sheets and the downstream layout. Send your application details.
Questions engineers ask
Do inverters contribute to the breaker’s duty?
Only slightly. The main concern is protection operation, not equipment rating.
What is grid-forming?
An inverter that sets voltage and frequency itself, like a generator, rather than following a grid.
Why does a UPS go to bypass in a fault?
To pass more current from the upstream source to clear the fault.
How do I get the data?
From the manufacturer’s data sheet or the supplier on request.
Values vary by manufacturer. Use the actual data sheet.