Different Power Source Behavior and Effect on the Power Distribution

Overview

The paper first tabulates typical fault levels. At 400 V a 1000 kVA transformer gives about 24 kA, 1600 kVA about 39 kA, 2000 kVA about 48 kA, 2500 kVA about 60 kA and 4000 kVA about 96 kA. LV generators deliver roughly 10 x In for the first 100 ms and then collapse towards nominal (about 28.9 kA for 2000 kVA).

A UPS under a downstream short circuit either transfers to static bypass when available or limits inverter current to only a few times nominal for a limited time. Inverter short-circuit current is typically 2-3 x nominal (r.m.s.); the product standard allows up to 5 s, while Eaton units feed it for 300-400 ms to align with IEC 60364-4-41 disconnection for circuits up to 32 A. Static bypass typically withstands about 10 x In for one cycle.

A worked example shows seven 10 A C-curve MCBs per phase needing 100 A (141 A peak) for instantaneous trip, which requires a UPS of at least about 39 kVA and a fault-loop impedance below about 2.31 ohm. Adding inverter capacity does not raise fault current when it is limited by circuit impedance; shorter cables or larger conductors are needed. A pre-arcing I2t ratio of 1.6:1 or more between same-type fuses typically gives selectivity.

IEC 62040-1 Amendment 1:2013 (mandatory February 2016) requires suppliers to declare the maximum prospective fault current at the bypass input. The paper also covers transformer winding-ratio effects, static-switch semiconductor protection, parallel-UPS bypass timing and modular UPS scalability.

Key points

1. 400 V transformer fault levels: 1000 kVA ~24 kA, 1600 kVA ~39 kA, 2000 kVA ~48 kA, 2500 kVA ~60 kA, 4000 kVA ~96 kA.
2. LV generators give about 10 x In for 100 ms then collapse towards nominal (2000 kVA ~28.9 kA).
3. A pre-arcing I2t ratio of 1.6:1 or more between same-type fuses typically gives selectivity.
4. UPS inverter short-circuit current is typically 2-3 x nominal; the standard allows 5 s, Eaton feeds 300-400 ms (IEC 60364-4-41 for circuits up to 32 A).
5. Example: seven 10 A C-curve MCBs per phase need 100 A (141 A peak); UPS of at least ~39 kVA and fault-loop impedance below ~2.31 ohm.
6. More inverter capacity does not raise fault current when impedance limits it; shorter cables or larger conductors are needed.
7. Static bypass withstands about 10 x In for one cycle; IEC 62040-1 Amendment 1:2013 (February 2016) requires the maximum prospective fault at bypass input to be declared.

Using it in design

• Treat the UPS in inverter mode as a current-limited source in the short-circuit engine, with catalogue fields inverterIscMultiple, inverterIscDuration_ms, bypassIcw and maxProspectiveFaultAtBypass.
• Verify downstream breaker instantaneous trip using inverter-only fault current (battery mode, bypass unavailable) and flag fault-loop impedance that is too high.
• Check the prospective fault current at the UPS bypass input against the declared rating.
• Check that same-type fuses have a selectivity ratio of at least 1.6.

Who should read it

Electrical designers doing selectivity and short-circuit studies on UPS-fed LV boards.

Related standards

• IEC 60364-4-41
• IEC 62040-1
• IEC 60076-1
• IEC 60269-1
• IEC 60909

Publication details

Authors

Janne Paananen; Martijn Imming

Publisher

Eaton

Year

2017

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation