Replacing a transformer can invalidate downstream short-circuit ratings even when the new unit has the same kVA. A lower impedance, stronger upstream source, additional parallel transformer or changed cable arrangement can increase bus fault duty. Before replacement, recalculate the credible maximum fault current and compare it with breaker interrupting/making ratings, busbar withstand, switchboard ratings and protection settings.
A same-capacity replacement can increase current
For an illustrative 1 MVA, 400 V transformer fed from an ideal stiff source, reducing impedance from 6% to 4% raises the simplified symmetrical terminal-fault estimate from about 24.1 kA to 36.1 kA: an increase of 50%. The nameplate MVA is unchanged. An actual study must include source and connection impedances, tolerances and other infeeds. The example explains why “same capacity, better voltage regulation” is not enough to establish compatibility with an existing switchboard.
Compare like-for-like short-circuit ratings
Breaker interrupting duty, peak making duty, busbar short-time withstand and assembly ratings describe different conditions. A kA figure without RMS/peak identification, time duration and the applicable standard is incomplete. Obtain the switchboard's actual documentation, including any conditional rating that depends on a specified upstream protective device. A replacement device with a similar current label may not preserve that condition. Include neutral and earth paths where relevant and check the permitted clearing time against the assembly withstand duration.
Review coordination before deciding the upgrade scope
Higher fault current can change instantaneous operation, selectivity and the fault energy evaluated in a separate safety study. Lower minimum current under an alternative source can be equally important for reliable clearing. Review normal operation, bus-tie closure and maintenance supply before deciding whether protection changes, current limitation or physical equipment replacement is required. Keep the approved fault study with the replacement data sheet. Do not issue a blanket “busbar upgrade required” conclusion from the transformer-only estimate without checking the installed system and its verified ratings.
Technical reference framework
[1] IEC 60076-1:2011 — Power transformers, General.
[2] IEC 60076-5:2006 — Ability to withstand short circuit.
[3] IEC 60076-8:1997 — Application guide.