Fault-current capability of a standby generator — why protective devices can behave differently on generator power

The guide on standby generators (§551) covers the mutual exclusion between mains and generator and star-point earthing during parallel operation, and the guide on generator load testing covers "wet stacking" and the load-bank test. This article covers a third, functionally separate concern: how much fault current a generator can actually deliver during a real short circuit, and why this is fundamentally different from what the same installation would see from the public grid.

The grid delivers far more fault current than a generator

A grid connection is fed through a transformer with a relatively low internal impedance: during a short circuit the grid can therefore deliver a multiple of the rated current for the first few cycles — often an order of magnitude above the rated load current, which is the basis of the regular prospective fault-current calculation (see the [guide on Icu sizing](/guides/nen-1010/prospectieve-kortsluitstroom-icu-dimensionering)).

A synchronous generator on a diesel set cannot approach that level: its fault-current capability is limited by the sub-transient and transient reactance of the machine itself, and — in the absence of active field forcing from the control system — the available fault current decreases as the fault persists. A commonly cited rule of thumb: a standard standby generator without extensive field forcing typically delivers, for a three-phase fault, on the order of 250-300% of its rated current, for at most a few seconds — after which its own generator protection (undervoltage, overcurrent) usually intervenes. The exact percentage and duration vary by manufacturer, rating and excitation system.

Why this affects protective-device coordination

An installation circuit breaker with a magnetic instantaneous trip (for example a curve C or D, which only trips instantaneously above a multiple of rated current) is typically sized and verified against the fault current the grid can supply. During a fault fed exclusively by the generator (for example during a mains outage, with the transfer switch on generator), the actual fault current can remain below that breaker's instantaneous trip threshold — the protection then falls back on the much slower thermal (overload) curve, or in the worst case does not trip at all before the generator itself is shut down by its own protection.

Practical relevance

When designing or reviewing an installation with a standby generator as backup, protective-device coordination must therefore be checked separately for the "generator only, no grid" scenario — not solely for the scenario with the grid as source. This requires data from the generator manufacturer (sub-transient reactance, fault-current decay over time) in addition to the regular grid data used for the prospective fault-current calculation. For critical loads (for example a hospital or data centre) this is a materially different calculation than the standard Icu check on grid supply.

Common mistakes

1. Assuming that a protective device correctly sized for the grid's fault current will also trip within the same time during generator-only operation — the fault current a generator can supply is typically an order of magnitude lower than the grid's.
2. Not requesting data from the generator manufacturer (sub-transient reactance, fault-current time profile) when assessing protective-device coordination for the generator scenario.
3. Confusing the load-bank test (against "wet stacking") with a verification of fault-current capability — these are two separate checks: one tests the engine's thermal operation, the other the protective-device coordination during a fault.
4. Assuming the generator's own protection (undervoltage/overcurrent) will always intervene in time without verifying this for the specific make and the specific load.

Related

• Standby generators (§551)
• Generator load testing — load-bank test
• Prospective fault current and Icu sizing
• Automatic transfer switch (ATS) for standby power