IEC 61660 Explained: Calculating DC Short-Circuit Currents
A short circuit on a DC bus behaves very differently from one on an AC grid. IEC 61660 is the standard engineers use to calculate those DC fault currents in battery storage systems and substations. That way, protection devices get sized right.
| Quick Answer IEC 61660 is a three-part IEC standard for calculating short-circuit currents on DC auxiliary buses. It covers rectifiers, batteries, capacitors, and DC motors. BESS engineers still use it today, even though it predates lithium-ion and was written for lead-acid batteries. |
What Is IEC 61660?
IEC 61660 first appeared in 1997. Its full title is “Short-circuit currents in d.c. auxiliary installations in power plants and substations.” IEC Technical Committee 73 developed it. That same committee owns IEC 60909, the equivalent standard for AC systems.
The standard exists because DC faults don’t behave like AC ones. An AC fault current oscillates and decays in a set way.
A DC fault current rises and falls on its own curve instead. Each source shapes that curve in its own way.
IEC 61660-1 is still listed by the IEC as active and current, even though no revision has replaced it since 1997.
This matters for BESS design. Every battery rack, busbar, and DC disconnect on the storage side of the inverter sits on a DC bus.
So when a fault happens there, protection devices must clear a current whose shape this standard was built to predict.
The Three Parts of IEC 61660
The standard is split into three linked documents. Each one covers a different piece of the fault-current picture.
| Part | Title | What It Covers |
|---|---|---|
| Part 1 | Calculation of short-circuit currents | The core method: peak current, quasi-steady-state current, and how to combine several sources |
| Part 2 | Calculation of effects | Mechanical and thermal stress on rigid conductors and busbars, caused by the current from Part 1 |
| Part 3 | Examples of calculations | A technical report with worked examples, so engineers can check their own math against a known result |
Part 1 does the heavy lifting for most BESS projects. From there, Part 2 turns that current into a mechanical design check, while Part 3 serves as a reference for checking the numbers.
Part 3 wasn’t even finished when Part 1 published in 1997. The original foreword lists it as “in preparation,” and it only appeared a few years later as a technical report.
How IEC 61660-1 Calculates DC Short-Circuit Current
Two values matter most to a protection engineer.
Two Values That Matter: Peak and Quasi-Steady-State Current
The peak short-circuit current, written as ip, is the highest instant current a fault ever reaches. So this value sets the rating for breakers, fuses, and busbars, since they must survive that first spike without failing.
The quasi steady-state current, written as Ik, is the current level one second after the fault starts.
Engineers then use this lower, settled value to set fuse and relay trip points, since it reflects what a slower device actually has to clear.
Between those two points sits the time to peak, tp. It tells engineers how fast the current climbs before it starts to fall.
Maximum and Minimum Short-Circuit Current
The standard actually calls for two separate calculations, not one. For equipment ratings, engineers work from conditions that produce the highest possible fault current: cooler conductor resistance and a fully charged battery.
For fuse and protection settings, they work from conditions that produce the lowest fault current instead: conductor resistance at the system’s maximum operating temperature, and a battery closer to fully discharged. Each case still has its own peak and quasi-steady-state current, so a full study runs through both.
Four Sources of DC Fault Current
The method models four types of equipment that can feed a DC short circuit:
- Rectifiers in a three-phase AC bridge connection
- Stationary lead-acid batteries
- Smoothing capacitors
- DC motors with independent excitation
Each source has its own current shape. A rectifier’s fault current follows the AC network behind it.
Meanwhile, a battery’s current rises with a time constant set by its own resistance and inductance, and a capacitor discharges fast, then decays.
A motor is different again, since it keeps feeding current for a short time while its stored mechanical energy converts back into electrical energy.
When more than one source can reach the same fault point, a correction factor (the standard calls it sigma) combines their peaks into one worst-case total. This matters because the individual peaks don’t always line up in time.
Picture a substation battery room with a rectifier charger and a battery bank feeding the same DC bus. The rectifier’s current might peak in a few milliseconds, while the battery’s current keeps climbing for longer, since its inductance slows the rise. Because of that timing gap, the correction factor keeps the combined peak from being just a simple sum of two separate maximums.
Why IEC 61660 Still Matters for Lithium-Ion BESS
The standard was written in 1997, years before lithium-ion reached grid-scale storage. That gap still matters for BESS engineers today.
The Lithium-Ion Gap
The battery clause only models stationary lead-acid cells, and it gives no official method for lithium iron phosphate or other lithium-ion chemistries. So a literal reading of the standard doesn’t cover the battery technology in most BESS projects built today.
How Engineers Work Around It Today
In practice, the industry leans on two workarounds.
First, many engineers treat a lithium-ion string’s short-circuit response like a capacitor discharge. Both show a fast spike, then a decay, and the standard already has a capacitor method built in.
Second, most BESS integrators pull short-circuit current data straight from the cell or rack maker’s datasheet. They then feed those figures into the rest of the calculation, alongside the rectifier and capacitor terms.
Commercial short-circuit analysis software built around this method now adds lithium-ion battery models as an extension beyond the base document. Engineers keep the same combination approach while plugging in a chemistry-correct source model.
IEC 61660 vs. Related DC and BESS Standards
This standard doesn’t work alone, since a few other documents reference it or cover nearby ground.
| Standard | Relationship to IEC 61660 |
|---|---|
| IEC 60909 | The AC equivalent. This standard’s own foreword names IEC 60909 as a companion reference for the rectifier’s AC-side contribution. |
| IEC 62485-2 | Covers stationary battery installation safety and cites this method for the short-circuit figures that feed protection and ventilation design. |
| IEC 62933-5-2 | The BESS safety standard covers electrical safety more broadly, including how DC-side fault-current figures feed into protection design. |
| IEC 62619 | Covers cell and battery safety testing rather than system-level fault current, so it complements this standard instead of overlapping it. |
What This Means for BESS DC Bus Protection Design
For a project engineer, the takeaway is simple: first, size DC breakers and busbars to the peak current, ip.
Set fuse and relay trip points from the quasi-steady-state current, Ik. Then combine every source that can feed the same fault point, not just the battery.
Since the method predates lithium-ion, document which battery-current approach the design used. Note whether it was capacitor-equivalent modeling or manufacturer datasheet figures, so the calculation can be reviewed and repeated later.
Also check Part 2 once the current is known. A busbar sized only for steady-state load current can still fail mechanically under a DC fault it was never checked against.
Frequently Asked Questions
Does IEC 61660 apply to lithium-ion BESS?
Not directly, since the battery clause only covers stationary lead-acid batteries. Engineers commonly adapt the capacitor model instead, or use manufacturer-supplied short-circuit data for lithium-ion strings.
What’s the difference between IEC 61660 and IEC 60909?
IEC 60909 calculates short-circuit currents in three-phase AC systems. This standard calculates them on the DC side instead, where fault currents don’t oscillate and decay the same way.
Is there a newer edition of IEC 61660?
The first edition dates to 1997, with corrigenda issued in 1999 and 2000. Even so, no second edition has replaced it as of 2026.
What does Part 2 cover that Part 1 doesn’t?
Part 1 calculates the fault current itself, while Part 2 uses that current to calculate the mechanical and thermal stress it puts on rigid conductors and busbars.
Who uses IEC 61660 in a BESS project?
Protection and electrical engineers use it to size DC breakers, fuses, and busbars during the design phase, well before commissioning.
Further Reading
For more on how DC-side faults are handled in BESS design, see our guide to BESS short-circuit protection.
To see how DC bus sizing fits into overall system specs, read Understanding BESS Specifications.
For how a BMS ties into protection design, see BMS Functional Safety, HARA, and FMEA.
For the broader BESS standards landscape, see IEC 62933 Energy Storage Standards, IEC 62933-5 Safety Standards, and our BESS Certifications Guide.
References
IEC 61660-1:1997, Short-circuit currents in d.c. auxiliary installations in power plants and substations – Part 1: Calculation of short-circuit currents. International Electrotechnical Commission (with Corrigenda 1:1999 and 2:2000).
IEEE Industry Applications Society, “DC Arc Flash: History, Physics, and Modeling for Battery, Capacitor, and PV Systems,” first presented at the IEEE Electrical Safety Workshop (ESW) 2023 — covers DC incident-energy modeling approaches for large battery systems, including lithium-ion, and their relation to standards-based short-circuit current models.