A distribution box must survive a short circuit, not just carry normal current. The two ratings that express this are Icw — how much fault current the assembly can withstand for a stated time — and Ipk — the peak current it can survive mechanically without deforming. Selecting a board on its current rating alone and ignoring these is one of the most consequential errors in distribution design, because a board that fails its short-circuit withstand does not trip politely; it destroys itself. This guide explains what the ratings mean, how they are verified, and how to check them against your installation.
What the Ratings Mean
Icw — Rated Short-Time Withstand Current
Icw is the RMS value of current the assembly can carry during a short circuit for a specified duration — commonly 1 second or 3 seconds — without damage beyond defined limits. It is expressed in kiloamps with the duration stated, because the two are inseparable: a board rated 30 kA for 1 s is not the same as one rated 30 kA for 3 s.
Icw is primarily a thermal rating. The fault current heats the conductors and connections; the assembly must survive that heating without the bars annealing, insulation degrading, or joints welding.
Ipk — Rated Peak Withstand Current
Ipk is the peak instantaneous current the assembly can withstand mechanically. It is the first major peak of the asymmetrical fault current, and it is higher than the RMS value by a factor depending on the circuit’s X/R ratio — the ratio of reactance to resistance at the fault location.
Ipk is a mechanical rating. Currents of tens of kiloamps in adjacent conductors produce electromagnetic forces proportional to the square of the current, attempting to push conductors apart and bend them. The busbar system, its supports, and the enclosure must resist that without deformation.
Icu and Ics — Device Breaking Capacities
These belong to the protective devices, not the assembly, and are frequently confused with the assembly ratings:
- Icu (rated ultimate short-circuit breaking capacity) — the fault current a circuit breaker can interrupt, after which it may not be fit for further service.
- Ics (rated service short-circuit breaking capacity) — the fault current it can interrupt and remain serviceable. Usually expressed as a percentage of Icu.
All three must be considered: the board’s Icw and Ipk describe whether the assembly survives the fault; the devices’ Icu and Ics describe whether they can clear it.
How the Ratings Are Established
Type Testing vs Design Verification
Under IEC 61439-1 and IEC 61439-3, an assembly’s performance can be established by type testing or by design verification — which includes calculation and comparison against a tested reference design. Both are legitimate routes, but they produce different documentation, and you are entitled to know which was used.
- Type-tested (TTA) — the actual assembly was tested. Strongest evidence.
- Design-verified (PTTA under the current standard) — verified by calculation and comparison. Legitimate, but the verification must be documented and traceable.
Ask for the verification route and the evidence. A stated Icw with no traceable basis is a number, not a rating.
What the Standard Requires
IEC 61439-3 covers distribution boards intended for use in ordinary person’s hands or accessible to ordinary persons, and addresses requirements including short-circuit withstand. IEC 61439-1 sets the general rules and the verification methods applicable across the series. The ratings declared by the manufacturer should be stated with reference to the standard and to the verification performed.
Checking the Ratings Against Your Installation
Step 1 — Determine the Prospective Fault Current
The prospective short-circuit current at the point where the board is installed is calculated from the supply characteristics — transformer rating and impedance, upstream cable impedance, and any other source contribution. This is the fundamental input, and it must be calculated for the actual installation, not taken from a typical value.
The fault level is highest close to the transformer and falls with distance along the cable. A board 5 m from a large transformer sees a very different fault level from one 100 m away.
Step 2 — Compare Against Icw and Ipk
- Icw: the prospective RMS fault current at the board must not exceed the declared Icw, and the duration must be consistent with the protective device’s clearing time. If a device clears in 0.1 s and the assembly is rated for 1 s, there is margin — but confirm the comparison is made properly rather than assumed.
- Ipk: the peak asymmetrical current must not exceed the declared Ipk. The peak depends on the X/R ratio at the fault location; a highly inductive circuit produces a higher peak for the same RMS current.
Step 3 — Check the Devices Too
Every device fitted in the board must have a breaking capacity not less than the prospective fault current at its point of installation. It is a common and serious error to fit devices with a lower breaking capacity than the board’s own rating — the board survives and the breaker does not.
Step 4 — Consider Protection by Upstream Device
Some assemblies are declared as suitable for protection by an upstream current-limiting device, where the limiter restricts the let-through energy and the downstream assembly’s withstand requirement is correspondingly reduced. Where this arrangement is relied on, it must be declared by the manufacturer and the specific upstream device must be the one specified — substituting a different upstream device invalidates the arrangement.
Worked Example (Illustrative)
A distribution board is installed approximately 40 m from a 1000 kVA transformer with 6% impedance, on a 400 V system.
Transformer full-load current: 1,000,000 / (√3 × 400) ≈ 1443 A
Prospective short-circuit current at transformer terminals: approximately 1443 / 0.06 ≈ 24,000 A, i.e. roughly 24 kA (ignoring source impedance, which would reduce it somewhat in practice).
At 40 m downstream, the cable impedance reduces the fault level — the reduction depends on the cable size and construction, and must be calculated for the actual cable. As an illustrative outcome, the fault level at the board might fall to somewhere in the region of 15–20 kA.
Selection: the board’s declared Icw must be at or above the calculated fault level, and its Ipk must cover the peak. Every device fitted must have Icu at or above the same fault level where it is installed. If the calculated level is 18 kA, a board rated 25 kA Icw with devices rated at least 18 kA Icu is a defensible selection; a board rated 15 kA is not.
Illustrative example only. Prospective fault current must be calculated for the actual supply, cable, and installation. Confirm against manufacturer data and your project specification.
Application Case: Board Upgraded After a Supply Change
Scenario Constraints
An existing distribution board was installed when the site supply was a 630 kVA transformer. The supply has been upgraded to 1250 kVA to support new equipment, and additional load has been added to the same board. The board’s original short-circuit rating was adequate for the old supply. No changes have been made to the board itself.
Approach
This is the classic scenario where short-circuit ratings are missed, because nothing about the board changed and so nobody thought to recheck it. But the prospective fault current has increased substantially — roughly doubling the transformer capacity increases the fault level materially, particularly if the new transformer has a similar impedance percentage.
The required actions:
- Recalculate the prospective fault current at the board under the new supply, using the new transformer rating and impedance and the existing cable.
- Compare against the board’s declared Icw and Ipk. If the new fault level exceeds them, the board must be replaced or protected by a declared upstream limiting arrangement — not simply left in service.
- Check every device fitted. Devices selected for the old fault level may now be under-rated even if the board itself is adequate.
- Do not assume the existing installation is adequate because it has been running fine. Short-circuit withstand is not demonstrated by normal operation; a board can operate for years within its rating and fail catastrophically on the first fault.
Common Mistakes
- Selecting a board on current rating alone. A 250 A board and a 250 A board can have wildly different short-circuit withstand.
- Not recalculating after a supply change. Transformer upgrade, new generator, or additional parallel source all raise the fault level.
- Fitting devices with lower breaking capacity than the board’s rating. The board survives, the breaker does not.
- Comparing Icw values without checking duration. 30 kA for 1 s and 30 kA for 3 s are different ratings.
- Ignoring Ipk and looking only at the RMS value. The mechanical peak is what bends the bars.
- Relying on upstream current limiting without using the specified device. The arrangement is declared for a specific device; substituting another invalidates it.
- Accepting a stated rating without asking for the verification basis. Ask whether it is type-tested or design-verified, and to which standard.
Acceptance Checks
- Prospective short-circuit current calculated for the actual supply and installation, documented with the assumptions used.
- Board’s declared Icw — with duration — verified as not less than the calculated fault level.
- Board’s declared Ipk verified as covering the peak asymmetrical current.
- Every fitted device’s Icu verified as not less than the fault level at its point of installation.
- Verification basis obtained — type test or design verification — and the standard stated (IEC 61439-3 / IEC 61439-1).
- Where upstream current limiting is relied on, confirmation that the installed device is the one the declaration specifies.
- Ratings recorded on the board’s documentation and on its label, so a future modification triggers a recheck.
Frequently Asked Questions
What is the difference between Icw and Ipk?
Icw is the RMS fault current the assembly can withstand thermally for a stated duration — 1 s or 3 s, typically. Ipk is the peak instantaneous current it can withstand mechanically. Both must be checked: Icw against heating, Ipk against electromagnetic force.
Is a higher Icw always better?
It is better for your installation only if your fault level requires it. Over-specifying costs money; under-specifying is dangerous. Calculate the actual fault level and select to it with appropriate margin.
Does the device breaking capacity have to match the board rating?
Each must be adequate for the prospective fault current at its own location. A board rated 50 kA with devices rated 10 kA is not a safe installation — the devices must be rated for the fault level where they are installed.
What if my fault level exceeds the board rating?
Replace the board with one adequately rated, or use a declared upstream current-limiting arrangement with the specific device the manufacturer specifies. Do not leave an under-rated board in service on the basis that it has not failed yet.
How do I know if a stated rating is type-tested?
Ask. IEC 61439-1 permits verification by testing, calculation, or comparison with a tested reference design, and the manufacturer should be able to state which was used and provide supporting documentation.
Do I need to recheck ratings after adding load?
Adding load does not by itself raise the fault level, but it changes the operating current and may change the devices fitted. Recheck whenever the supply changes — transformer upgrade, new generator, additional source — because that does change the fault level.
Does cable length reduce the fault level enough to ignore?
Cable impedance does reduce fault level with distance, and sometimes substantially. But it must be calculated for the actual cable, not assumed. A board close to a large transformer sees a very high fault level regardless of any short cable run.
Specify Verified Short-Circuit Ratings with BANGE Electric
BANGE Electric supplies distribution boxes and assemblies with declared Icw and Ipk values and documented verification to IEC 61439-3 and IEC 61439-1. Send us your supply characteristics — transformer rating and impedance, cable sizes and lengths, and any other source contribution — and we will calculate the prospective fault current and confirm the assembly and device ratings required at your board.
Compare distribution box types or contact our technical team.
Requirements may vary by application, market, and applicable standard. Figures given here are illustrative examples; always confirm against manufacturer data and your project specification.