A busway has to survive the short-circuit current available at its feeding transformer without fire, ejected parts, or loss of the protective circuit. The two ratings that express this are Icw (r.m.s. short-time withstand current, normally declared for 1 s) and Ipk (rated peak withstand current). Whether a declared Icw is real depends on conductor section, bracing, joint design, and support spacing. This article explains how to read, specify, and verify these values to IEC 61439-6.
What short-circuit withstand means for a busway
A busbar trunking system (busway) is a passive conductor path: it does not interrupt fault current, it carries it until the upstream protective device clears it. Its short-circuit performance is therefore described by withstand ratings, not by breaking capacity.
IEC 61439-1 divides design verification into numbered checks; the relevant ones here are:
- 10.10 — verification of temperature-rise limits (the rating you use every day)
- 10.11 — verification of short-circuit withstand strength (the rating that keeps the installation safe on the worst day)
- 10.9 — dielectric properties, which is re-checked after the short-circuit test sequence
IEC 61439-6 adds the particular requirements for busbar trunking systems, including tap-off units, joint arrangements, and the marking of ratings on each length. In North America the equivalent product standards are UL 857 (busways) and NEMA BU 1 (busway construction and ratings). Older projects may still reference IEC 60439-6, the predecessor to IEC 61439-6.
One point buyers often miss: IEC 61439-1 does not require the assembly to be fully operational after the short-circuit test. The acceptance criteria focus on no danger to persons, no fire or explosion, retention of covers and live parts, and continuity of the protective circuit. Deformation of bars and insulators is acceptable within those limits. A busway that has been through a real fault is a replacement item, not a repaired item.
The ratings: Icw, Ipk and the peak factor
Rated short-time withstand current (Icw) — the r.m.s. value of current the assembly can carry under fault conditions for a declared duration. Unless the manufacturer states otherwise, the declared duration is 1 s; 0.5 s and 3 s values are also used and must be stated on the rating plate.
Rated peak withstand current (Ipk) — the peak value of the first major loop of fault current. It drives the electromagnetic forces between conductors. IEC 61439-1 fixes the relationship between the two through a peak factor n taken from the standard’s table:
| Icw (r.m.s., kA) | Peak factor n | Resulting Ipk | Assumed power factor |
|---|---|---|---|
| Icw ≤ 5 | 1.5 | e.g. 5 kA → 7.5 kA | 0.7 |
| 5 < Icw ≤ 10 | 1.7 | e.g. 10 kA → 17 kA | 0.5 |
| 10 < Icw ≤ 20 | 2.0 | e.g. 20 kA → 40 kA | 0.3 |
| 20 < Icw ≤ 50 | 2.1 | e.g. 50 kA → 105 kA | 0.25 |
| 50 < Icw | 2.2 | e.g. 65 kA → 143 kA | 0.2 |
Example only: Icw 50 kA / 1 s gives a minimum Ipk of 2.1 × 50 = 105 kA. Always confirm the values on the manufacturer’s rating plate and type-test report.
Duration conversion — a common trap in procurement
Because thermal damage follows I²t, ratings at different durations can be compared approximately with I²t = constant:
Icw(1 s) ≈ Icw(t) × √(t / 1 s)
Example: a busway declared at 70 kA for 0.5 s corresponds to roughly 70 × √0.5 ≈ 49.5 kA for 1 s. A project specification that simply says “50 kA Icw” is ambiguous unless the duration is written too. Treat this conversion as an engineering approximation — the manufacturer’s declared values take precedence.
Rated conditional short-circuit current (Icc)
IEC 61439-1 also recognises a conditional rating, where the busway relies on a current-limiting device (fuses or a current-limiting MCCB) installed in the assembly to limit both let-through energy and peak current. If a quotation quotes “Icc” rather than “Icw”, the rating is only valid with the named device, at the named rating, and with the limiting characteristic documented.
Where the mechanical stress comes from
Two conductors carrying fault current in opposite directions repel each other. For parallel bars the force per unit length is:
F/L = 2 × 10⁻⁷ × i² / d (N/m, with i in amperes and d in metres)
Example (illustrative): Ipk = 105 kA, phase centre distance d = 80 mm = 0.08 m:
F/L = 2 × 10⁻⁷ × (105 000)² / 0.08 ≈ 27 600 N/m ≈ 2.8 tonnes-force per metre, applied as a hammer blow within the first half-cycle.
That is why short-circuit strength is a mechanical design problem as much as an electrical one. The load path runs: bars → insulating supports/bracing → housing → supports → building structure.
What carries the force
- Conductor section and bar spacing: compact (sandwich) construction, with laminated bars held in a tight insulating stack, resists deformation better than widely spaced air-insulated bars of equal section. See the dedicated comparison of sandwich-type vs air-insulated busway.
- Bracing / insulator spacing: the declared Icw is valid only for the bracing arrangement that was type-tested.
- Joints: bolted joints carry the full force in tension as well as current. Joint design, plating (tin or silver), and torque are part of the tested construction.
- Support spacing: typically 1.5 m to 3 m on horizontal runs for industrial ratings; the declared maximum spacing is part of the rating and must not be increased on site to save hangers.
- Tap-off units: plug-in contacts and tap-off units carry their own short-circuit rating, frequently lower than the trunking itself. They must be specified separately — see the busway tap-off unit selection guide.
Sizing Icw from the transformer: worked example
The prospective short-circuit current at the busway origin is dominated by the transformer. A first estimate (ignoring upstream source impedance and the impedance of the connection, so the real value is lower):
Isc ≈ In / (Uk/100) where In is the transformer rated current and Uk is its short-circuit impedance in %.
| Transformer | In at 400 V | Uk | Indicative Isc | Typical Icw specified |
|---|---|---|---|---|
| 630 kVA | 909 A | 4 % | ≈ 22.7 kA | 30 kA / 1 s |
| 1000 kVA | 1443 A | 6 % | ≈ 24.1 kA | 30 kA / 1 s |
| 1600 kVA | 2309 A | 6 % | ≈ 38.5 kA | 50 kA / 1 s |
| 2000 kVA | 2887 A | 6 % | ≈ 48.1 kA | 50 kA / 1 s |
| 2500 kVA | 3608 A | 6 % | ≈ 60.1 kA | 65 kA / 1 s |
Example only. The figures ignore source impedance, cable or busway impedance, motor contribution, and temperature effects. The final value must come from the project’s short-circuit study and be confirmed against the manufacturer’s data.
Application Scenario: Busway Feeding a Production Line in an Assembly Plant
Scenario constraints
A 1600 kVA, 400 V, 6 % transformer feeds a busway running 30 m along an assembly line. Ambient temperature near the process reaches 40 °C. The line has welding equipment, so the load is unbalanced and harmonic-rich. Tap-off points move when the line is re-balanced. Production stops are short, so any work on the busway must be planned and quick.
Selection approach
- Prospective fault ≈ 38.5 kA → specify Icw ≥ 50 kA / 1 s, Ipk ≥ 105 kA, verified to IEC 61439-6 design verification 10.11.
- Trunking rated current: In chosen above the calculated load with the manufacturer’s derating for 40 °C ambient applied (see the busway derating guide); oversizing the conductor also raises Icw headroom.
- Neutral conductor sized for the actual harmonic load — 100 % or 200 % of phase section depending on the measured or calculated third-harmonic content.
- Degree of protection IP54 to handle coolant mist and conductive dust (IEC 60529).
- Supports at the declared spacing (≤ 2 m in this case), with flexible/spring supports in the vertical section and an expansion unit at the building movement joint.
Common mistakes
- Comparing a 0.5 s Icw figure with a 1 s project requirement.
- Specifying the trunking rating but not the tap-off unit rating, then fitting tap-offs with lower withstand.
- Increasing support spacing by 30–50 % to save hangers, which changes the mechanical boundary condition used in the type test.
- Mixing joint kits or busway lengths from different manufacturers; the tested joint is a specific combination of bar geometry, plating, bolt grade, and torque.
- Bolting joints without a calibrated torque wrench and without recording the value.
- Omitting the expansion unit across a building movement joint, so thermal and structural movement is taken up by the joints.
Acceptance checkpoints
- Rating plate on each length states In, Ue, IP, Icw (with duration), Ipk, and the referenced standard.
- Type-test report / design verification record references IEC 61439-6 and clause 10.11; verify the tested arrangement matches what is being delivered (bar section, bracing, spacing, joint type).
- Routine verification as required by IEC 61439-1 Clause 11, including the dielectric test, is documented per manufacturing batch.
- Protective-circuit continuity measured; joints torqued with recorded values.
- Insulation resistance measured and recorded before energisation.
- After energisation at load, first thermographic survey: any joint reading markedly hotter than comparable joints under the same load is investigated before handover. (A difference-based screening criterion is common practice; the absolute limits are set by IEC 61439-1 and the project specification.)
Selection checklist
- Prospective short-circuit current at the point of installation, from a study — not from a rule of thumb alone.
- Icw with the duration stated (e.g. 50 kA / 1 s) and the matching Ipk.
- Whether the rating is unconditional (Icw) or conditional (Icc, tied to a specific device).
- Declared support spacing and bracing arrangement.
- Tap-off unit rating and its conditional device, if applicable.
- Degree of protection (IEC 60529) and impact rating (IEC 62262).
- Conductor material and plating (see copper vs aluminium conductor considerations), joint torque values.
- Derating for ambient temperature, altitude above 2000 m, and enclosure — IEC 61439-1 sets the standard service conditions (ambient up to +40 °C, 24 h average ≤ +35 °C, altitude ≤ 2000 m).
Quality inspection checkpoints at manufacture and delivery
- Type test evidence: IEC 61439-6 short-circuit test report with test circuit parameters, duration, peak current, and post-test condition. Ask for photographs or the recorded test sequence of the actual arrangement supplied.
- Dimensional and material check: conductor section and material verified against the drawing; plating thickness measured on sample bars.
- Joint assembly: bolt grade, washer arrangement, and torque verified on sample joints; torque wrench calibration certificates current.
- Routine verification: dielectric test per unit/batch, protective-circuit continuity, visual and dimensional inspection.
- Marking: rating plate content complete and durable (legible after handling and cleaning).
- Packaging: bar ends protected; joint contact surfaces protected from contamination and mechanical damage.
How to evaluate a supplier’s short-circuit claim
- Ask which arrangement was tested and whether the supplied arrangement is identical — bar section, spacing, bracing, joint, and support distance.
- Ask whether the claim is verified by test, by comparison with a reference design, or by calculation. IEC 61439-1 permits certain verifications to be met by comparison with a reference design under defined conditions; the rules must be stated, not assumed.
- Ask for the tap-off unit rating separately, with its conditional device.
- Ask who performed the test and whether the report is issued by a third-party laboratory.
- Ask for the declared support spacing in writing and check it against the ceiling/floor structure available on site.
- Ask what changes when the rating is conditional — device model, setting, and the limiting curve.
Frequently Asked Questions
1. Is a higher Icw always better?
Not automatically. A higher Icw usually means a larger conductor section, tighter bracing, or a different construction — all of which affect price, weight, voltage drop, and sometimes the temperature-rise performance. Specify the value the short-circuit study requires, plus the margin the project standard demands.
2. Can I use a 0.5 s Icw rating for a 1 s requirement?
Only by conversion, and only as an approximation: I²t ≈ constant gives Icw(1 s) ≈ Icw(0.5 s) × 0.707. Confirm with the manufacturer and record the conversion in the submittal.
3. Does the tap-off unit inherit the trunking’s Icw?
No. Tap-off units and plug-in contacts have their own ratings, often expressed as a conditional rating with a specific protective device. Specify them explicitly.
4. What happens to a busway after a real fault?
Even if it looks undamaged, treat it as suspect. IEC 61439-1 acceptance after the short-circuit test does not require continued operation. Inspect joints, measure insulation resistance, and replace affected lengths according to the manufacturer’s instruction.
5. How does support spacing affect the rating?
The declared Icw applies to the tested mechanical arrangement, including the distance between supports. Increasing spacing on site invalidates the tested condition. Keep the declared spacing or obtain written confirmation of an alternative.
6. Is UL 857 equivalent to IEC 61439-6?
Both cover busways, but they are different standards with different test regimes and marking requirements. For projects in North America specify UL 857 (and NEMA BU 1 where referenced); for IEC markets specify IEC 61439-6. Do not assume a crosswalk without the manufacturer’s documented evidence.
7. What should appear on the rating plate?
Manufacturer and type, rated operational voltage and frequency, rated current In, degree of protection, Icw with its duration and Ipk (or the conditional rating and device), and the reference standard.
Need Help Specifying a Busway for Your Fault Level?
BANGE Electric supplies busbar trunking systems for industrial plants, high-rise risers, and data halls, and can provide the rating data, type-test references, and joint torque schedule your submittal needs. Send your single-line diagram, prospective fault current, ambient conditions, and support layout to our engineering team via the busway systems page or the contact page.
Requirements may vary by application, market, and applicable standard.
