Fault rating is the single most important electrical specification on a switchboard. Get it wrong and the assembly cannot safely contain a short-circuit. Get it right and the board sits comfortably inside its design envelope for the life of the installation.
Quick answer
A switchboard’s fault rating must be at least equal to the prospective short-circuit current (PSCC) at its point of installation, with both short-time withstand (Icw, 1 second) and peak withstand (Ipk) capability assigned. Typical NZ ratings sit between 25 kA and 65 kA on industrial sites, dropping below for small commercial boards. Confirm the upstream transformer impedance, fault contribution and upstream protection settings before selecting.
In this article
This guide explains what fault rating means on a switchboard, the difference between Icw, Ipk and PSCC, how upstream transformer impedance sets the fault level at your point of installation, and what to specify on an NZ project. Clive Wilson Switchboards, based in Invercargill, has been designing and verifying LV switchboards to AS/NZS 61439 for over 55 years.
A switchboard’s fault rating is the maximum short-circuit current the assembly can safely carry and clear without sustaining damage that would compromise its safety or its ongoing service. It is a kA value, declared on the rating plate, assigned through type-testing under AS/NZS 61439.
A fault inside the assembly, between a phase and earth or between phases, drives an extremely high current through the busbars, terminals and protective devices. Without an adequate fault rating, the energy released can rupture the enclosure, melt conductors and pose a real arc-flash risk to anyone nearby. The fault rating proves the assembly was designed and verified to handle that current up to its declared level.
Fault rating is not a single number, it is a paired specification: the short-time withstand current (Icw) and the peak withstand current (Ipk). Both must be assigned, and both must be at least equal to the prospective fault levels at the point of installation.
Three different kA values come up in any conversation about switchboard fault rating. They sound similar but they describe different things.
PSCC is the fault current that would flow at the point of installation if a bolted short-circuit happened there. It is a property of the network and the upstream impedance, not the switchboard. The switchboard must be rated to withstand it.
Icw is the RMS fault current the assembly can carry for a defined time (usually 1 second) without damage. It is the figure that appears most often in fault rating discussions and is what most people mean when they say a board is “rated to 50 kA”.
Ipk is the instantaneous peak current the assembly can handle during the first half-cycle of a fault, before the RMS settles. It is typically a multiple of Icw, often around 2.1 to 2.2 times for industrial systems, set by the X/R ratio of the upstream network.
The PSCC at any point in the network is set by four things, working back from the fault location toward the source.
Our NZ Transformer Fault Current Calculator gives a first-pass estimate of PSCC from transformer kVA, secondary voltage and percent impedance. It is a starting point for early sizing, not a substitute for a full network study on a complex site.
Typical kA ratings we see across the LV switchboards built at Clive Wilson Switchboards:
These are general ranges, not a substitute for site-specific calculation. The right number on your project comes from the network operator, the consulting engineer, or a fault study, not from a category lookup.
A switchboard rated too low for its PSCC is a safety hazard. A switchboard rated significantly too high is overspec, wastes capital, and rarely justifies the cost. The right answer is the actual fault level at the point of installation, with a sensible margin.
The fault rating you specify needs to satisfy two tests: it must be at least equal to the calculated PSCC at the point of installation, and it must coordinate cleanly with the upstream protection.
Confirm the prospective short-circuit current at the switchboard’s point of installation with your network operator or a consulting engineer. For new installations, the network operator usually issues a fault contribution letter. For existing sites, an updated fault study or a worst-case calculation off the transformer rating gives a usable figure.
It is common practice to specify a fault rating slightly above the calculated PSCC. A typical margin is the next step up in the standard kA ratings, for example a 30 kA PSCC site specifying a 36 kA / 1s board. Specifying 65 kA on a 25 kA site is wasteful.
Upstream protection has to clear a fault inside the switchboard’s Icw withstand time. If the upstream breaker takes longer than 1 second to clear, the switchboard’s 1-second Icw is no longer adequate. This is where discrimination studies and protection coordination matter. Current-limiting devices on the upstream side can dramatically reduce the let-through energy the downstream board needs to handle.
The rating plate on the finished assembly carries the declared Icw and Ipk, the test voltage, the rated insulation voltage and the form of internal separation. All of those come from the AS/NZS 61439 type-test pack the platform was verified against, not from a re-calculation done at build time.
For projects where the fault rating still needs to be established, our in-house design team can work through the PSCC calculation, coordinate with the network operator, and produce a fault rating recommendation in the quote pack.
The kA fault rating, technically the short-time withstand current (Icw), is the maximum RMS short-circuit current the assembly is verified to carry for a defined time (typically 1 second) without sustaining damage. It is set by type-test under AS/NZS 61439, declared on the rating plate, and must be at least equal to the prospective short-circuit current (PSCC) at the point of installation.
A short-circuit current has two components, the RMS current that flows for the duration of the fault and the higher instantaneous peak that occurs in the first half-cycle. The assembly’s busbars and bracing must handle both. Icw covers the RMS withstand, Ipk covers the peak. AS/NZS 61439 verifies both during type-test.
The network operator or distribution company usually provides the PSCC at the point of supply. From there, the design engineer calculates the PSCC at each downstream switchboard by accounting for cable impedance and any in-line transformers. For existing sites without documented values, a fault study may be needed.
Yes, with care. Current-limiting devices (fast-acting fuses or current-limiting breakers) reduce the let-through energy reaching the downstream board. The assembly must still be verified to handle the limited fault per AS/NZS 61439, and the coordination has to be documented. This is sometimes called a “conditional short-circuit rating”.
An under-rated switchboard cannot safely contain a short-circuit at its installed location. The risk is enclosure rupture, conductor damage, arc-flash escape, and harm to anyone nearby. Under-rating is a code non-compliance and an unacceptable safety risk. If you find a board under-rated for the current PSCC, isolate, escalate to the engineer of record and plan a replacement or an upstream fault-limiting strategy immediately.
Our Simotrol and Logstrup platforms are AS/NZS 61439 type-test backed across the standard kA bands. The Simotrol envelope covers the most common NZ industrial MSB and MCC requirements; Logstrup is specified on projects that need a particular busbar architecture or withdrawable module option. We confirm the rated Icw and Ipk for each project at quote stage based on the site PSCC.
Reviewed by Chris Wilson, Co-Director, Clive Wilson Switchboards. Registered electrician, 15+ years in LV switchboards. Updated May 2026.
