Switchboard Specification
March 2026 · 7 min read
A switchboard needs a fault rating that matches the actual fault conditions at its point of installation. This is not something that should be guessed, copied from a previous job, or treated as a generic default. This guide explains what fault rating means in practice, why it matters, and what information should be confirmed before pricing or manufacture begins.
Fault rating is a core technical requirement that affects safety, equipment suitability, protection strategy, switchboard construction, device selection, quotation accuracy and compliance. If the required fault level is unclear at the enquiry stage, the board may be priced on assumptions. If those assumptions later change, the design and commercial basis can change with them.
In practical project terms, fault rating refers to the prospective short-circuit current (PSCC) that the switchboard assembly and its associated protective devices must be capable of withstanding or interrupting under fault conditions.
This figure is determined by the electrical system upstream of the board, including the supply network, transformer impedance and the distribution hierarchy. It is not selected by the switchboard manufacturer. It is a characteristic of the system at the point of installation, and it must be provided as a design input.
The switchboard must then be specified and built so that every component in the assembly is rated to safely withstand or interrupt that level of fault current. This includes the busbars, incoming device, outgoing protective devices and the enclosure assembly itself.
A switchboard may appear physically suitable for a project and still be wrong if the fault rating basis has not been correctly established.
Every component in the switchboard, including the incomer, outgoing circuit breakers, busbars and isolators, must have an interrupting or withstand rating that meets or exceeds the prospective fault current at the installation point. If any device in the assembly is underrated for the fault level present, it may fail to operate safely under fault conditions.
Fault rating sits alongside the broader protection philosophy for the project. Understanding the fault level at each board in the system is part of ensuring that protective devices are correctly selected and coordinated so that only the device closest to a fault operates, leaving the rest of the system unaffected.
Higher fault duties can influence switchgear selection, the busbar system specification, internal construction detail and overall quotation value. These are not minor variations. An incorrect fault rating assumption can require significant redesign if corrected late in the project.
New Zealand’s mandatory standard for low-voltage switchgear and controlgear assemblies requires that the assembly be verified as suitable for the electrical conditions of the installation. The fault level at the point of supply is a fundamental input to that verification. A board that has not been assessed against the correct fault level cannot be reliably demonstrated as compliant.
The required fault rating is determined by the characteristics of the electrical system at the point where the board will be connected. The main factors include:
From a manufacturer and quotation perspective, the fault level should be provided by the project’s electrical designer or confirmed from documented network information. It is not something the switchboard manufacturer can calculate from the project description alone.
Not every board on a site requires the same fault rating. The available fault current decreases with distance from the source due to the impedance of the cables and conductors in the path. A main switchboard connected close to the incoming supply will typically face a higher prospective fault current than a distribution board fed from it downstream.
This is one reason a main switchboard and a sub-distribution board may need to be assessed separately, even on the same project. The closer the board is to the source, the more critical it becomes to correctly establish the fault level at that point in the system.
A common source of confusion is the assumption that the interrupting rating of the circuit breaker alone answers the whole question. It does not.
The interrupting capacity of a protective device is the maximum fault current that device can safely break. But the switchboard fault rating covers the entire assembly, including busbars, connections, enclosure construction and all other components, not just the protective devices.
In practice:
Treating a single device rating as the whole answer can result in an undersized or non-compliant assembly.
Fault rating has direct commercial implications. Depending on the project it can affect device selection, busbar construction, internal assembly design, enclosure arrangement, compliance documentation and overall cost.
That is why fault rating should be treated as a core enquiry input alongside main rating, outgoing schedule, board role in the system, segregation requirements and enclosure IP requirements. If any of these are missing, the quotation is more likely to rely on assumptions that may need to be resolved later.
The correct fault rating for a switchboard is not a generic choice. It should reflect the actual prospective short-circuit current at the point of installation and be confirmed clearly in the project documentation before pricing or manufacture proceeds.
Where the fault level is uncertain, clarifying it early supports safer design decisions, clearer equipment selection, more accurate quotations and fewer avoidable revisions later in the project.
Once a board is built and installed, the confirmed fault rating should be clearly and permanently identified on the assembly. For engraved fault rating labels and circuit identification plates, see Clive Wilson’s engraving service.
Need help confirming fault rating for your switchboard project?
Clive Wilson Switchboards · 31–33 Clyde Street, Invercargill · 03 218 3459
