Short Circuit Withstand in Switchboards: Icw, Ipk and Conditional Rating

Short Circuit Withstand in Switchboards: Icw, Ipk and Conditional Rating

The short circuit withstand of a switchboard means that the enormous current released by a fault, and the electrodynamic forces that come with it, can be carried without tearing the structure apart. Specifications usually state a single kA figure, yet the standard defines several distinct quantities, and confusing them is the most common source of incorrect sizing.

The key quantities

Quantity	Definition	What it drives
Icw	Rated short time withstand current, usually the rms value for 1 s	Busbar section, insulator spacing, structural reinforcement
Ipk	Rated peak withstand current, the peak of the first half cycle	Busbar supports and mechanical strength
Icc	Rated conditional short circuit current, valid only with a stated protective device	Withstand achieved with a current limiting breaker or fuse
Icp	Prospective short circuit current imposed by the supply at that point	The target the design has to meet
n factor	Ratio between Ipk and Icw, depending on power factor	Peak force calculation

How the value is determined

The chain starts at the transformer. Its rated power, short circuit impedance and the contribution of the upstream network give the prospective short circuit current at the point where the board is connected. Cable and busbar length reduce that current, which is why the figure at the main distribution board differs from the one at a sub board. The design objective is that the verified Icw of the assembly equals or exceeds the prospective current at its point of connection.

What is a conditional short circuit rating for?

Building every board to withstand the full prospective fault current is expensive and often unnecessary. A current limiting breaker or fuse sharply reduces both the peak current and the energy let through. In that case the figure declared for the assembly is a conditional short circuit current, valid only in combination with that protective device. Replacing that breaker on site with a different model effectively invalidates the withstand documentation, a detail that must be communicated to maintenance teams.

Design points to watch

• The neutral bar must withstand at least half the short circuit rating of the phase bars
• Earth bar and PE connections are sized for the same fault current
• Insulator spacing follows from Ipk; increasing the cross-section alone is not enough
• Bolt torque values at busbar joints are part of the rating and assembly records should be kept
• Verification can be achieved by following the rules of a design that has passed type testing; testing every board is not mandatory

Key takeaways: short circuit withstand

• How the value is determined — The chain starts at the transformer.
• What is a conditional short circuit rating for — Building every board to withstand the full prospective fault current is expensive and often unnecessary.
• Standard source — IEC publication catalogue: iec.ch.

Frequently asked questions

Is there a downside to specifying a higher Icw than needed?

Cost and volume grow. A higher Icw means a larger busbar cross-section, more frequent insulators and heavier structural reinforcement. The right approach is to calculate the prospective current and match it with a reasonable margin. Its effect on the busbar side is covered in our article on busbar material selection.

Related articles

• What is Short Circuit Withstand (ICW)? Short Circuit Protection in Electrical Panels.
• Cable Sizing and Voltage Drop Calculation
• IK Impact Rating Explained: IK07, IK08 and IK10 Compared
• All technical articles

At Devpan we determine short circuit withstand together with transformer data, cable routing and protection coordination, and whenever a conditional rating is used we record the valid breaker model in the switchboard documentation. Share your single line diagram and transformer data and we can clarify the required Icw together.