IEC 61117 — Assessing Short-Circuit Withstand Strength of Partially Type-Tested Assemblies (PTTA)

In the low-voltage distribution industry, the Partially Type-Tested Assembly (PTTA) represents one of the most prevalent product categories. Unlike a fully Type-Tested Assembly (TTA), a PTTA allows partial modifications, extensions, or customizations to be made on top of a base design that has already passed type tests. This flexibility is invaluable for panel builders who must tailor distribution boards to specific project requirements. However, flexibility introduces an engineering challenge: how does one reliably demonstrate that a modified assembly still possesses adequate short-circuit withstand strength without subjecting every variant to a full type test?

IEC 61117 was developed specifically to address this problem. Although the standard has now been withdrawn, its core methodology — comparison verification, engineering calculation, and extension rules — continues to serve as a crucial reference in the design and certification of low-voltage assemblies. This article provides a deep technical interpretation of IEC 61117 from an engineering practitioner’s perspective, and examines its relevance within the modern IEC 61439 framework.

1️⃣ The Three Pillars of PTTA Short-Circuit Withstand Assessment

The central contribution of IEC 61117 is a systematic framework for verifying the short-circuit withstand strength of PTTA designs. This framework rests on three complementary technical pillars, each suited to different verification scenarios.

1.1 Comparison Method

The comparison method is the most intuitive and widely used approach in IEC 61117. The principle is straightforward: the candidate PTTA design is systematically compared against a reference design that has already passed a full type test. If all critical parameters of the candidate design are equal to or better than those of the reference, its short-circuit withstand strength is considered verified.

Parameters that must be compared include at least the following dimensions: rated peak withstand current (I_pk) and rated short-time withstand current (I_cw) of both main and distribution busbars; busbar material type, cross-sectional dimensions, and spacing; busbar support type, material, and mounting pitch; type and interrupting capacity of incoming and protective devices; enclosure sheet-metal thickness, stiffener arrangement, and ingress protection rating.

1.2 Calculation Method

When the candidate design deviates significantly from any tested reference — making direct comparison impractical — IEC 61117 permits the use of engineering calculation. This method is rooted in the fundamental physics of electromagnetic force and thermal effects, applying closed-form equations to compute the mechanical and thermal stresses imposed on the busbar system and support structures under short-circuit conditions.

The core equation for electromagnetic force is: F = 2 × 10⁻⁷ × (I_p)² × (L/d), where I_p is the peak short-circuit current, L is the conductor length between supports, and d is the conductor spacing. The resulting force must remain below the maximum mechanical withstand capability of both the supports and the busbar conductors themselves.

1.3 Extension Rules

Extension rules constitute the most practically valuable section of IEC 61117. They define the specific types of modifications that are permitted on an already-verified assembly without requiring a fresh short-circuit withstand evaluation. Typical permitted extensions include: adding additional outgoing circuits provided the total current does not exceed the rating; replacing protective devices with equivalents of equal or higher breaking capacity; increasing busbar cross-section provided the rated current capacity is not exceeded; and modifying enclosure dimensions as long as the structural stiffness is maintained or improved.

2️⃣ Engineering Pitfalls and Practical Considerations

2.1 The Dynamic Nature of Short-Circuit Electromagnetic Forces

Short-circuit current is not a steady-state phenomenon. The peak current (the “prospective peak current”) can reach up to 2.5 times the symmetrical RMS value, depending on the system X/R ratio and power factor. This means the mechanical shock load on the busbar system at the instant of a fault is dramatically higher than what a steady-state RMS calculation would suggest. IEC 61117 places strong emphasis on evaluating forces based on peak current rather than RMS current — a distinction that carries profound engineering consequences.

A common design error is selecting busbar support spacing based solely on the rated short-time withstand current (I_cw) while neglecting the instantaneous impact of I_pk. In systems with high X/R ratios (e.g., industrial distribution fed by large transformers), the I_pk/I_cw ratio can exceed 2.5, resulting in electromagnetic forces more than six times those estimated from RMS values alone.

2.2 Thermal Effects and Coupled Temperature Rise

Short-circuit withstand verification must address not only mechanical forces but also thermal effects. IEC 61117 requires evaluation of the I²t (Joule heating) stress on busbars and connection points, ensuring that conductor temperatures remain below levels that would cause significant loss of mechanical strength or insulation damage during the fault duration.

For copper busbars, the typical permissible maximum temperature under short-circuit conditions is approximately 300°C; for aluminum busbars, this limit is around 200°C. Connection points (bolted joints or welds) are the thermal weak spots because contact resistance produces localized hot spots. Engineering best practice calls for silver-plating critical connections and using calibrated torque wrenches to ensure consistent contact pressure.

2.3 Arc Flash and Internal Fault Considerations

Although IEC 61117 focuses primarily on the short-circuit withstand strength of the busbar system, internal arc faults represent a consequential risk that cannot be ignored. When busbar supports fail under short-circuit stress, phase-to-phase arcing can occur — with consequences far more severe than simple busbar damage. The arc plasma generates extreme temperatures (up to 20,000 °C), pressure waves, and toxic byproducts.

Modern distribution board design should incorporate arc flash mitigation measures in line with IEC 61439 series and IEC/TR 61641: arc-resistant partitions, pressure relief channels, and arc flash detection relays. While these measures fall outside the strict scope of IEC 61117, they represent the practical engineering extension of short-circuit withstand philosophy — an assembly that can survive a short-circuit current but cannot contain an internal arc is still an unsafe design.

3️⃣ The Legacy of IEC 61117 in the Modern Standards Landscape

IEC 61117 was formally withdrawn in 2008, its core technical content absorbed into the “Design Verification” clauses of IEC 61439-1 (General Rules) and IEC 61439-2 (Power Switchgear and Controlgear Assemblies). This transition does not render the IEC 61117 methodology obsolete. On the contrary, IEC 61439 has preserved and generalized the comparison and extension-rule philosophy, embedding it within a broader and more rigorous certification framework for low-voltage assemblies.

Viewed from a broader perspective, IEC 61117 embodies an important principle in standardization: adequate verification should not become a barrier to innovation. By providing a scientifically grounded framework for comparison and calculation, the standard creates a rational engineering pathway for product diversification and customization without compromising safety. This principle remains directly relevant to today’s rapidly evolving modular distribution systems and prefabricated data center solutions.

It is worth noting that despite its withdrawn status, certain regulatory bodies and certification agencies — particularly in emerging industrial economies — may still reference IEC 61117 as a benchmark for PTTA short-circuit verification. Therefore, retaining IEC 61117 references in manufacturer technical documentation can be strategically advantageous in some markets.

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