Short-circuit aspects: NEC vs IEC

Last edit: 09/05/2023


There are several differences between the North American and International electrical standards (published by IEC).

One of those differences is the way they deal with the short circuit aspects.

There are five important parameters, linked to a short circuit current, that machinery manufacturers should know. The first two are referring to the value of short circuit current that the distribution network can generate in any point, the last three are referring to the component capacity of withstanding such a current. 

Ik: steady-state short-circuit current

IEC 60909-0] 3.10 Steady-state short-circuit current. Ik. Rms value of the short-circuit current which remains after the decay of the transient phenomena

Therefore, according to IEC 60909, Ik is the steady state short-circuit current. It can be defined as the r.m.s value of the fault current once the transitional period has ended (a few cycles). It is “applicable” in any point of the circuit. That means it is the generic term to indicate the value of the short circuit current. The figure details the concept.

Please consider that, at the very first moment a short circuit is established, based upon the inductive part of the line, an overshoot is normally present, which value is indicated as Ip (Peak short-circuit current). 

Icu Ultimate short-circuit breaking capacity

[IEC 60947-2] 2.15.1 ultimate short-circuit breaking capacity – Icu. A breaking capacity for which the prescribed conditions according to a specified test sequence do not include the capability of the circuit breaker to carry its rated current continuously.

Therefore, the Icu is the maximum value of the short-circuit current that the component can interrupt. The testing conditions are specified in the standard and are based upon an Open (under short circuit), elapsed time, Close and again short circuit sequence (O-t-CO).

Icw Rated short-time withstand current

[IEC vocabulary 441-17-17]: short-time withstand current Icw. The current that a circuit or a switching device in the closed position can carry during a specified short time under prescribed conditions of use and behaviour.

According to EN 61439-1, Icw is the rated short-time withstand current; it is defined as the r.m.s value of a.c. or mean value of d.c. short-time current, declared by the ASSEMBLY manufacturer, that can be withstood under specified conditions, defined in terms of a current and time.
Only Open Air circuit breakers have a Icw value. It is instead the reference value for Disconnectors.

SCCR and/or Icc

SCCR (EN 60204-1) is the short-circuit current rating value of prospective short-circuit current that can be withstood by the electrical equipment for the total operating time (clearing time) of the short-circuit protective device (SCPD) under specified conditions.

Icc (EN 61439-1) is the rated conditional short-circuit current of a circuit of an ASSEMBLY is the value of short-circuit current, specified by the manufacturer, which that circuit, protected by a current limiting SCPD specified by the manufacturer, can withstand satisfactorily for the operating time of the  device under the test conditions specified in Clause 10. 
The breaking capacity and current limitation characteristic (I²t, Ilt) of the specified short-circuit protective device shall be stated by the ASSEMBLY manufacturer, taking into consideration the data given by the device manufacturer.
The value must be indicated, at least, on the electrical drawings of the Industrial Control Panel or Electrical Assembly. 

USA terms

In USA they use mainly three terms:

Available Fault Current AFC

[NEC 2020] Fault Current, Available (Available Fault Current). The largest amount of current capable of being delivered at a point on the system during a short-circuit condition. 
Informational Note: A short-circuit can occur during abnormal conditions such as a fault between circuit conductors or a ground fault. See Informational Note Figure 100.1.

That would be the equivalent of Ik according to IEC definition


Interrupting Rating IR

[UL 489] 2.47 INTERRUPTING RATING – the highest RMS symmetrical current at rated voltage that a device is intended to interrupt under standard test conditions.

That would be the equivalent of Icu, according to IEC definition. However the IEC and UL standards have different testing procedures; therefore the two terms do not mean the same testing procedure.


Short Circuit Current Rating SCCR

[UL 508A] 2.44 SHORT CIRCUIT CURRENT RATING – The prospective symmetrical fault current at a nominal voltage to which an apparatus or system is able to be connected without sustaining damage exceeding the defined acceptance criteria.

Despite being the same concept as Icc and SCCR according to IEC standards, for Industrial Control Panel, the way to calculate the SCCR is quite different between IEC and North America.

From 2017 to 2020 NEC edition

Please be aware that in the 2020 edition of the NEC (NFPA 70), article 408.6 was added. It requires that for Switchboards, switchgear, and panelboards that the value of the AFC be indicated on the panel. That makes the work of the Authority Having Jurisdiction easies when they have to check the panel safety.

408.6 Short-Circuit Current Rating. Switchboards, switchgear, and panelboards shall have a short-circuit current rating not less than the available fault current. In other than one and two family dwelling units, the available fault current and the date the calculation was performed shall be field marked on the enclosure at the point of supply.

In the 2017 edition of the NEC, for the first time, that requirement was stated for Industrial Control Panels for industrial machinery. It is article 670.5:

(1)  Industrial machinery shall not be installed where the available short-circuit current exceeds its short-circuit current rating as marked in accordance with 670.3(A)(4).

An example of field marking with AFC value for Switchboards, switchgear, and panelboards is reported.

No significant changes have been made to the article 670.5 Short-Circuit Current Rating in the 2020 edition:

(1) Industrial machinery shall not be installed where the available fault current exceeds its short-circuit current rating as marked in accordance with 670.3(A) (4).
(2) Industrial machinery shall be legibly marked in the field with the available fault current. The field marking(s) shall include the date the available fault current calculation was performed and be of sufficient durability to withstand the environment involved.

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