Siemens NEC Article 409 - Table of Contents

Short-circuit current rating (SCCR) of industrial control panels
Reference Manual, 06/2013, A5E31896849002A/RS-AA/001
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Table of contents
1 Introduction ............................................................................................................................................. 7
2 Overview of NEC Article 409 ................................................................................................................... 9
3 Overview of Supplement SB from the UL 508A standard ....................................................................... 11
4 Basic steps for calculating the SCCR of a control panel ......................................................................... 13
4.1 SB4.2 – Determining the short-circuit current ratings (SCCR) of individual power circuit
components .................................................................................................................................. 13
4.1.1 Basic: Device markings or component instruction sheets ........................................................... 13
4.1.2 Basic: Assumed short-circuit current rating ................................................................................. 14
4.1.3 Basic: Tested combination of devices according to UL508 ......................................................... 14
4.2 SB4.3 – Adaptation of the available short-circuit current rating based on the peak let-
through currents when using current-limiting devices in the feeder circuit .................................. 18
4.2.1 Current-limiting transformers ....................................................................................................... 18
4.2.2 Current-limiting circuit breakers (according to UL489) ................................................................ 18
4.2.3 Current-limiting fuses ................................................................................................................... 20
4.3 SB4.4 – SCCR calculation of the industrial control panel ............................................................ 23
5 Flow chart for calculating the SCCR of a component ............................................................................. 25
5.1 Calculation of the short-circuit current rating of an individual power circuit component
according to UL 508A SB4.2 ........................................................................................................ 25
5.2 Calculation of the short-circuit current rating of current-limiting feeder components .................. 26
5.3 Calculation of the short-circuit current rating of a control panel in accordance with
UL 508A Supplement SB 4.4 ....................................................................................................... 28