National Electrical Code 2005: Unifying Control Panels
Editor’s Note: This is the seventh in a continuing series covering significant new issues raised by the 2005 Edition of the National Electrical Code.
Industrial control panels, used to control such systems as lighting, conveyor systems and air conditioning, in many cases, are manufactured in the field. Even thought the individual devices used in a system might be Underwriters Laboratories (UL)-listed, the panel itself is not. This has been a concern for both installers and inspectors, especially because these panels have been increasingly misused.
In an effort to close existing gaps in safety requirements for industrial control panels, as well as to advance safety in the industry, NEC's new Article 409, “Industrial Control Panels,” details requirements for the installation and inspection of “control panels that are intended for general use and that operate at a voltage of 600 volts or less.”
These assemblies of industrial control components are intended to provide control logic and distribution of power to various external motor loads, non-motor loads or a combination of both. Article 409 is not limited to industrial occupancies; rather, it has general application. Industrial control panel is defined in Section 409.2 as “an assembly of a systematic and standard arrangement of two or more components such as motor controllers, overload relays, fused disconnect switches, circuit breakers and related control devices such as pushbutton stations, selector switches, timers, switches, control relays and the like with associated wiring, terminal blocks, pilot lights and similar components. The industrial control panel does not include the controlled equipment.”
Control panels do not exist in isolation, of course; they are inextricably integrated with the circuits and systems for which they are used. Article 409 affects the way equipment is designed and constructed. In order to unify applications, a reference chart (Section 409.3) lists all of the other NEC articles that contain requirements applicable to the circuits and equipment discussed in Article 409, such as Article 440 for air-conditioning and refrigeration equipment. This provides a roadmap through the code to which a designer, installer, manufacturer or inspector may need to ensure full compliance.
The main feature of Article 409 is its provision of minimum requirements to facilitate safe installation and inspection of these panels. The industrial control panel must be marked with a short circuit current rating (SCCR) established using an approved method. The entire panel and all components inside must meet and be marked with the appropriate SCCR. UL 508A Supplement SB is one of the three methods that can be used to determine the SCCR of an industrial control panel.
This code change addresses the fact that if an overcurrent situation arises, the energy level may be higher than the lowest level SCCR on a component within the panel. When ratings on components are less than the available fault current, the safe performance of the panel comes into question. Further evaluation of the panel and component combinations may be necessary to ensure a safe electrical installation.
Another point of interest is the exception in the Installation section.
Exception: Where one or more instantaneous-trip circuit breakers or motor short-circuit protectors are used for motor branch-circuit short-circuit and ground-fault protection as permitted by 430.52(C), the procedure specified above for determining the maximum rating of the protective device for the circuit supplying the industrial control panel shall apply with the following provision: For the purpose of the calculation, each instantaneous trip circuit breaker or motor short-circuit protector shall be assumed to have a rating not exceeding the maximum percentage of motor full-load current permitted by Table 430.52 for the type of control panel supply circuit protective device employed.
Where no branch-circuit short-circuit and ground-fault protective device is provided with the industrial control panel for motor or combination of motor and non-motor loads, the rating or setting of the overcurrent protective device shall be based on 430.52 and 430.53, as applicable.
Simply stated, within the control panel, all of the protective components cannot have a fault current rating higher than the motor full-load rating. This is to ensure that the protective devices do their task: Protect the motor and circuit before they fail.Basically, the breaker or trip device will be actuated to protect the equipment--the motor or circuit devices will not fail or “open” the circuit before the devices “upstream” of their tasks.