INTRODUCTION

The performance of an Ethernet hub, switch or media converter is always of interest, but little is said of the mundane task of powering these devices. In order for performance objectives to be met, a successful installation depends upon properly applying power to these devices. Incorrect powering can lead to unreliable operation, equipment damage or possibly a safety hazard.

The power for Industrial Ethernet devices is ultimately derived from mains power which can be as high as 480 V. Typically, there is a transformer or distribution system that provides mains voltages in the range of 100–240 VAC at 50–60 Hertz. This is still much too high to power the 3.3–5 VDC electronics used within the devices. What will be discussed are techniques to power these low-voltage devices.


National Electric Code

The governing electrical wiring standard in the United States (US) is the National Electric Code (NEC), but there are similar electrical codes throughout the world. Published by the National Fire Prevention Association (NFPA), the main purpose of the NEC is safety and fire prevention. Industrial Ethernet hubs, switches, and media converters are low-energy devices requiring power inputs in the range of 4 to 20 VA so it would appear that safety hazards would be minimal. However, wiring of these devices must still conform to the NEC when installed in jurisdictions within the US that have adopted the code. The NEC is over 600 pages in length and can be a confusing document when trying to find that one reference that pertains to a particular installation. Both high- and low-power installations are addressed in the code, but what part is relevant?

Industrial Ethernet equipment usually resides in control panels and, although it may not appear evident, article 725 of the code has application. Article 725 is entitled “Remote-Control, Signaling, and Power-limited Circuits.” A signaling circuit supplies energy to an appliance or device that gives a visual and/or audio indication. This could be a control circuit such as an interlock or an alarm circuit for an annunciator. A power-limited circuit could be used to power low-power devices such as Industrial Ethernet hubs or switches. Article 725 defines three classes of wiring designated Class 1, 2 and 3.



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