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Black Box Explains...Power over Ethernet (PoE).

What is PoE?
The seemingly universal network connection, twisted-pair Ethernet cable, has another role to play, providing electrical power to low-wattage electrical devices. Power over Ethernet (PoE) was ratified by the... more/see it nowInstitute of Electrical and Electronic Engineers (IEEE) in June 2000 as the 802.3af-2003 standard. It defines the specifications for low-level power delivery—roughly 13 watts at 48 VDC—over twisted-pair Ethernet cable to PoE-enabled devices such as IP telephones, wireless access points, Web cameras, and audio speakers.

Recently, the basic 802.3af standard was joined by the IEEE 802.3at PoE standard (also called PoE+ or PoE plus), ratified on September 11, 2009, which supplies up to 25 watts to larger, more power-hungry devices. 802.3at is backwards compatible with 802.3af.

How does PoE work?
The way it works is simple. Ethernet cable that meets CAT5 (or better) standards consists of four twisted pairs of cable, and PoE sends power over these pairs to PoE-enabled devices. In one method, two wire pairs are used to transmit data, and the remaining two pairs are used for power. In the other method, power and data are sent over the same pair.

When the same pair is used for both power and data, the power and data transmissions don’t interfere with each other. Because electricity and data function at opposite ends of the frequency spectrum, they can travel over the same cable. Electricity has a low frequency of 60 Hz or less, and data transmissions have frequencies that can range from 10 million to 100 million Hz.

Basic structure.
There are two types of devices involved in PoE configurations: Power Sourcing Equipment (PSE) and Powered Devices (PD).

PSEs, which include end-span and mid-span devices, provide power to PDs over the Ethernet cable. An end-span device is often a PoE-enabled network switch that’s designed to supply power directly to the cable from each port. The setup would look something like this:

End-span device → Ethernet with power

A mid-span device is inserted between a non-PoE device and the network, and it supplies power from that juncture. Here is a rough schematic of that setup:

Non-PoE switch → Ethernet without PoE → Mid-span device → Ethernet with power

Power injectors, a third type of PSE, supply power to a specific point on the network while the other network segments remain without power.

PDs are pieces of equipment like surveillance cameras, sensors, wireless access points, and any other devices that operate on PoE.

PoE applications and benefits.
• Use one set of twisted-pair wires for both data and low-wattage appliances.
• In addition to the applications noted above, PoE also works well for video surveillance, building management, retail video kiosks, smart signs, vending machines, and retail point-of-information systems.
• Save money by eliminating the need to run electrical wiring.
• Easily move an appliance with minimal disruption.
• If your LAN is protected from power failure by a UPS, the PoE devices connected to your LAN are also protected from power failure.
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The ANSI/ISA Standard and Hazardous Locations

Fires and explosions are a major safety concern in industrial plants. Electrical equipment that must be installed in these locations should be specifically designed and tested to operate under extreme... more/see it nowconditions. The hazardous location classification system was designed to promote the safe use of electrical equipment in those areas “where fire or explosion hazards may exist due to flammable gases or vapors, flammable liquids, combustible dust, or ignitable fibers of flyings.”

The NEC and CSA define hazardous locations by three classes:
Class 1: Gas or vapor hazards
Class 2: Dust hazards
Class 3: Fibers and flyings

Two divisions:
Division 1: An environment where ignitable gases, liquids, vapors or dusts can exist Division 2: Locations where ignitables are not likely to exist

Hazardous classes are further defined by groups A, B, C, D, E, F, and G:
A. Acetylene
B. Hydrogen
C. Ethlene, carbon monoxide
D. Hydrocarbons, fuels, solvents
E. Metals
F. Carbonaceous dusts including coal, carbon black, coke
G. Flour, starch, grain, combustible plastic or chemical dust

ANSI/ISA 12.12.01
Our line of Industrial Ethernet Switches (LEH1208A, LEH1208A-2GMMSC, LEH1216A and LEH1216A-2GMMSC) is fully compliant with ANSI/ISA 12.12.01, a construction standard for Nonincendive Electrical Equipment for Use in Class I and II, Division 2 and Class III, Divisions 1 and 2 Hazardous (Classified) Locations. ANSI/ISA 12.12.01-2000 is similar to UL1604, but is more stringent (for a full list of changes, see Compliance Today). UL1604 was withdrawn in 2012 and replaced with ISA 12.12.01.

The standard provides the requirements for the design, construction, and marking of electrical equipment or parts of such equipment used in Class I and Class II, Division 2 and Class III, Divisions 1 and 2 hazardous (classified) locations. This type of equipment, in normal operation, is not capable of causing ignition.

The standard establishes uniformity in test methods for determining the suitability of equipment as related to their potential to ignite to a specific flammable gas or vapor-in-air mixture, combustible dust, easily ignitable fibers, or flyings under the following ambient conditions:
a) an ambient temperature of -25°C to 40°C.
b) an oxygen concentration of not greater than 21 percent by volume.
c) a pressure of 80 kPa (0.8 bar) to 110 kPa (1.1 bar).

The standard is available for purchase at www.webstore.ansi.org. To learn more about ANSI/ISA 12.12.01 and hazardous location types, visit https://www.osha.gov/doc/outreachtraining/htmlfiles/hazloc.html. -- collapse

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