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A dry contact, also called a volt-free contact, is a relay contact that does not supply voltage. The relay energizes or de-energizes when a change to its input has occurred. In other words, a dry contact simply detects whether or not an input switch is open or closed.

The dry contacts in the ServSensor Contact provide a simple two-wire interface that can be easily adapted to third-party sensors and devices. Because you define what the open or closed condition means, dry contacts are infinitely adaptable.

Use dry contacts to monitor alarms such as fire alarms, burglar alarms, and alarms on power systems such as UPSs. A very common use for dry contacts is to detect whether a cabinet door is open or closed.

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 Institute 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, 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.

A dry contact, also called a volt-free contact, is a relay contact that does not supply voltage. The relay energizes or de-energizes when a change to its input has occurred. In other words, a dry contact simply detects whether or not an input switch is open or closed.

The dry contacts in the ServSensor Contact provide a simple two-wire interface that can be easily adapted to third-party sensors and devices. Because you define what the open or closed condition means, dry contacts are infinitely adaptable.

Use dry contacts to monitor alarms such as fire alarms, burglar alarms, and alarms on power systems such as UPSs. A very common use for dry contacts is to detect whether a cabinet door is open or closed.