Power Over Ethernet (PoE) FAQs

Power over Ethernet (PoE) technology allows electrical power to travel through an existing data connection with a normal Ethernet cable. Convenient and cost-effective, PoE has revolutionized modern network infrastructure.

Implementing a PoE network that provides the required power and data performance requires careful planning and testing. Here are some of the questions we encounter most often.

Q: “Can a network have both PoE and non-PoE devices?”
Yes, PoE is a safe technology. Unlike an AC connection, PoE technology uses DC current. If there is no device present, or communication links have been lost, then there is no power.

Q: “What about using Cat 7 in lieu of Cat 6A? Will it improve PoE?”
Cat 7 can provide some benefits with regards to PoE, but not strictly because it is Cat 7… Rather, the construction of the cable allows for better heat dissipation if it is of the S/FTP type. This is more fully explained in the next question below.

Q: “What is the relation of Heat Dissipation in SFTP Cables? (Individual and overall screened cables)”
Shielding offers the benefits of significantly improved pair-to-pair crosstalk performance, alien crosstalk performance, and noise immunity that cannot be matched by any other cabling design strategy. ANSI/TIA-184-A clearly states that thermal dissipation can be improved by selecting cables with metallic elements, such as a shield or a screen. When you do not have a shield or a screen, heat can only be dissipated by air, which is poor conductor for heat.

Q: “Do we need a special tester to say that a system is PoE compliant when running a specific test?”
No, if the standards for product, design, installation, configuration and testing are followed, along with the Molex PoE Calculator, then there is sufficient buffer in there to be effectively protected. However, there are some good reasons for using testers:

• Using a cable certification field tester which includes these resistance measurements allows for a quick and easy test for DC resistance unbalance within a pair and between pairs. This provides assurance that the cable plant deploys will perform in two- and four-pair PoE applications.
• Knowing the capacity of the Power Sourcing Equipment (PSE) and the requirements of the Powered Device (PD) make installation and troubleshooting much simpler. In the real world, technicians supporting PoE powered devices may not have access to this information. They can easily check the requirements of an EA (Ethernet Alliance) Certified PD, but in most cases, the technicians work quite a distance from the PSE, so they’re faced with a long walk back to the telecommunications closet or data center to find out the capabilities of the switch. Then they must work out which cable goes to the relevant PD. In many instances, they might not have access to the PSE and would need to contact the IT team to find out. A technician could waste half a day tracking the cable and accessing the switch. Testers, such as the Fluke Networks’ MicroScanner PoE, are available to solve this problem.
• Testers will also help identify the speed of the port up to 10 Gbps. A slow port may limit the performance of an access point or camera. If the cable has been damaged, the tester will display the length of each pair, potential breaks, or other failures. Cables can also be unplugged or misrouted – these PoE testers can act as a tone source for tracing the cable. Identifiers can be connected to remote cables to determine where they go.

Q: “Is it necessary to use a PoE calculator all the time?”
Use of the Molex PoE calculator is highly recommended, as it helps in the following areas:

• Ensures the cabling architecture is compliant with the latest PoE Standards and Molex requirements
• Ensures the number of cables in each bundle does not adversely affect your PoE application performance
• Provides options on how to fix issues and achieve compliance with applicable Standards
• Provides the max lengths of the channel, permanent link and combined length of patch cords and work area cords to ensure they are within the limits of all applicable international cabling Standards
• Ensures that the customer benefits from the optimum cabling architecture, to support their desired PoE applications
• Provides more information on potential heat issues that could arise based on length, bundle size, cable diameter, ambient temperature and PoE operation
• Additional analysis on whether existing legacy infrastructures can support the demand of current and future PoE applications
• Cable design feedback and confirmation of coverage through the Molex PoE application assurance warranty.

The Molex PoE Calculator can be downloaded free of charge from our Customer Support Portal (CSP), at csp.molex.com.

Q: “How far can PoE travel on a cable?”
All standard PoE has Ethernet cable distance restrictions of 100 meters (328 feet) for data and power transmission, from type 1 to type 4 (90W). The limitation is not the power but the Ethernet cabling standards that limit the total length of cabling to 100 meters. The furthest distance a PoE switch can transmit simple data over Ethernet is 100 meters.

Q: “Can we use an LAN extender to increase distance between the PoE device and the device that needs power?”
A PoE Ethernet Extender can increase that span up to 4000 feet (1.2km). Extenders are useful for larger distances such as, hotels, shopping malls, business and academic campuses, and sporting venues. The extended distance will depend on power level needed, with most PoE extenders supporting up to 100Mb/s over a max distance of 2000 feet (610 meters).

Q: “What can you advise in a situation where the Permanent Link cannot be reduced to less than 95 meters?”
It is important to do upfront planning at the beginning. The only recommendation is to limit or reduce the combined length of the patch cords and work area cords. A PoE Extender could be an option, but would limit the data rate to less than 1Gb/s.

Q: “Can PoE power a TV?”
Yes, Power over Ethernet plays a role for a growing range of devices, such as: security cameras, access points, PTZ conferencing cameras, energy efficient flat screens, thin client computers.

Q: “Is PoE technology safe to use? Can it damage my equipment?”
IEEE 802.3af/at/bt compliant PoE technology is safe. PoE injectors and switches will not damage any equipment, even if the equipment is not designed for PoE applications. Before the PSE sends any power to a connected PD, the PSE initiates a handshake procedure that establishes how much power the connected device requires. This procedure uses low voltage and is harmless to any connected device, PoE or non-PoE. If that handshake is not completed for any reason, the PSE never sends any power. It is this built-in feature of all IEEE 802.3af/at/bt-compliant devices that makes PoE technology inherently safe.

Q: “My cable is UL LP rated… Will it safely support all the latest PoE applications?”
Cable heating can be managed by four main factors:

1. increased AWG size,
2. cable design variations,
3. material selection,
4. installation practices.

This led to the development of test-based requirements for Limited Power (LP) cables. If the cable temperature rise is not controlled, then this will affect cabling/networking performance. UL standards address this challenge from a safety standpoint but disregard the challenges of network performance.  An LP UL certified cable will not exceed its temperature rating under certain conditions, but regardless of the cabling (LP certified or not), if higher temperatures are introduced into the cable, then the cable’s reach and performance will be negatively impacted due to insertion loss and the accelerated physical wear of the cable.

There are also some important inconsistencies between UL, ANSI/TIA and the IEEE. The bundle sizes are different, the ampacity is different, and the temperature reference is different. For more information, contact your local sales team.

Download the full guide
This post is an excerpt of the full “Power over Ethernet: Frequently Asked Questions” guide, which can be downloaded from the Customer Support Portal. Log in or apply for access here.

PoE Solutions from Molex
Find out more about our PoE solutions here.