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A remote powering primer

When telecom cabling is used to power and control devices such as alarms, knowing the code regulations is crucial.

September 1, 2004  

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I am frequently asked to provide guidance on the use of 4-pair telecommunications cabling for remote powering and control of electrical devices such as security cameras, different sensors, access controllers and alarms.

I have already written a column on the new standard for remote powering over Ethernet, and undoubtedly the industry will be migrating toward IP-enabled devices that use DTE (Data Terminal Equipment) powering.

For other types of devices, it is important to know the code regulations and standard requirements that apply when telecommunications cabling is used for powering and control purposes.

This is for reasons of safety and to ensure system compatibility.

Article 725 of NFPA 70, the National Electrical Code (2002) covers remote-control, signaling and power-limited circuits.

It specifies requirements for Class 1, 2 and 3 circuits. Class 2 and Class 3 power sources are defined in Table 11 (A) of NEC (2002) for AC power and Table 11 (B) for DC Power.

FCC Part 68

Another standard that is relevant and provides specific guidance for powering over telecommunications cables is FCC Part 68.

This covers rules and regulations for the protection of the telephone network from harms caused by the connection of terminal equipment and associated wiring.

FCC Part 68.215 limits the current on individual wiring conductors to values, which do not cause an excessive temperature rise with due regard to insulation, materials and ambient temperatures.

The table below assumes a 45 degrees C temperature rise for wire sizes 22 AWG or larger, and a 40 degrees C rise for wire sizes smaller than 22 AWG, for poly-vinyl chloride insulating materials.

This should be regarded as establishing maximum values, to be derated accordingly in specific installations where ambient temperatures are in excess of 25 degrees C.

It is important to note that the total current in all conductors of multiple conductor cables may not exceed 20% of the sum of the individual ratings of all such conductors.

This means that for a 24 AWG, 4-pair cable the maximum total current is limited to (20% x 8 x 2.1) = 3.36 A due to temperature rise considerations.

TIA has also published a standard for Building Automation Systems ANSI/ TIA/EIA 862.

Normative Annex A covers “Power feeding BAS devices over twisted-pair cabling.”

This also takes into consideration the limitations of 8-pin modular connecting hardware and higher ambient temperatures of up to 55 degrees C.

Under the worst case environmental conditions, the maximum current is limited to 0.75 amperes per conductor and a total of 1.68 A for the cable.

This would provide the capability of delivering up to 40 watts of power using a 48 volt DC power source and using all 4-pairs in a simplex powering configuration.

A new addendum

There is a new Addendum to TIA 568-B.1-6 that is in the final stages of the balloting process, which applies to inserting DC power onto structured cabling for low voltage applications, such as, but not limited to, IEEE802.3af DTE Power.

It also illustrates the different implementations to deliver up to 40 W of DC power using a circuit formed by using either one or two sets of conductor pairs (1,2-3,6 and 4,5-7,8).

Finally, the question of system compatibility needs to be considered if powering, control and data or video signals share the same sheath, for example, in the case of a remote powered camera with pan, tilt and zoom control.

In such cases it is important to verify that there is no interference due to electrical transients on the power and control signals, that the signal levels and power spectra are compatible and that the crosstalk isolation is sufficient.

More information on system compatibility can be found in Annex B of ANSI/ TIA/EIA 568-B.1 standard.

Paul Kish is Director, IBDN Systems & Standards at NORDX/CDT. He is also vice chair of the TR-42 engineering committee.

Disclaimer: The information presented is the author’s view and is not official TIA correspondence.