Remote Powering over communications cabling (part 2)

Remote powering implies that the power equipment is not local but some distance away. It is an alternative to commercial power, which can be hard to get and/or expensive. Remote means that the powering and powered equipment are in separate buildings or at different external locations inside same building.

Line powering is a way to power remote equipment from a central location using existing cable pairs in the copper network. Remote powering over communications cabling is not something new. Over the years, various choices of operating voltage and practices such as utilizing only negative voltages have served the telecom industry well. Line power has been used as far back as the early 1960s by Telcos for T1 power and then HDSL powering. Telecom business is changing, and new services utilizing various forms of DSL now require increased power levels. Higher power remote powering has now applications such as fiber-to-the-node (FTTN), fiber-to-the-home (FTTH), DSLAM powering and powering cell phone network base station components.

Plain old telephone service is low power

Normal analogue telephone line (POTS) supplies power to the line that is used by the telephone device on the use end.The current which powers your telephone typically comes from the 48V battery in the central office. The + side of the 48V battery set is connected to ground so the supplied is -48V DC. The -48V voltage was selected because it was high  enough to get through kilometers of thin telephone wire and still low enough to be safeThe line feeding voltage was selected to be negative to make the electrochemical reactions on the wet telephone wiring to be less harmful.

The 48V voltage is sent to the telephone line through current limiting components (from few hundred ohms to few kilo-ohms). There are current limiting components on both TIP and RING wire side. The telephone cable typically has loop resistance of around 90-140 ohms per kilometer.

When your telephone is in on-hook state the “TIP” is at about 0v, while “RING” is about -48v with respect to earth ground. When you go off hook, and current is drawn, TIP goes negative and RING goes positive. A typical off hook condition is TIP at about -20v and ring at about -28v. This means that there is about 8V voltage between the wires going to telephone in normal operation condition. The DC-resistance of typical telephone equipment is in 200-300 ohm range and current flowing through the telephone is in 20-50 mA range.

There is not a lot of power available on normal PSTN line. At 8 volts and 50 mA the power would be  400 mW, and the power can be often lower than that, especially on the end pf long lines.  Maxim has published an application note how to to Draw 150mW of Isolated Power from Off-Hook Phone Line, thus eliminate the need for batteries and ac adapters by drawing power from ordinary phone jacks without interrupting the voice signal.

Telephone ringing voltages can be as high as 90 volts AC.

Other signals on phone line

However there are other signals present in the telecommunications access network that can also deliver electric shocks on contact with conductive parts of the circuit e.g. ringing on PSTN or Telex signalling using ±80 V (probably now historic). Telex used current levels of 20 mA, 40 mA or 60 mA.

ISDN needs more power over telephone cable

When technology moved to digital world, there was need to supply more power over telephone cable to user. Integrated Services Digital Network (ISDN) is a set of communication standards (defined in 1988) for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network. The key feature of ISDN is that it integrates speech and data on the same lines, adding also extra  features. The entry level interface to ISDN is the Basic Rate Interface (BRI), a 128 kbit/s service delivered over a pair of standard telephone copper wires. BRI-ISDN was popular in Europe. Remote powering of the NT1 from the network is preferred but could be that uses remote powering in absence of local power. BRI-ISDN was connected from central office to home using one wire pair that supplied data and power (U-interface). A number of feeding voltage ranges is defined for different applications (from 51Vto 115V DC). The used voltages I have seen has been typically around 90V DC . Sources with a fixed current limitation between 40 mA and 55 mA. Higher voltage allowed more power to be transported.

T1 first used high voltages

Transmission System 1 (T1) was introduced in 1962 in the Bell System to transmit up to 24 telephone calls simultaneously over a single transmission line of copper wire. The T1/DS1 data rate was 1.544 Mbit/s. When a DS1 is transported over metallic outside plant cable, the signal travels over conditioned cable pairs known as a T1 span. T1 span repeaters are typically engineered up to 6,000 feet (1,800 m) apart.

T1 Remote Modules may be T1 line (span) powered or locally powered (usually 48VDC). For remote powering a T1 span can have up to +-130 Volts of DC power superimposed on the associated four wire cable pairs to line or “Span” power line repeaters, and T1 NIU’s (T1 Smartjacks). Different variations exists, for example only -130V, +-130V and some repeaters are set to provide 60 mA simplex current to the span line. Because higher than normal telephone applications, T1 equipment with span power are intended to be installed in Restricted Access Areas only.

Reverse power feeding

Reverse power feeding is feeding power through telephone wiring from end customer home towards telecom network. Reverse Power Feed (RPF) is a new technology being standardized by ETSI  (TS 101 548, TR 102 629) and the Broadband Forum (TR-301). It allows sending power from the customer premises to a Distribution Point (DP), in order to power the Distribution Point Unit (DPU). The DPU includes typically a DSL Access Multiplexer (DSLAM). RPF is a critical technology for the upgrade of VDSL2 subscribers to the new G.FAST standard, which has a maximum range (at high speeds) of 250m. At this distance the device that coverts fast fiber data to G.FAST signal needs to be quite near to customers and fiber run, possibly in small cabinet somewhere near road. This topology as also known as fiber to the distribution point (FTTdp). Reverse Power Feeding reduces installation cost by removing the need to connect the DPU locally to the power grid, and to monitor its power consumption with a smart meter.

Reverse Power Feeding Constraints:

Primary Constraints
•Local safety standards: If user can be exposed, voltage must be <60V
•Wires may have 0.4 mm diameter: 26.78 ohms at at 100m
• CPE devices at various ranges from DP: from10m* to250m**
•Power available to power DP depends on distance and wire type
Additional ETSI standard constraints
•21W,15W and10W maximum power output classes
•POTS interoperability

The power is fed through normal telephone wire, usually through one wire pair. The basic idea is similar to PoE, but now the power is fed through less wires and to longer distance. Unlike PoE, which defines worst case power losses on a cable that is well defined, RPF defines a maximum safety power envelope, and allows implementations to squeeze as much power as possible from the line. What is best power feeding arrangement depends on the case. In case the drop cable is just tens of meters long,  it is not necessary to operate at high voltages in order to reduce copper losses to an acceptable level and therefore it is possible to operate at SELV levels (60 V dc) in order to achieve a reasonably efficient reverse powering scheme. If there are two pairs available, some kinds of modified PoE type powering arrangement could be possible well to 100 meters or so (may be necessary to increase the existing PoE current limit beyond 400 mA). Proprietary solutions using up to 720 mA of current per line pair to provide 36 W of remote power feed on a single pair have also been devised.

For short lines (up to 200 m) then the standard described in ETSI TR 102 629 clause 4.7.1 should be followed. This is essentially a voltage limited standard (60 V) which is also current limited (300 mA). The maximum voltage/current that is injected into the network before a suitable sink has been recognised is 50 V@40 mA.

There are security reasons why it would be a good idea to supply power to line only when there is powered device connected (compare to PoE). According to Power Matters Reverse Powering Its Benefits and Constraints slide set from Microsemi the ETSI recommended method is to use ETSI RPF Power Up protocol. ETSI RPF Power Up protocol has two options for detection: Active handshake (handshaking protocol over data) and Passive handshake (resistive signature based). Some system s Force Power (non-ETSI)

For longer lines (with associated larger copper losses), a lower current limit is necessary (60 mA) which necessitates a higher maximum injection voltage (200 V) (RFT-C Circuit current limited (EN 60950-21 [i.10], clause 6.1). The maximum voltage/current that is injected into the network before a suitable sink has been recognised is 50 V@40 mA. A range of approximately 1 600 m is possible before the power dissipation in the line reaches 1 W. With those higher voltages, it must be installed so that end user can’t come in touch with those high voltages.

The RPF system is designed to power a DPU with a set number of lines, typically 1, 4, 8, 12 or 16. The system must allow the DPU to work with any number of active users at a point in time, even a single one.

RPF creates challenges for delivering POTS service and metallic line testing (MELT). You can’t provide standard POTS on the same pair as used for reverse powering (operators are expected to use VoIP up to the end user in combination with FTTdp).

Information sources:


  1. Tomi Engdahl says:

    Telephone cable is usually thought of only as a communication medium operating at a
    normal 48 Vdc level. But telephone cable is regularly used by telephone operating companies
    to supply power to remote telephony equipment such as: PCM (Pulse Code Modulation)
    repeaters, FDM (Frequency Division Modulation) repeaters, telegraph and various types of
    control circuits.
    The normal talk battery voltage is regularly exceeded in the case of long subscriber loops
    utilizing loop extenders and on dial long line circuits which operate in the 70 to 100 Vdc
    range. For these applications, the referenced devices are generally current limited to
    approximately 150 milliamps. In all such cases, normal telephone industry practice usually
    limits pair-to-ground applied voltage to 150 Vdc, and pair-to-pair voltage is usually limited to
    300 Vdc.

  2. Tomi Engdahl says:

    What is a Limited Power Source?

    There are many technical details regarding Limited Power Sources (often referred to as an LPS) covered in the IEC60950-1 safety standard, involving a variety of applications

    What is, and why are Limited Power Sources important? Simply put, if a piece of electrical or electronic equipment supplying DC power to external devices is to be installed by a third party, such as an electrician, the risk of wiring fires & electrical shock needs to be minimized. That electrician will not be expected to know all the potential fault scenarios and use the appropriate cable thicknesses and insulation to cover those hazards. By using a Limited Power Source, the system wiring can also be reduced, saving cost.

    If a Limited Power Source is used, then the electrician’s job is simplified, even if there is a (single) fault inside of the power supply.

    The conditions for a “Limited Power Source” AC-DC power supply are:

    1. For a power supply rated at 30V or less, the following must be met even with a single fault condition:
    a. The output current must not exceed 8A
    b. The output power must not exceed 100W
    2. For a power supply rated above 30V, but not exceeding 60V:
    a. The output current must not exceed 150 ÷ Vout
    b. The output power must not exceed 100W

    60950-1 Limited Power Source (LPS) Rated Power Supplies

    A Limited Power Source (LPS) as defined in 60950, is a secondary circuit with an open circuit output voltage, UOC, not exceeding the SELV circuit limits of 42.4 VPEAK or 60 VDC. The maximum apparent power, S, available on the output under any load condition, and the maximum fault current, ISC, available on the output under any load condition, (including a short-circuit), are limited to magnitudes not likely to cause ignition under fault condition in components mounted on, or circuits constructed from, suitably rated materials.

  3. Tomi Engdahl says:

    Using PolySwitch PPTC Devices for LPS Compliance
    in Li-ion and LiP Cell Battery Applications

    Battery packs marked “LPS” are safe for consumers to
    replace and access, negating the need for specialized
    electrical technicians to perform basic battery maintenance
    procedures. Conversely, if the source is not a Limited Power
    Source, the live voltage on that circuit must be contained in
    an environment where a tool is needed to access a secured
    panel. The benefit of an LPS circuit is that connectors and
    cables connected to the active circuit may require fewer tests
    and eliminate the need for certification.

    A key step in meeting LPS requirements is by testing at
    the maximum power point load. The value of this load is
    determined prior to commencement of the tests. To pass
    the LPS test, the source should be tested under the normal
    load, at the maximum current load fault condition and at
    the maximum power load fault condition.

  4. Tomi Engdahl says:
    NEC Class 2:
    Power-limited Circuits max. 8A and 100VA

    According to article 725-121 NEC Class 2 circuits are only allowed to be powered
    from a power source with an output power of lower than 100VA and an output
    current of lower than 8A. The power source needs to be listed as an UL 1310 po-
    wer supply device or must be approved as a Limited Power Source (LPS) according
    to IEC 60950-1. The NEC Class 2 limits need to be fulfilled even under overload
    or during fault conditions of the unit. Extensive tests and approvals are therefore
    NEC Class 2 circuits are considered to be safe from a fire ignition standpoint and
    provide an acceptable protection against electric shock. The benefits of using NEC
    Class 2 circuits are reduced and less expensive requirements regarding wiring me-
    thods and over-current protection. Furthermore, the agency testing and approval
    process of the end-application is much easier.


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