Remote feeding is a technology which enables operators to power several remote sites from a central location. Remote powering implies that the power equipment is not local but some distance away. Remote means that the powering and powered equipment are in separate buildings or at different external locations inside same building.It can be used as an alternative to commercial power, which can be hard to get and/or expensive on remote locations. By delivering that power via twisted pair cables telecommunications operators can reuse their old copper telephone wiring (that is being at many locations be replaced with fiber optics for high speed data) for powering different devices on the field.
My earlier postings Remote Powering over communications cabling (part 1) and Remote Powering over communications cabling (part 2) describe methods to provide power over telephone wiring. New services utilizing various forms of DSL now require increased power levels, so new powering technologies needed to be developed.
The transmission of DC power over copper cables, a technique called Remote Line Power, is rapidly emerging as a means of improving the reliability and resiliency of distributed telecommunications networks. Energizing the remote devices in Fiber-to-the-Home (FTTH), DSLAM, and DAS networks using the robust power of a Central Office (CO) can ensure that services run even during extended commercial power outages. Extending as far as six (6) kilometers into the network, Remote Line Power eliminates batteries at the edge of the network.
Telephone cable has limitations on how much current it can handle and how much loss the cable resistance causes. The telephone cable typically has loop resistance of around 90-140 ohms per kilometer. So if you run 100 mA current through one kilometer of thin telephone cable, you can get 14V of voltage drop.
Also safety regulations limit the voltages and current that can be run on the telephone cable. On the lines accessible to end users the voltages are typically limited below 60V DC, normal voltage being 48V. In applications where all the wiring is inside telecom company premises, higher voltages could be used. 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. The used current is generally current limited to approximately 150 milliamps.
Oxidation or corrosion of conductors or connection strips can also be found on circuits carrying higher than the normal ~50 V commonly used for the PSTN service. Operators using remote power feeding would need to be aware of the limitation of the insulation of the cable and other access network plant (e.g. over voltage protection) in the network.
The most common type of telecommunications cable in use today employs solid copper conductors with a diameter of
between 0,4 mm and 0,9 mm. In modern cable the conductors are insulated using polyethylene which has either a
cellular, foam skin or solid construction. Older cables that are in service would have paper or PVC insulation.
Telephone wire is typical AWG 22, 24, or 26. According to IEC 60950-1 the maximum current should not exceed a current limit for specific wire gauge. The current limit is 1.3A if such wiring is not specified. The maximum amperage in PVC insulated multicore cables (43 and above wires) for AWG 22 is 1.5A and AWG 24 1.0A. Here is some technical data on different telephone cable resistance and amperage for power:
AWG 22 53 ohms/km 0.92A for power
AWG 24 106 ohms/km 0.577A for power
AWG 26 134 ohms/km 0.361A for power
According to those data the telephone wiring could handle more than typical 150 mA. Can standard phone line carry 300-400 mA? Most probably it can. If we presume 24 AWG wire, you have an area of 404 circular mil, and a resistance of 0.027 ohm/foot. At 400mA you are producing about 4mW of heat per foot, which should not be a problem at all. If you increase current, the losses on cable losses increase according to equation P = R * I * I (where I=current and R=resistance). It is clear that the losses will increase very quickly, so using high current does not seem to be the best idea. Maybe if we could increase the voltage safely and keep current at reasonable level.
Even more power over telephone cable
New services utilizing various forms of DSL now require increased power levels. Also fiber optic systems need power. While fibre optic cable can carry signals as fast and far as needed, it is too expensive to run it to everyone’s home, so many telephone companies are implementing an FTTH (fibre to the hub) solution. In this case, the hub is a Remote Concentrator, and it is located at an OPI or JWI, as these are generally within 1 km of people’s homes. A new fibre optic cable is then only needed from the Central Office to the remote concentrator.
One idea to feed the needed power is to do remote powering (different variations) with +-190 over telephone pair (0.25A) or dedicated shielded power cable (up to 5 km). This approach has safety and regulatory approval (at least in USA) including the recently released UL/CSA 60950-21. The solution operates within the limits of both the EN60950-1 for TNV networks and the EN/IEC 60950-21 RFT-C standards. The technology can be installed by telecom technicians over existing copper networks
That +-190 at 0.25A over telephone pair pushes just slightly less than 100W power to the cable, so can be considered at low energy circuit. ADSL2+ DSL Remote Concentrator web has this image showing one remote powering implementation.
Here is a close-up of the power pairs. The green wafer (here terminating the 25 pairs numbered 1,926 through 1,950) has the power from the Central Office. The blue wafers terminate the five pairs that provide the power to the remote concentrator. The label is on the inside of the door, and gives some detail on the power supply. It states that it is powered by five lines at ±190 volts DC, 0.25 amps each line. The datasheet here notes that each line is limited to 100 VA, and the entire chassis draws up to 260 watts. At 0.25 A and 190 volts, each line can supply 47.5 watts, for a total of 237.5 watts for the five lines. The label also indicates that this power must be supplied by a “Remote Feeding Telecommunication (RFT-V) rated source“.
Remote feeding is a technology which enables operators to power several remote sites from a central location by delivering that power via twisted pair cables. The solution operates within the limits of both the EN60950-1 for TNV networks and the EN/IEC 60950-21 RFT-C standards. The technology can be installed by telecom technicians over existing copper networks.
Using the HVDC Power Feed solution, mains DC power is converted to 380-400VDC – a voltage level that can be transmitted long distances with very low losses – at the CO. The 380VDC power then passes through a distribution box providing the necessary protection and safety functions, before being distributed to the load in the remote location. A second voltage conversion takes place at the point of use, where DC/DC converters transform the 380VDC /400VDC back down to 54VDC /48VDC.
Besides providing power to the sites where mains power is not easy to get, remote power feeding (RPF) is a new way of achieving cost efficient back up for telecom. Great savings are possible by decreasing the amount of batteries needed at every telecom location, and limiting the numbers of reserve power installations. In a pre-study by Telia and Ericsson it was concluded that great cost savings are achievable.
Line Powering vs. Commercial Power article describes case where telecom operator decided to utilize vacant copper cables to deliver power to the DSLAMs. The power comes from the 48Vdc plant in central office. The line power equipment installed in the CO consists of a shelf of modular up-converters, which are DC-DC converters that take the 48Vdc up to ±190Vdc. All the DSLAMs have built-in down-converters (±190Vdc to 48Vdc). For line power this application uses the Cordex HP LPS36 manufactured by Alpha Technologies. The operator reuses the copper cable pairs that were replaced by fiber that serves the DSLAMs.There is no need to coordinate power with utility power companies and there is no need for batteries at the DSLAM. And, best of all, the remote line power solution is less expensive than commercial power.
Remote Powering Ericsson Small Cell from a Centralized Power Source video shows how Alpha’s line powering equipment is used to remotely power an Ericsson Small Cell device that can be located up to 15kft away from the nearest power source. First AC power is converted to high-voltage DC (+/-190Vdc) following the RFT-V standards. It is then transported via standard twisted copper pair cable to the remote node where Alpha’s down-converter unit (LPR48-150) converts the high-voltage DC to a working voltage of -48VDC to power the Small Cell device.
The Eltek 380V Remote Power solution page shows a solution that begins with the existing 48VDC power system and battery in the central site. From there, the 48VDC power is converted to 380VDC through Flatpack2 HE DC/DC converters. The 380VDC power then passes through a distribution box providing the necessary protection and safety functions, before being distributed to the load in the remote location. At the remote site, another set of Eltek DC/DC converters brings the voltage down to 48VDC for the telecom equipment. Depending on the power consumption in each site, and number of remote sites connected to each 380VDC cable, the distance from the central site to the remote site can be up to 5km.
ITU-T Recommendation K.50: Safe limits of operating voltages and currents for telecommunication systems powered over the network is one relevant standard for remote powering. Voltages and currents that may be applied to a telecommunication network by equipment forming part of a subscriber’s installation are covered in [IEC 60950-1] and [IEC 62368-1].
Telecommunication networks sometimes use equipment that generates voltages and currents that exceed TNV-1, TNV-2, TNV-3 (defined in [IEC 60950-1]) and ES1, ES2 (defined in [IEC 62368-1]) to provide power to remote equipment over paired-conductor network telecommunications cable. The voltages and currents that power devices inside telecom operator network can differ from those of telecommunication services provided to end users or subscribers.
Typical remote power sources are voltage-limited remote feeding telecommunication (RFT-V) or a current-limited RFT (RFT-C) circuits. The standard IEC 60950-21 describes two different remote power architectures:
• RFT-C: Remote Feeding Telecommunication Current circuit
• RFT-V: Remote Feeding Telecommunication Voltage circuit
Both technologies have in common that they were designed to use existing twisted pair cables from existing telephone
cable networks. The main difference between the two technologies is their different safety concept.
The safety concept of RFT-V systems allows a maximum voltage of 200V against ground and a maximum power of 100 VA per channel. RFT-V, which starts as 60 V d.c., limits the touch voltage (and above 80 V d.c. limits the current), so that the insulation or the resistance of the body limits the current conducted by service personnel to tolerable levels. To avoid an accidental current flow through human bodies above 60mA, additional earth fault detectors are needed. In case of a single fault, the current limits are reduced to lower values.
Both methods have been used and defined for many years by various operators. RFT-V is used in North America and is based on the US and Canadian Electrical codes and Electrical safety codes. RFT-C and RFT-V are typically >120 V d.c. levels in most countries, but could be as low as 60 V d.c. No more than 100 W (PS2) can continuously be delivered by any single powering circuit under normal operating conditions.
Under normal operating conditions, an RFT-V circuit can have voltage greater than 80 V d.c. but less than or equal to 200 V d.c. and be current limited to earth to 5 mA d.c. with a monitoring and control device (i.e., an RCD circuit). For RFT-V circuits whose (open circuit or terminated) voltage exceeds 80 V d.c. under normal operating conditions, the current between the other conductors and earth, measured through any resistance value from 0 Ω to 40 kΩ, under any external load condition, shall not exceed 5 mA d.c. after 10 ms.
The [IEC 62368-1] safety standard does not allow instructed persons to access voltages above 120 V d.c. (ES2). Service and skilled persons may access any voltage provided that adequate safety precautions are taken. Because high voltages are potentially dangerous, suitable safety labels should, as necessary, be attached when operating or working on electrical installation. All cables shall be continuously labelled with the symbol for the telephone and the telephone adapter (EN 60417-1  – Symbol number 5090) the symbols for hazardous electrical voltage (EN 60417-1  – Symbol number 5036; lightning symbol without triangle) has to be added.
The maximum continuous current that may be applied to communication wiring under normal conditions shall be consistent with its power rating, but shall not exceed 1.3 A unless the current capacity of the network wiring and current rating of network connectors and other network components is specified at a higher value and is controlled.
While EN 60950-21 tends to focus on physiological damage as a direct result of an electric current passing
through the human body, there are other health and safety issues that may need to be considered for personnel working on access network plant that carries high voltages (for example spontaneous reaction to electric shock may result in injury).
ETSI EN 302 999 document presents safety requirements for the erection of information technology installations with remote power feeding at an operating a.c. voltage exceeding 50 V(rms value) or an operating d.c. voltage exceeding 120 V, conductor to conductor and/or conductor to earth. It applies in addition to EN 60950-1 and EN 60950-21 and contains terms, requirements and tests. In USA the relevans standard is UL 60950-21: Information Technology Equipment – Safety – Part 21: Remote Power Feeding
- Remote means that the powering and powered equipment are in separate buildings (external feed) or at different external locations in cabinets.
- Remote can also be expressed as the powering and powered equipment do not share a common earth point.
- RFT voltages exceeds the limits for TNV CIRCUITS or in hazard speak exceed ES2 voltages(=ES3) (The IEC 60950-21 scope is “This part of IEC 60950 applies to information technology equipment intended to supply and receive operating power via a TELECOMMUNICATION NETWORK, where the voltage exceeds the limits for TNV CIRCUITS.”)
- These high voltages automatically means that users be prevented access to RFT voltage
There are some ideas around to improve safety. Highly safe remote power feeding system for high-power broadband network paper describes concept called Tele-Differential© insures to have a safety level comparable to the SELV 48 V while working with voltages close to 400 V. This leading technology overcomes the requirements of the standard EN60950-21 describing the RFT-C/RFT-V circuits by the way, meeting the IEC 479-1 about electric shocks and resulting physiological effects. There iscommunication channel needed for the safety protections.
Remote power causes some special demand for copper infrastructure and cable pairs. The power pairs must withstand voltages up to ±190Vdc.
Verification of suitability of telecommunications cables and wiring is the responsibility of the telecommunications service provider. In all cases, potential power pairs should be Megger tested by applying a high voltage of at least 500 VDC Tip to Ring. This test will test the insulation resistance of the potential power pairs. Test the pair for Insulation Resistance: “Connect a MEGGER or equivalent insulation resistance tester to the potential power pair. Do not terminate the far end of the pair. For the Tip to Ring test, use 500 VDC as the source voltage. For Tip to Ground and Ring to Ground tests, use 250 VDC as the source voltage.”
Most of the cables that are in good condition will be OK for remote powering with ±190Vdc. Old telephone cables often do not carry formal ratings, or have a 300 V rating from a historical perspective (ringing voltages can be up to 300V peak to peak). Paper insulation is normally rated for greater than 500 VDC. Pulp cable insulation was tested to withstand DC voltages up to 500 VDC. Plastic Insulated Conductors (PIC) insulation is tested during the manufacturing process with a minimum of 2,200 VDC or higher.
Please note that oxidation or corrosion of conductors or connection strips can also be found on circuits carrying higher than the normal ~50 V commonly used for the PSTN service. Operators using remote power feeding would need to be aware of the limitation of the insulation of the cable and other access network plant (e.g. over voltage protection) in the network.
I think it would be also a good idea to verify the loop resistance also that it matched to the expectations.
Even more power of thicker cables
Besides remote powering through existing thin telephone wire pairs, there has been work going on DC power for remote powering through dedicated power cables. With this arrangement it is possible to transport more than 100W power to remote site. For example Emerge Alliance has worked on HVDC transmission at 380-400V voltage range for remote powering. The claim is that If you need to power a DSLAM up to 5000 meters away from an existing backed up power system, a HVDC remote power solution is the way to go. The downside of higher power is that dedicated cables are needed and you might also need an electrician for installing the system.
High efficiency remote power distribution for telecom infrastructure paper presents HE-Remote Power System that uses 400V voltage for connection between the CO and the distributed DSLAMs. The underground cable is a two-pole, shielded cable. A key safety element built into the concept is the use of a floating IT (French: Isolé Terre) DC transmission network. The 400Vdc section is designed as an IT (French: Isolé Terre) system and is isolated against earth potential. The system is designed to offer protection against multiple errors. Since the current source is not grounded, no closed circuit is formed for the first fault (fault loop), the fault current is low. A shutdown for protection against electric shock is not required. According to HD 60364-4-41 , Section 418.104.22.168, the occurrence of a first fault between a live part and a body or against earth must be reported. For this purpose, an insulation monitoring device (IMD) is used. In the case of a second earth fault in the IT system, the 400Vdc on power distribution cables is removed in 0.4 to 5 seconds galvanically and bipolar, according to HD 60364-4-41. Protection is built using very sensitive Hy-Mag circuit breakers.
Eltek HE Remote Power System is designed for the safe feeding of telecom equipment that is distributed in many
decentralized outdoor locations.Remote end sites are connected to central office power supplies via a cable which
is dedicated for remote power. A dedicated long-distance power cable (up to 5 km) connects the central site and the remote sites. The underground transmission cable is a two-pole, shielded cable, which is suitable for ground installation and voltages above 380 VDC. The power is transmitted to cable at 380V DC voltage (+-190V arrangement) to minimize transmission losses. At the remote sites the 380 VDC power is converted back to 48 VDC power for the DSLAM equipment. A key safety element built into the concept is the use of a floating IT (French: Isolé Terre) DC transmission network.
The HE-Remote Power System is expected to give better efficiency and longer transmission distance on the same cable compared to traditional AC mains power (120V or 230V). If you assume for HE-Remote Power System a 500W load at the remote end of the cable and a 2km copper cable. We see that for a 1,5mm2 cable, only the 400Vdc can provide power over the 2km cable, with the required minimum terminal voltage. With a 2,5mm2 cable, the power loss of the cable is 11% with the 400Vdc solution whereas it is 25% with the 380Vdc solution.
Besides remote powering, 380V DC is used in data centers for local powering of servers. 380V DC Power is Shaping the Future of Data Center Energy Efficiency. Deploying DC power distribution in the data center instead of using the traditional AC design is one way to reduce power loss, eliminate unnecessary conversions and, ultimately, lower energy costs. It can also make it easier to provide battery backed up power and connect alternative energy sources to data center (for example solar power is naturally DC). Electric Power Research Institute (EPRI) has teamed with the EMerge Alliance to advance the adoption of the 380-volt DC UPS solution. 380V DC has been installed in many data centers around the world and acceptance of the technology has gained momentum (more momentum than 300, 400, and 575 V that have been proposed in various forums). There are essential standards and interoperability specifications, such as the DCG+C [DC Components and Grid] consortium, the ITU [International Telecommunications Union] via standard L.1200, ETSI [European Telecommunications Standards Institute] via EN 300 132-3-1, the IEC [International Electrotechnical Commission], NTT/Japan [Nippon Telegraph and Telephone], and the IEEE.
ITU L.1200 recommendation specifies a power feeding system for ICT equipment with a DC voltage of up to 400 V (target voltage range 260-400 V). In the proposed international ETSI standard for DC distribution for data centers, the 380V DC system is actually created with the midpoint at ground potential to keep the maximum system voltage to ground to within +/- 190 V. It provides similar safety as 250 VAC. Cutting the DC at high currents is harder than with AC, but it seem that arc flash is not unacceptable hazard at 380V with right components. The connectors have been specifically designed with features to fully enclose the arc for at least 5-10A currents and the standard connector is already rated for 20A. IEC-309 connector is already fully rated for up to 450 Vdc (IEC 60309).
It seems that 380V DC distribution is becoming a necessary choice for new telecom and datacenter power infrastructure. Many applications in the industry are based on the level of 380 – 400Vdc. Due to that all relevant components for power modules, safety building blocks, cables and distributions are available on the market with reasonable costs and the requested safety approvals. The potential for off-the-shelf computer equipment that can accept either 380 V DC or AC is a realistic possibility. There are industrial power supplies that can accept both normal mains AC (110-260V) and also 380V DC.
Besides those DC based remote powering method, there is also a different somewhat related technology called “Digital Electricity” supported by at least two companies. Articles What is digital electricity? and Amtrak HQ renovation employs passive optical LAN technology for long-term campus network connectivity present the new “Digital Electricity” concept. Amtrak HQ renovation employs passive optical LAN technology for long-term campus network connectivity article mentions that PON network was wired with a hybrid cable that has a single strand of bend-insensitive single-mode fiber that will provide unlimited bandwidth, plus two 18-gauge copper wires to carry electrical power to each work area outlet. Because of the inherent drop in voltage that occurs over copper wire, Reale’s team also incorporated digital electricity to enhance transmission performance. This emerging technology combines DC power and data into packets which are transmitted and received in a manner that is somewhat analogous to how information packets are conveyed over networks.
The article says: “This emerging technology combines DC power and data into packets which are transmitted and received in a manner that is somewhat analogous to how information packets are conveyed over networks. Digital electricity allows us to push power out to much longer distances without having to plan for the normal voltage drop – and without having massive copper wire size; we incorporate this technology into our designs when centralized power is a must and on projects when the facility’s design doesn’t support traditional cable lengths – such as rail stations, airports and sports venues.”
EDN blog posting What is digital electricity? tries to dig into what technology is behind this “Digital Electricity” marketing term. There are apparently two vendors prominent in this area: VoltServer Inc. and JMA Wireless.
The stated benefit is that the energy is so low that the wiring does not need an electrician to do the installation, so it can be done by regular construction crews. It is claimed to be lower cost in materials and highly efficient. It also meets all relevant UL and IEC regulatory requirements for low-power safety. A complete system requires what are called digital electricity transmitters and complementary receivers. VoltServer claims about 200 installations in various commercial buildings as well as named sports stadiums. It also meets all relevant UL and IEC regulatory requirements for low-power safety.
There is a hour-long VoltServer presentation “Touch-Safe, High Voltage Digital Electricity Transmission using Packet Energy Transfer,” which you can view.
The basic idea is that you send enough energy, via short pulses, and they are integrate and convert into a fairly large amount of output power. The system is designed to detect if something is wrong, and stop sending power to line immediately then something wrong is detected.
One EDN blog comment said: “The power is said protected not by the cabling or fusing, but by monitoring every 1.5ms and shutting down when they detect a fault. If their transmitter is designed with appropriate hardware and software it can be very safe. One nice feature is that you get built in ground fault and arc fault detection. Even better than ground fault, it can detect a person from “hot” to “neutral”, not just hot to gnd .”
The pulses are halted within 3 msec if there’s a break in the cable, someone touches it, or any other irregularity. So the damage on short on circuit or touching wire should be pretty small, because energy on one or two pulses quite low. There is some data transfer after every power pulse.
Image source: EDN article What is digital electricity? got it from VoltServer.
From the EDN blog comments say “Seems to me it’s just a 700Hz high-voltage square wave.” and “I see this poorly named mechanism (?) as 666 hz power with data injection.” Those comments are pretty consistent on the other documents on the same technology. I can agree the comment “the naming of the base technology is hokey, as much marketing tends to be”
EDN blog comments said” high voltage spikes would capacitively couple to other conductors in the wire bundle, creating an EMC issue.” and “Biggest problem: (as others have stated) radiated emissions of a high voltage square wave”. That could be an issue on some applications. The radiated emission is less than with just square wave signal, because the signal does not seem to be exactly square wave, but somewhat smoothed with limited rise and fall times. This was maybe done for EMC reasons and maybe can do something with the fault detection. It seems that this method is more noisy than DC or traditional mains AC, but is it “too noisy” I can’t say for sure.
One EDN blog comment said on he voltage level: “As I understand this, it is 336Volts DC @ 73% duty cycle (1.1msec on 0.4msec off) or 246V average. At 300 watts per pair that comes to 1.22 amp.” It seems that 336Volts DC level was selected because it is at such voltage level that there are many ready suitable power supply designs for this. For example most universal switch mode power supplies (90-250V AC or so) would nicely run on 336V DC, because in their design they first rectify the incoming AC to DC (230V AC peak voltage is 320V). For simple rectifier it does not matter if the incoming power is AC, DC or pulsed DC (at about 700 Hz repeat rate).
It seems that besides thin telephone cable there are systems aimed to thicker cables and higher power. One comment said: “They said they spec larger gauge wires for longer runs and higher power. 16ga (or was it 18) is common in the security business and therefore is inexpensive, so they use it in many applications.” Another comments said “”Oxymoron? Limited power source of 1 kW? That does not qualify as limited power suitable for limited power wiring” and “Sounds like they’re circumventing the code.” I don’t know all the fine details of electrical code in USA to be exactly sure if they are circumventing the code, using some loophole in it or not.
The are several patents related to this technology.
The basic patents tells that this is a powerline carrier data transmission system with intermittent DC power transmission, with built in fault diagnostics to allow for safe transfer of higher DC voltages. The author declares the periodic transmission of DC supply voltage qualifies as “digital power” and then uses a powerline carrier technique to transfer data over the power leads at different points in time. The OFF time for the power transmission is also used to detect fault conditions on the leads.
There is also a patent allows for a dynamic power distribution network by controlling networked nodes with the transmitted data.