Using Switch Line Cards

Some of our customers would like to use switch line cards for powering loop extenders. We strongly recommend against the use of switch line cards for this purpose. A telephone switch is not designed to be a general purpose power supply and its use as such creates some concerns for the service and the switch. These physical limitations are summarized in the table below.

Maximum Line Power Available. All switches are designed to limit the power provided to the telephone line to protect against shorts and abuse. A line powered telephone requires 20mA to power the microphone and speaker in the handset, so this defines the minimum power specification of a switch. Most new generation switches are designed to provide precisely that -- 20mA -- as maximum current although some older switches are reported to offer as much as 50mA of power. The 20mA draw of a telephone set is also the signal to the switch that the subscriber is off-hook. To avoid causing the line to appear to be off-hook, a loop extender must draw significantly less than 20mA. The combined current of telephone set & loop extender also must not exceed the maximum rated switch current (20-50 mA) for all devices to operate correctly.

The voltage on the line when the phone is off hook may drop significantly (as low as 8v) making it difficult to maintain the rated voltage of a line powered loop extender. When that voltage drops, the current demands of a loop extender will rise. For this very reason Charles Industries requires 35mA on its line powered loop extenders vs. 10mA on express powered lines. This 35mA power draw added to the off hook current used by the telephone (20mA) is usually higher than the rated power available from the switch (20-50mA). Given this extra power draw and low voltage, both the phones and the loop extender are underpowered and may force the switch past its rated power output, exhibit problems with false off-hook and ADSL stability. Even if the switch can produce this power on a line by line basis and gracefully current limit itself for line card protection, the extra power demand on a permanent basis may be well over the design requirement for the switching system when many loop extenders are used.

Our in-house lab testing with the Charles Industries loop extender confirms these problems. When the line powered loop extender is used with an off hook phone, the line voltage will crash below acceptable levels. T1.418 specification requires that the line voltage never be pulled below one half of the open circuit voltage. This is a limitation of the physics of power distribution and should not be violated for a stable service. There is a good discussion about why this limit is so important at http://www.commsdesign.com/showArticle.jhtml;?articleID=54800658

When using the Charles line powered extender, frequent resync issues occur when the phone goes off-hook and the voice service performs below expectations as well. In the field, customers also confirm these observations when using new technology switches. When using old technology switches, short loops, a limited number of line powered phone extensions and a limit to the total number of loop extenders per switch, this may be a workable solution. However, we assume this is an unrealistic environment.

Human Safety Factors

It is generally accepted that a low-voltage (110 to 220 V), AC current traveling through the chest for a fraction of a second may induce ventricular fibrillation at currents as low as 60mA. For DC power, 300 to 500 mA is required for the same affect. AC ring voltage powering 5 REN is actually closer to this hazard threshold than the 50mA DC loop power. By raising the voltage we are able to send more power down the line yet stay well within the safety limits of UL making the solution safer for both plant and personnel.

Underwriters Laboratories (UL) specifies that RFT-C circuits must have a way to limit the current to 60 mA. No maximum voltage limit is specified for RFT-C circuits, except that the voltage must be limited to the rating of the wiring insulation. Telco wire PVC insulation is rated for 600v (or more) and 26 AWG wire is rated for 363mA. The Widearea loop extender power supply is limited to 115v and 50mA DC, well within safe limits and has been approved by UL.

Key Recommendations

Based on all these issues, power on telco lines should be limited to 60mA and 600v to maintain safety of equipment & personnel. Operating line powered repeaters from a switch line card is sometimes possible, but the current draw will exceed the rated power of the card and will often result in unstable power when line powered phones go off hook. Safety and service reliability argue strongly against using switch provided power. Some means to limit current below 60mA is necessary. Using an auxiliary power supply in the central office to increase the voltage and limit current is the best and safest way to deliver power to remote equipment.