From Power Protection Basics for Competitive Carriers: Every CLEC and Reseller Should Know What can Cause Metallic Plant to Waiver,
by Russ Gundrum, published by America’s Network
on October 15, 1997.
“Competitive local exchange carriers (CLECs) and resellers that connect their facilities to incumbent local exchange carrier (ILEC) networks can benefit greatly by understanding the electrical aspects of wire-line (metallic) telecommunications facilities. Most of the services that CLECs and resellers provide involve a pair of copper wires that run from the customer’s telephone jack and network interface device (NID) to an ILEC’s central office (C.O.). When troubleshooting a customer problem, CLECs and resellers should know a circuit’s make-up, whether it’s served by a digital C.O. and whether an analog or digital subscriber loop carrier (SLC) is involved in providing the service. These companies should also know whether the SLC service is being delivered via fiber optic or copper cable.”
“This level of induced AC also can cause intermittent outages to analog or digital loop carrier (DLC) systems that are served over long, metallic facilities. The induction literally ‘starves’ the repeaters of DC power, and shuts down the systems. Also, the amount of induced AC current used to be important at the C.O. end of subscriber pairs, because the C.O. provided a low impedance path-to-ground (the positive terminal of the talk battery was always grounded) for the current to flow. However, too much current would saturate the line relay equipment and cause noise problems, or the switch would just take the circuit out of service and disconnect any customer on the line at the time. Digital C.O.s solved this problem by ‘floating’ the battery from ground, which eliminates any AC current flow. However, we now have AC induced voltage appearing at the switch, and as little as 20 volts-to-ground may cause the switch to disconnect from that circuit and deny service to the customer. The unfortunate thing about this ‘self-protecting’ feature is that a surge that small will cause a disconnect! If this is steady-state induction, the C.O. will be out of service until this condition is removed from the circuit. Induction usually affects an entire cable route—meaning, more than one subscriber can be affected.”
“AC induced voltage generally is greatest near the exchange boundaries of the C.O. serving area—not close in. Historically, that’s where ILECs have made voltage-to-ground measurements, because the C.O. end was grounded and the far end was an open circuit. However, if the far end happens to be a PBX trunk circuit that has ground-start signaling, then once the circuit goes to ground, an AC induced current flowing to ground can be high enough to seize the trunk and keep it from operating properly.”
“Incidentally, this same induction phenomenon—either voltage or current—can cause innumerable false alarms on fire or security alarm circuits that either want to see battery or not see battery. Because AC is sinusoidal, the induced AC will add voltage to the circuit during the positive portion of the sine wave to cause the alarm, or subtract from the battery during the negative portion of the sine wave to cause the voltage to disappear from the circuit, which also can cause an alarm.”
“The things to remember about noise and possibly coordinating with the ILEC in solving problems it causes, is the location of the imbalances in the circuit. Of course, this assumes that the CLEC has access to the ILEC’s serving-C.O. quiet termination number to make power influence and circuit noise measurements. It also would be helpful to have the ILEC’s milliwatt supply (1000 Hz tone) number to do basic transmission measurements. In fact, many CLECs will have sophisticated test equipment that they can use to pinpoint what the problem is on an ILEC’s facility; thus, when the trouble is reported, this information could be passed along to expedite the resolution of the customer’s line trouble.”
“Just as AC induced voltage is usually the greatest at the far end of a metallic facility, so is the power influence measurement that is made with a noise measuring set. Assuming that the overall circuit balance (C.O. and station equipment, plus the ILEC’s cable and the customer’s inside wiring) is at an industry acceptable level of 60 decibels (dB), then an industry acceptable level of power influence of 80 dBrnc will cause 20 dBrnc (decibel reference noise C-message weighted) of noise. However, if the overall circuit is quiet when the CPE is removed, but becomes noisy when reconnected, the circuit imbalance is in the inside wiring or the CPE (which is usually the case). The ILEC has no responsibility under this situation, and the CLEC can place a mitigative device to reduce the influence, or it can reconfigure or change the equipment.”
“However, if the CLEC discovers more than 90 dBrnc of power influence at the customer’s demarcation jack, then the ILEC should be contacted about this unacceptable level of interference, because it can cause unacceptable levels of noise, even if the customer’s equipment is well balanced. In other words, 90 dBrnc of power influence, acting on a circuit balance of 60 dB, will generate an unacceptable level of noise of 30 dBrnc. It’s the gray area of these three circuit indicators that generally create the most controversy.”
“A typical cause of an imbalance in physical plant is moisture in a cable. Because this is a capacitive imbalance, moisture generally causes more of a noise problem near the end of a cable route, because that’s typically where the voltage and power influence are greatest. A customer may have purchased a mitigative device from a CLEC to solve voltage problems, but he may not get a satisfactory reduction in circuit noise, because the power influence may have already acted on the circuit imbalance and converted to metallic noise, passing right through the mitigative device. If the ILEC treats its cable with a similar mitigative device ahead of the imbalance, then the customer should receive a substantial noise reduction. Of course, if the ILEC had done this initially, then the customer would not have needed a mitigative device at his end of the circuit in the first place!”
“This is the ‘real world’ of telephony. CLECs and resellers that understand these basics and implement them on a daily basis and on every trouble call will go a long way toward making themselves winners in the marketplace.”