In many telecommunications applications, repeaters and other electronic devices are housed in remote units scattered throughout a geographical region in the vicinity of a central office. In one example, a remote unit communicates with the central office and also receives power from the central office through the same cable or other communication medium. This cable is also referred to as a “span cable,” “plant,” or “cable plant.” An example of a span cable includes a set of twisted-pair conductors over which telecommunications data is transferred between the central office and the remote units, and over which DC power is supplied by the central office to the remote unit.
The remote unit typically utilizes the power received from the central office over the span cable to power one or more electronic devices within the remote unit. The power delivered via a span cable is often susceptible to disturbances (such as faults, voltage spikes and surges) caused by environmental factors such as lighting and nearby electrostatic discharges. Left unmitigated, such power disturbances can interrupt telecommunications operations and permanently damage equipment.
Many electrical protection and personnel safety systems have been developed to detect these disturbances. One such system is generically referred to as ground fault detection system. With ground fault detection, the system looks for excessive current flowing to ground. When such current is detected, the ground fault detection system takes appropriate action such as shutting down the power supply that transmits power over the span cable.
AC power lines are often located within the vicinity of the span cable or plant of the telecommunications network. The signals on the AC power lines can adversely affect signals on the span cable through a phenomenon known as “AC induction.” With AC induction, an AC signal from the power lines or other source of AC power is induced onto the copper plant. When the electronic devices of the network are separated by a large distance, the plant is more susceptible to AC induction.
AC voltages typically are induced longitudinally upon span cables which cause currents to flow through the longitudinal noise filter circuits to ground at both the Central Office Terminal (COT) and Remote Terminal (RT) equipment. The earth ground maintained between the COT and RT installation completes the circuit, allowing the induced voltage to maintain current flow in the communication systems grounding path. The longitudinal noise filter circuits present a relatively high impedance to ground at the AC power line frequencies to avoid large currents from flowing in the filters ground path, as would be the case in a direct contact of an AC power line with the span cable (known as a power cross event). The ground fault detection circuit is designed to monitor the level of DC current flowing in the grounding system as the result of leakage currents to ground along the cable span and equipment. AC induction currents are imposed on the DC leakage currents and can look like a ground fault to the ground fault detection circuit during the half of the AC cycle which is additive to the DC current. Thus, the AC induced signal could trip the ground fault detection circuit causing the power supply to be inadvertently turned off. This could be compensated for with a large filter, e.g., a large capacitor, in the ground fault detection circuit to filter out the AC signal. However, the filter would have to be prohibitively large and expensive due to the large voltages involved. Further, if a large capacitor is incorporated into the ground fault detection circuit, any alternating longitudinal voltage on the span would be exposed to a low (longitudinal) impedance to ground. If the power lines came into direct contact with the cable plant of the telecommunications network, the power lines would be shorted to ground through the network device. Software filters have also been used to attempt to address this phenomenon. However, the effectiveness of software filters tend to roll off at higher frequencies. It has been discovered that some of the most relevant frequencies for AC immunity are harmonics that fall outside the effective range of traditional software filters.
Therefore, there is a need in the art for enhanced AC immunity in ground fault detection.