Triple-Play Service Deployment

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Chapter 4: Troubleshooting the Copper Plant for IP Services
Foreign voltage
Typically, the first physical layer test checks for the presence of
foreign voltage, either DC or AC, on an open pair. Excessive foreign
voltage on an open pair can be a safety hazard and is also a sign
that the loop is in poor health.
Foreign DC voltage
DC voltage on an open pair typically comes from the central office
battery. This happens when a pair is crossed with one or more
working pairs as a result of physical cable damage, water in the
cable, or faulty splices. This cable damage can result in excessive
noise or attenuation on an xDSL circuit.
To accurately test for DC voltage, the technician must use a 100K
Ohm impedance digital voltmeter and test on a dry pair, with the
circuit open on the far end. Ideally an open pair should have no DC
voltage, but xDSL can tolerate some amount—generally less than
3V DC tip to ring, tip to ground, and ring to ground is considered
acceptable. If a DC voltage measurement is taken on a wet pair
connected to the central office, it should range between 48V and
52V DC tip to ring, -48V to -52V DC ring to ground, and less than 3V
tip to ground.