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There are several types of electrical faults. The most common are opens, shorts
and grounds. Other common telephony faults include resistance faults, noise,
line imbalance, crosses, splits, power influence, induction, circuit loss, and low
loop current. Multiple faults can occur simultaneously. Each of these fault types
are discussed in the following paragraphs.
Opens
Typically, an open is a physical break in the transmission line that prevents any
signal or electrical current from flowing past the break. An open can occur on
either the Tip or Ring line, or both at the same time. Usual symptoms are no dial
tone or the line is out of service. Extremely high resistance can also appear as an
open. An open can occur when other faults are present.
Shorts
A short usually occurs when two conductors physically touch each other. But a
short can also occur when water, dust, or other matter causes the signal or current
to divert onto another path. The affect on service depends on the severity of the
short. A short can occur when other faults are present.
Grounds
A ground usually takes current or a signal out of the loop; it can occur on
either the Tip or Ring line, or both at the same time. The usual symptom is a loud
hum noise. A ground fault can occur when other faults are present.
Resistance Faults
Resistance affects the ability of current to flow in a conductor. Excessively
high resistance will appear as an open effectively blocking the flow. High
resistance can be attributed to a breakdown in the electrical characteristics of the
conductor or physical damage to the conductor.
Noise
Noise in a communications system can be any unwanted signals or disturbance.
Noise is typically heard as any detriment to the clarity of a signal usually as static
or hum. In an unbalanced Tip and Ring pair, current levels are unequal which can
induce audible noise.
Line Imbalance
When the electrical characteristics of each conductor in a Tip and Ring pair are
not the same, the pair is unbalanced. The imbalance of a pair can cause noise in
the circuit.