Specifications

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6.1 Network Interface Device
Back in the bad old days when the phone company rented you a
phone and did inside wiring they made no provision to install
customer supplied equipment, commonly called Customer
Premise Equipment (CPE). With the advent of
telecommunication deregulation the local telephone companies
were prohibited from being in the equipment business. This
caused a dilemma because there is a need to demarcate between
the customer and Phone Company responsibility. Everything
outside the demarcation point is the responsibility of the Telco;
anything inside is the customer’s.
The specific embodiment of the Network Interface Device (NID)
has changed over the years but the basic purpose remains the
same. The Telco installs a device that terminates outside wiring,
and provides lightning protection. The customer side has
terminals to connect inside wiring and a test connector to quickly
disconnect inside wiring from the Telco. Some NIDs include half-ringer test circuit. The half-ringer creates
a unique test signature to allow test equipment to determine if faults exist on the Telco or customer side of
the demarcation point.
The picture at right shows a typical multiline NID. Telephone company wiring terminates under the
protective cover on the left. The Telco side contains protection circuits that divert lightning surges to earth
ground. The right hand side has provisions to connect inside CPE wiring and a test disconnect. Opening the
cover exposes a RJ11 single line phone test jack. Plugging a phone into the test jack, automatically
disconnects the inside wiring. If the test phone works the problem is the inside wiring, if it does not the
problem is with the Telco.
6.2 Secondary Lightning Protection
The phone company provides lighting protection as part of the Network Interface
Device. Electronic devices are more fragile then electromechanical phones; this is
especially the case with computer equipment because they have multiple connections,
power, phone, DSL and Ethernet. This makes the equipment susceptible to line surges.
Adding secondary protection minimizes the risk of damage. The best location for
lightning protection is the building entry point. That allows all equipment to be bonded
to the same low impedance ground connection minimizing voltage difference between
different conductors. Lightning protectors do not absorb energy they divert it
somewhere else. If the diversion path is not low impedance a substantial voltage
difference is created. This is what kills electronic gear.
The EDCO TSP-200 series protectors add very little capacitance to the line. This is
critical so the protectors do not interfere with the high frequencies used by DSL. The
protector clips to a 66 style split block. The Surge protector acts like a bridging clip
between the left side (Telco) and right side (Phone). With the protector removed inside
wiring is completely isolated from the external conductors. A grounding bar runs down
the left side of the block. This is connected to a high quality earth ground, the same used
by the NID and the power service entrance. When the protector fires damaging voltage
is shunted to ground.
One protector should be used on each telephone line. Additional protectors should be used on any lines that
connect to outbuildings.
Figure 18 Telco NID
Figure 19
Lightning
Protection