Telephone protection circuit needed

[bud--]

Reviewing the schemtic posted by Andy, those fuses also do nothing
useful for a 4800 volt surge. Fuses are only rated for 600 volts -
remain conductive when trying to stop many thousands of volts.

I agree that there is a good chance fuses will not provide protection
unless they are rated a lot more like 4800V. Fuses also have an fault
current interrupt rating that may be exceeded.

What is two stage protection? A system that has layers of
earthing. Same exists with AC electric. 'Whole house' protector
installed in a mains box makes a short connection to building's
earthing electrode. That is secondary protection. Primary protector
provided by the utility is demonstrated in pictures in:
http://www.tvtower.com/fpl.html

This is for power wiring and is irrelevant for telco. If you are
lucky, a service panel surge protector will cause a high enough earth
current to blow the fuses on a 4800V line. Surge protectors are
usually based on MOVs which work well for short lightning events. But
a crossed power line may be long enough time to exceed the energy
rating of the MOV.

Meanwhile, the telco installs a 'whole house' type protector on all
subscriber lines where their wires meet a homeowner's. Again, this
protector is only as effective as its short ('less than 10 foot')
connection to a single point earthing electrode. A protector
installed for free because it is so effective and so inexpensive. A
protector often unknown to those who somehow want to stop or block
surges (ie those fuses). Surges are not stopped or blocked. Anything
that would do that stopping is already inside phone appliances.

I don't think a telco protector would be effective against crossed
power wiring. The clamping element will be burned out by high
sustained current and fuses, if any, will have the same high voltage
arc--across problem. Actually my old technology telco protector has
fuses in long tubes that might work.

Surges must be diverted to what surges seek. Either a surge is
earthed before it can enter a building OR surge will seek earth ground
destructively via household appliances. So that protection already
inside appliances is not overwhelmed, the effective protector makes
that short connection to earth.

In general, protection can be provided inside a building with plug-in
surge protectors which work by clamping, not earthing. (All power and
signal wires to a set of equipment need to pass through the surge
protector.) For a crossed distribution wire, a plug-in surge protector
may be protected by wiring impedance, but may well not.

An excellent guide from the IEEE on surges an surge suppression in
general is at:
http://www.mikeholt.com/files/PDF/LightningGuide_FINALpublishedversion_May051.pdf

And a similar one from the NIST:
http://www.nist.gov/public_affairs/practiceguides/surgesfnl.pdf

The IEEE guide was targeted at people who have some (not much)
technical background.



I have some doubts how practical it is to protect against power line
crosses, which are quite rare. The available energy is likely very
high. Depending on what the power line hits, it may stay live. And
distribution protection schemes may include reconnecting several times
before finally disconnecting.

Gas discharge tubes commonly used in telco protectors and MOVs
commonly used in power line surge protectors act like crowbar
circuits. Gas discharge tubes, like crowbar circuits, have current
limits that could be exceeded by a crossed power line. MOVs can have
very high surge current ratings but may be killed by energy - single
event or cumulative.

Adding high energy rating MOVs to phone lines may work. Depends on the
distribution voltage, circuit impedance and time before the
distribution line gets disconnected.