On Thu, 01 Jan 2009 18:45:36 -0800, Archimedes' Lever
:On Fri, 2 Jan 2009 02:25:48 -0000, "Phil McKerracher"
:
:>
:>It's true that isolation is not important for a well-insulated telephone.
:
:
: A 10kV arc from a 2MV lightning strike *COULD* make it all the way into
:the handset, and OUT of the perforations in the handset, through the
:earpiece or mouthpiece, and hit the user. The microphone and the
:earpiece transducer both use metal cans, making the distance to the user
:a mere 1/4" through air. Not good.
Let's get one thing straight about the old POTS telephone. It has never been
designed to include an "isolation transformer" - where "isolation" means
galvanic isolation....
A plain old POTS telephone is basically sacrosanct in terms of its lack of
requirement to include galvanic isolation to anything. All it has to withstand
is the normal voltage potentials found on the line itself.
However, ANY Customer Equipment (CE) which is powered from or has any connection
to the mains supply, and which interfaces to a telephone line, MUST have
galvanic isolation. The POTS telephone itself DOES NOT.
CE which is connected to the mains supply MUST have an approved mains
transformer (or SMPS) with the required galvanic isolation, and separation of
the telephone line side from the mains powered side using an approved line
interface transformer (600:600 usually). Also the spacing between any conductors
on the mains powered side and the telephone line side of the circuitry must be
at least 6mm (iirc). This includes any opto isolators used to provide signal
interface from the mains side through to the telephone line side. These devices
must have physically wide separation between terminals on the the input and
output sides, and they must a high voltage breakdown rating (eg. 7.5kV). In this
way 3 levels of galvanic isolation are incorporated in mains powered CE to pride
galvanic isolation "from the mains powered side".
:
: This is ONE of the many reasons that isolation elements are
:incorporated at VARIOUS locations in the system. One of which is at the
:CABLE connection to the phone itself, which is why isolation elements can
:be found at these positions. This is a standard element of device design
:where human contact is present, and has nothing to do with it being in a
lastic case. It isn't your Dad's AC fed two wire drill motor with an
:un-phased power cord and metal case. It is, however, in close
electrical) proximity (potentially) with lightning events, and that is
:why arresting elements have been incorporated.
In the old days of magneto, central battery manual and even central battery auto
equipment there was no isolation in the terms you mention above. Yes, they did
have "protection" devices on the telephone line to limit voltage surges and high
current ingress onto the line, but the telephone line today is essentially a
straight connection from the line interface equipment (which these days may have
VDR's and that's it) through to the telephone at the other end. The types of
protection which were once used on the POTS line were; fuses to isolate the line
in the event of a heavy current surge, heat coils to break the circuit if an
aerial line came into contact with a foreign potential which did not produce
sufficient current to rupture the line fuses, and lightning arresters (spark gap
or gas discharge). These devices were provided at both the exchange end (on the
MDF) and the customer end of the line (eg. protector #1). After the 1960's these
items were rapidly disappearing from the telephone environment due to the
majority of line plant being underground where it had almost zero chance of
coming into contact with foreign potentials.
In modern days where most cabling is underground, these devices are dispensed
with - no heat coils (which were only really required for pole mounted telephone
lines), no fuses and no lightning arresters. Even today a 3 stage surge
protector (gas discharge tubes, fusible resistors and VDR's) is only required to
be connected to a telephone line at the either the customer end or the exchange
end where part of the construction is aerial, and/or the local area is prone to
constant lightning activity. Due to the requirement for customer premises
cabling and CE to conform to standards which provide for adequate separation
from foreign voltage sources there should be little chance of a breakdown in
galvanic isolation from these sources to the telephone line.
