> > > > > > > You might want to seriously concider adding current limiting on those lines
> > > > > > > current over 35 mills is a potental source of trouble, with relay contacts,
> > > > > > > hook switches, etc. dr.d
> > > > > > No such problem. Our PBX handles the incoming CO lines.
> > > > > > And it's under maintenance contract. :-)
> > > > > > The diff between 27 and 55 mA isn't enough diff to cause any shortening
> > > > > > of the lifespan of a relay.
> > > > > > --
> > > > > I beg to differ I've seen and had to service relays, hook switches and
> > > > > selector switches that excessive loop current fried, whall not as common in
> > > > > relays, it is still a cause of many mystery problems in C.O. circuits, having
> > > > > a maintance contract is well and good but for a small additonal cost you can
> > > > > prevent unesssary down time and frustration, of course there is the
> > > > > possablity that your switch has a currrent limiting circuit as an intrigral
> > > > > part of the C.O. circuits, if so you can count you self truly blessed. Most
> > > > > manufacturers dont bother. dr.d
Quote:> > considered low current. The Art of Electronics, by Horowitz and Hill,
> > has a graph, I believe from the Bell Labs, which gives the permissible
> > voltages and currents for a POTS line. It also shows the areas where
> > currents are NOT allowed. Some of these areas are on the _low_ end of
> > the graph. In other words the low currents are not allowed because they
> > resemble leakage currents and may cause the CO to go off hook. On the
> > high end, there are areas where the current is limited by the CO
> > equipment, so the line should not go over this. So, assuming that these
> > Bell System maximum currents are taken into account when the phone
> > instruments were designed and built, then there should not be any
> > problem with excessive current.
> > --
> > I grant you that low current can be a source of problems, it however is not common, the FCC, and Belcore standards require a minimum of 22 ma be supplier to the terminal equipment, this is based on several conventions that the standard, battery feed resistance is 400ohms, that the battery is 48 volts, that the line resistance is 1500 ohms or less, that the terminal resistance is 200 ohms. Obviously there is some vareation possable especaly with loop legnth/resistance
Fix your long line problem. This above paragraph was on ONE single
line! Limit it to 80 characters.
Quote:> and feed voltage, however keep in mind there is no mandated loop current
You, yourself just specified, above, that there was a maximum. You said
48 volts divided by the sum of 400 ohms plus 200 ohms. That gives 80
mA. Don't try to weasel out of that one!
Quote:> As long as they deliver a minimum of 22ma their job is done, if
> your equipment is damaged it's not their problem. If you have a C.O. that
> will limit the current great, a lot of us don't. I've messured loop current
> of 54 ma with a phone in the loop. As far as low current resembling *leekage*
> this would be a senereo where a marginal loop current will cause unreliable
> opperation, hence the minimum of 22ma. Personaly I like to see a solid 27 ma
> minimum in the loop.
Then by your judgment all of our lines are marginal. Our location has
lines that get a consistent 27 mA from the CO at the MPOE. Maybe
slightly less when the currents travel thru the lines across our campus.
Quote:> Unfortunitly telephones and other terminal equipment are
> often designed by engineers who have no idea what the REAL world is like and
> got their paramiters from a book. I personaly think that any one who is going
> for an engineering degree should be required to work in the real world for
> two years fixing other engineers junk befor they get their deploma. dr.d
Maybe one should get an engineering degree before criticizing engineers
for their inexperience with the real world. Walk a mile in their
shoes. That way, when you criticise them, you'll be a mile away, and
have their shoes. ;-)
There's nothing wrong with parameters from a book, provided they mirror
the real world. Unfortunately some of the parameters have to do with
"engineering economics" that, when emphasized, often cause shortcuts to
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