Newsgroups: sci.electronics.basics,sci.electronics.misc
Subject: Re: *TOTALLY* isolating phone from line, electrically?
References: 
Don Bruder  writes:

> OK folks, here's a little something that just blew through this disaster 
> area I call my mind. Tossing it out for the idea of determining 
> practicality:
> 
> Lightning kills computers and computer gear. We all know this to be a 
> pretty well established fact. We also know that, in most (not all, but a 
> very large percentage) cases, the lightning damage didn't come from the 
> power lines, which are, in general, pretty well protected, either by 
> compliance with well-researched/written building codes, requirements for 
> grounding a specific way, surge suppressors (both on the pole and within 
> the house) and so on. Indeed, most lightning damage seems to come into 
> the building by way of the phone line, which is nowhere near as heavily 
> "regulated and sheilded" by building practices. Phone wire runs the 
> lightning right into the house, cooking off whatever is attached 
> directly, and sometimes nearby items. Usual scenario: phone line takes a 
> hit, modem fries, takes computer's serial ports (or even more of the 
> motherboard) with it. 

It might look that the telephone line get the lightning into house, 
but thigns are not as straighforward.  
What kills the device connected to the telephone line and the mains 
power is the huge voltage difference that happens between mains power 
and telephone line. This voltage difference can be cause by either:
1. Lightning hitting the telpehone calbe, telephone central office 
   where the cable goes or nearby the central office. This causes 
   the telephone line potential to raise
2. The lighting hits the electrical distirbution network. Your whole 
   ground potential in your house raises to very many kilovolts 
   This causes the voltage difference between the telephone wires 
   that are grounded on the one end at the telephone central 
   that is still the original ground potential (no raise there)

> With that in mind, I've just had something resembling a brainstorm, and 
> want to bounce it off this merry band of electrical lunatics to see if 
> it's at all practical.
> 
> Since the phone line is (or at least, for the sake of this discussion, 
> I'm "ASS-U-ME"-ing that it is...) the most frequent route for a 
> lightning hit to follow and cause damage, 

It is the combination of mains connection and telephone connection 
that causes the msot disasters. 

> it seems to me that 
> electrically isolating the phone line from the house and contents would 
> be the best route to take. 

The telephone modems are already isolated somewhat (1.5 - 4 kV osolation). 
This is not enough for ay real lighting protection. 
�ou need a very high isolation to make the isolation work. 
There are some special products that convert telephone line to 
fiber and back, that could help in some this kind of cases. 
Quite expensive special solution. 

Usually the good enough protection can be made with proper 
whole house protection plan, where there are surge protectors 
for mains and telephone line, and both of those are connected 
togerher and wored to good ground connection. This kind of 
arrangement usually limits the voltage differences to something 
the equipment can handle.

> As of right now, I'm *REALLY* hazy on the 
> details, but the seemingly ideal implementation would be a box (black or 
> otherwise...) which plugs into the demarcation point (the place where 
> the wire from the pole connects to your house, if you're not up on 
> phoneco lingo) then everything else in the house that needs/uses a phone 
> line plugs into that.
> 
> Anybody ever heard of/encountered such a beast? 

Here is one isolator:
http://www.harksystems.com/tower/oc100.html 
It promises up to 75,000 Volts isolation (4 inch air gap).

Here are links to some other products:
http://www.optelator.com/

> If one exists, what would you expect/be willing to pay for it, and who 
> would you look to in order to source it?
> If no such thing exists, has anyone got a good explanation for why not?

The existing products on the market I mentioned are from around 
370 USD and up.

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/