ECN Forum Forums Code Related Discussion NEC & other Code issues Phone line Protection

Just a curious Q...

Driving down Rte 60 I noticed a few places that the phone lines were perpendicular and intersecting the power lines. With my "worst case scenario" mentality, I pondered what would happen if the 4160V line came down onto the phone line, and what dangers to the public would be possible...

What is the voltage rating of outdoor phone cable? (They're currently converting to fiber optic, but I'm pretty sure the older ones are still copper)

Assuming the phone cable doesn't have the dielectric strength to hold back 4160V, would the grounded phonelines have enough "continuity" to open an OCPD?

Just where would this energy go if unprotected?

Good argument for cordless phones if my suspiscions are true!

Phone lines are heavily protected against overvoltage, for both utility line contact and lightning protection. The energy would end up going into the ground at the nearest telco "can" or junction box, through a gas tube type of surge arrester. Assuming a utility line contact, the thin telco wires would disappear in a flash of copper vapor in a few milliseconds.

Surge arresters are also installed where the telco wires enter buildings, usually at the "network interface", and are supposed to be bonded to the electrical service grounding system.

Not much is going to protect against a direct contact with distribution lines (or a lightning strike) immediately outside a residence, but in general, the long wire runs and small conductor sizes effectively limit the energy "let-through" by the time it gets into your home.

Just where would this energy go if unprotected?

can it go to telemarketers first???

Whatever you do, don't count on the Telco surge arrestors to protect anything except an old fashioned rotary dial telephone and maybe some of the old pushbutton touchtones. Your modem, ansering machine, fax machine, and computer can all go up in the proverbial puff of smoke if you do not install additional protection.

I had an answering machine back in the 80's that ate some lightning. Luckily, the manufacturer thought they had enough additional protection built in & they gave me a new one free just so I'd send them the dead one so they could do a post mortem. The jolt even blew a few of the covers off the machine.

If you're smart, you won't talk on anything except a cordless phone during an electrical storm.

Tom

[This message has been edited by Tom (edited 10-04-2001).]

The exchange (central office) end of lines here are generally fitted with discharge-type surge arresters, except where the cables are all underground. Sometimes arresters are used at strategic points elsewhere.

Protection at residences is poor to non-existent these days. The current jack units have a simple spark gap, but wired across the line rather than to ground.

Sparky,

"can it go to telemarketers first???"

I'm with you!!!

Bill

The best protection is from a TVSS that has common power - phone - and coax protection this way they are all at the same potential in a surge. There are some hard wire units like this and surge strips too. To do it right you need both, more is better in surge supression.