Working on antenna ground, ended up with phone NID question

[qdude]

Hi, kind and knowledgeable people. I was working on getting an antenna outside, doing things the right way, running ground wire, etc. I noticed that the ground wire for the network interface box for the landlane phone was in bad condition. I replaced it and got curious about how the phone ground works. You can see the (new) green ground wire. It seems to have connectivity only to an isolated black wire. Does that black wire simply provide a big surface, a (hopefully) more attractive place where stray voltage could jump if it got into the phone line? I know the phone wires themselves are balanced pairs that can't function if grounded. Thanks.

 

[mmckenna]

Would help to have the whole photo of the network interface.

Works similar to coax suppressors. Carbons or gas tube protectors in the network interface shunt some of the energy to ground.
I can't see the whole photo, so hard to tell what's going on. Usually the cable coming in from the phone company enters one side of the box that's not easily accessible to the customer. Inside that part is the carbons or gas tubes. Idea is the customer shouldn't be messing with them, as it's something the I&R tech would troubleshoot.

Sometimes there's an extra ground point on the customer side. Some phones at one point used a ground connection to signal the switch to utilize certain functions, but most of that is long gone.

 

[wowologist]

I dint think that's a ground wire at all. Years ago, like 30ish when DSL first arrived mainstream the telco would split the box and send one side to the modem and the other to the interior phone system. Then they came out with the actual filter/splitters inline and I think that was used to put the dial tone on the other side of the box. Might be that, might not.

 

[mmckenna]

know the phone wires themselves are balanced pairs that can't function if grounded. Thanks.

Yes, they are a balanced pair (or should be…) however one side is at ground potential back at the central office or remote terminal. You'll see something like -48vdc on the ring (red wire) and ground/return on the tip (green wire).

 

[qdude]

Here's the whole box. The line comes down the house, goes behind the box, and then loops up into the box. (By the way, the DSL comes inside the house on a dedicated line, so no need for filters on the phone lines.)

 

[mmckenna]

OK, thanks, that helps.
The ground stud on the left connects to a buss where the entry protectors are. The two modules are held in with a nut onto a grounded stud. If you checked continuity between your ground rod and the small nut holding the module in place, you should see continuity. The extra black thin ground wire routed over the top of the NI is there for the customer to connect their phone lines to.

 

[mmckenna]

"Molded in ground strap commonly grounds and bonds all protectors together while eliminating telco wiring clutter."

https://www.tiitech.com/repository/datasheetlibrary/3700.pdf

As for the customer grounding point:

• For special customer ground situations (such as partyline service, coin phone), a ground termination point is located in the upper most position of the customer compartment

 

[qdude]

"Molded in ground strap commonly grounds and bonds all protectors together while eliminating telco wiring clutter."

https://www.tiitech.com/repository/datasheetlibrary/3700.pdf

As for the customer grounding point:

• For special customer ground situations (such as partyline service, coin phone), a ground termination point is located in the upper most position of the customer compartment

Thank you for the education. Much appreciated.

 

[cmdrwill]

For special customer ground situations (such as partyline service, coin phone)

So the bobby pin would ground start the pay/coin phone.

 

[mmckenna]

So the bobby pin would ground start the pay/coin phone.

In some systems.
I still have a few ground start circuits at work, and keep a short jumper in my tool bag for tripping them.

Found out when I was new if you reversed the polarity, it would give dial tone and accept money, but never make a call. Coins would NOT be returned.

Found about $5.00 in change in one when I went out to fix it. Fix the polarity, and all the coins would dump out.