5 or 25 Ohms

Discussion in 'General Technical Questions and Answers' started by KF5LJW, Jan 16, 2021.

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  1. KF5LJW

    KF5LJW Ham Member QRZ Page

    That is the question. After working in Telecom for 40 years I have had experience working with many Grounding and Bonding Specs. I got into Telecommunications early enough to work with the pioneers of Bell Labs, and Western Electric. We had a huge conference every year up to the dot com bust in the early 2000s. During those times I learned the origins of Telecom’s 5-ohm spec and NEC 25-ohm spec.


    Sometimes it helps to know where things came from, and why. For the answer, you have to go back to the US Civil War and the telegraph. My history of the telegraph is not such I can recall model numbers or names but the long line telegraphs required a 5-ohm ground, which allowed up to 450 ohms for the overhead wire conductor. They used dirt as a conductor for one of the battery polarities.


    Fast forward to 1919 the first rotary dial telephone system was installed in Norfolk Virginia. In the very early days of the telephone was primitive requiring party lines placing up to 12 customers on a single pair of wires leaving the Central Switching Office. So how do you use a single Tip/Ring pair of 20 AWG wire to ring 12 different customer circuits individually with only a 48-volt battery and coffee grinder AC generator?


    To start the telephone talk circuit is DC, so the ring voltage is AC of roughly 90 to 100-volts. The first thing they did was how they applied Ring Voltage in both Differential and Common Modes. For Differential-Mode, they applied Ring-Voltage Tip-to-Ring. The two Common-Modes were applied Tip-to-Ground and Ring-to-Ground. To use dirt as a conductor for the Ring Circuit required the Central Office Ground to be 5-ohms, and the home to be 25-ohms. That gave the Telco 3 but needed 12. Did I mention Ring Frequencies of 20, 30, 40, and 50 Hz at roughly 90 to 100 volts? They used 4 selective Ring Frequencies x 3 modes = 12 circuits. In those days it required the Telco to come to your house, install the ringer correctly and run a 3-wire circuit of Tip, Ring, and Ground.


    In those days obtaining a 5 ohm or 25-ohm ground was as easy as falling off a bar stool passing out drunk. Bond to the water pipe and you were done. We were stupid and ignorant in those days. We did not understand how extremely dangerous using ground as a conductor was or how much chaos it creates with RFI and EMI. It took a tall stack of bodies to change things. Seems Plumbers and Water Utility Workers were not happy being shocked and electrocuted. Wives had a problem burying husbands. That is when Water utilities started using dielectric insulators on water meters to stop electric utilities and telephone companies from using water pipes and ground as a circuit conductor.


    On top of our ignorance, our homes' electrical systems were two-wire systems consisting of Line and Neutral where one of the conductors, neutral is also used as a Ground. Took a lot of bodies and destroyed equipment to change to a 3-wire system with a ground.


    Today you still see the shadows of the past. Many hams think they need a rod outside the shack, Neutral and Ground (Negative and Ground) are the same things, and AC ground is dirty. All true back in the 50/60’s before transistors and color TV came along when the world was 2-wire. You even see the 25-ohm verbiage in the NEC although is completely meaningless. Paraphrasing NEC 250.53 exception: if a Single Rod does not provide 25-ohms, an additional rod shall be driven. Drive two rods, pass inspection, collect a check, and call it a day. Both rods could be 1000-ohms making a 500 Ohm ground is good enough. You still see the 5-ohm verbiage in some specs for Elmers out there with a Party Line, in a home with 2-wire electrical systems, tube radios, and a grainy Black and White TV set.
     
    W6KCS likes this.
  2. W1BR

    W1BR Premium Subscriber QRZ Page

    I got caught using a TT phone on a party line. Ma Bell was not happy with me. Also got yelled at for cleaning the carbons back in the sixties; Bell guy really had a fit when he saw the phone patch on the service call.
     
  3. KE0ZU

    KE0ZU Ham Member QRZ Page

    In the very early days of Cordless phones, "you" couldn't attach anything to MaBell's lines. It took a few court cases to some extent the breakup of MaBell, to allow attaching "things" that met MaBell's previously secret specifications to the twisted pair. But here we are today, with twisted pair becoming extinct outside of the business world.
     
  4. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    5 ohms was the Motorola R56 standard
     
  5. WN1MB

    WN1MB XML Subscriber QRZ Page

    Balanced line antenna feeds and balanced audio lines are a form of twisted pair technology and far from extinct.
     
  6. NG1H

    NG1H XML Subscriber QRZ Page

    Even there it is going away. During the past 4 years all the small businesses I know of have all moved to a fiber link with VOIP instead of POTS. This is less because of lower prices for fiber (locally) but because POTS has become so d*mn expensive.
     
  7. KE0QQQ

    KE0QQQ Ham Member QRZ Page

    I used to service phone company trucks on the iron range of Minnesota in my youth. This line worker loved to tell the story of the dog tied up to the house to some pipe with a metal chain that howled every time the phone rang. Poor thing :(
    Once he told the owner why. The owner changed to rope
     
  8. KP4SX

    KP4SX Premium Subscriber QRZ Page

    Been hearing that one for years :)
     
  9. KE0QQQ

    KE0QQQ Ham Member QRZ Page

    You worked in Ely too? Geez its cold there this time of year. hi hi
     
  10. KP4SX

    KP4SX Premium Subscriber QRZ Page

    Actually no but I had a headhunter offer me a job there once. I looked at a map and said NO!
     

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