Don Boone wrote |
Rick you might be right about the 100v thing unless telephones were exempt. Talk battery is 48 volts and ring voltage is 90 volts. But I heard the 36 volt thing sometime during my education at the Burn & Learn School of Electronics.
|
Telco ring voltage maxes at 100 volts @ 20Hz AC on some systems, and I've never heard of a code that says telco lines have to run in conduit. The current is so low that it won't cause a 24-gauge wire to burn, and there are protections at the CO or switch to shut it off if an over-current condition is detected in the system.
Talk battery is typically between 40 and 50 volts DC if the phone is on hook, and it drops to a max of 9vdc if it's off hook. Most phones will work off hook with as little as 4vdc.
The issue I have with the 36-volt number in a 70.7-volt CV distributed sound system is that it's not a real-world scenario. The center tap of a transformer does nothing for the speaker circuit. (In fact, I've never even seen a center tap connected unless it's a school system, where a call switch in a classroom shorts a floating CT to ground to produce a call-in at the head end.)
In your example, there are three wires in the circuit. One is carrying the "hot" or in-phase signal. A second is carrying the "cold" or out-of-phase signal. A third is center-tapped and is referenced to ground. If you measure the voltage across the hot & ground or cold & ground, you'll see a 6dB voltage drop versus the measurement between hot & cold. That's because half of the voltage (-6dB) is being taken to ground by the center tap -- which typically isn't connected to anything in the field.
The real-world measurement of voltage is taken from the hot & cold, where a peak-to-peak reading on a 70.7-volt speaker line will be 100 volts.
Again, though, the vast majority of distributed systems never come close to the full 70-volt potential in day-to-day operation. The highest I've ever measured hit 28 volts on my trusty Simpson meter, and it was crankin' loud!
Regards,
Rick Johnston