[SI-LIST] Re: -48v

Let me correct a few misstatements.
While the use of -48v for telephone circuits is over a hundred years old 
and has evolved into many different systems, it does NOT have anything 
to do with ringing requirements. There are at least seven systems for 
ringing, the most popular being 16 to 20 Hz at 120 vpp. Note this an AC 
voltage that is super imposed over the regular telephone supervision  
(term for the current that is passed down the line from the CO (Central 
Office)).  In the old offices the ring voltage was derived from an 
alternator (mechanical).  Now they use solid state inverters.

-48v is obsolete.  Many of the smaller offices use -24v. (Our equipment  
would accept  anything from -18 to 72 V.)

Many of the newer systems do not feed power from the CO at all.  This is 
because the subscriber is feed by carrier and fiber to a location near 
the subscriber.  The subscriber is connected to the distribution 
terminal by copper where local power is applied. The system is known as 
"Last Mile Copper".

Finally there is nothing inherently safe about -48 volts. It is feed on 
buss bars larger than your arm in the CO. Since -48v was around before 
any safety standards it was grandfathered into many regulations. That 
doesn't mean it is safe. It is extremely dangerous.

Fred Townsend



Mahesh Linge wrote:

>Dear Santhosh,
>A large voltage is required to generate a ringing tone in telephone
>system.That's why it is recommended to have a 48 Volts.When phone rings it
>gives atleast 40-45 Volts across the line.
>
>Regards,
>Mahesh Linge
>----- Original Message -----
>From: "Santhosh E P" <setavala@xxxxxxxxx>
>To: <si-list@xxxxxxxxxxxxx>
>Sent: Wednesday, March 24, 2004 11:17 AM
>Subject: [SI-LIST] -48v
>
>
>  
>
>>hi all...
>>is there any specific requirement of having -48v for telephone lines. if
>>then whats the advantage? why we can't have other voltages to carry the
>>signal, not even +ve?
>>regards
>>santhosh
>>
>>
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