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Telephony: Minimum Loop Current on an Internal Telephone Line (FCC Specifications)

Posted on 2006-10-29
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Does anyone know if there are any FCC regulations specifying the minimum loop current on a telephone line. I am referring to the off hook current the is supplied to the telephone/fax by the carrier down the telephone lines. FCC part 68 specifies that the loop current should not exceed 100mA and there is reference to a minimum of 20mA, but I am not sure about this.
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Question by:Vespa72
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Lazarus earned 500 total points
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Currently there are no FCC rules as to Loop current.

At 20 mA or greater, it is offhook. At 18 mA or lower, it is onhook.
The switch should supply a minimum of 23 mA.
The absolute maximum allowed is 120 mA. The max recommended is 60 mA.
Typical values range from 23 mA to 55 mA, with 28 mA to 45 mA being the most common.

Perhaps the most notable thing about the above is that any resistance in series with a loop should be capable of dissipating the heat generated when 120 mA of current flows (and the assumption should be made that enough voltage can exist to cause that much current). That can be a significant amount of heat!

There is an ANSI standard though:
I cannot give a real answer to your problem since I have not seen the specifications. However, the ANSI/ISA S50.1-1972 standard specifies the use of 4-20 mA current loop to communicate the value of a process variable corresponding to a 0-100 percent full scale value. This is analog data transmission.

Another specification used in data communications is known as the Bell 202 (Bellcore) standard that calls for a constant current, usually 20 mA, to be carried on a two- or four-wire circuit. The data is communicated by use of FSK, Frequency Shift Keying, where one frequency, 1200 Hz represents a mark or 1, and another 2200 Hz represents a space or 0.


This little write up on Loop Curent might be of help to you though.
http://www.sandman.com/loopcur.html
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by:Vespa72
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That confirms my suspicion of the problem I am having. The clincher for me will be to try and hold my carrier to supplying greater than the 20mA they are currently giving. Thanks!
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by:Lazarus
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Good luck with that. It's to the telco intrest to keep them as low as possible. But it should be at around 23ma.
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