Ringer equivalency number
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In telecommunication, a ringer equivalency number (REN) is a somewhat arbitrary number which denotes the loading a telephone ringer has on the line. In the U.S., this is determined in accordance with the Code of Federal Regulations, Title 47, part 68.
A ringer equivalency number of 1 represents the loading effect of a single "traditional" telephone ringing circuit, such as that within the Western Electric Model 500 desk telephone. Note that modern telephone equipment may have a REN significantly lower than 1: as a rough guide, externally-powered digital-ring phones may have a REN as low as 0.2, while modern analog-ring phones (where the ringer is powered from the phone line) typically have a REN around 0.8.
The total REN for a subscriber's line is simply the sum of the RENs of all devices connected to the line; this number expresses the overall loading effect of the subscriber's equipment on the central office ringing current generator. The local telephone company usually sets a limit on the total REN, typically 5 or less.
If the total allowable REN load is exceeded, the phone circuit may fail to ring (or otherwise malfunction). In extreme cases, the telephone service provider may temporarily disconnect an overloaded line to reduce load.
While REN is a United States-developed yardstick, analogous systems exist internationally. In some countries, (particularly in Commonwealth nations?), the REN is better known as the ringer approximated loading number (RAL). In the United Kingdom it is called the ringer equivalence number and a maximum of 4 is allowed on any British Telecom (BT) line.
(Source: partly from Federal Standard 1037C)