In telecommunication, a ringer equivalence number (REN) is a somewhat arbitrary number which denotes the electrical load a telephone ringer has on the line. In the U.S., this is determined in accordance with ANSI/TIA-968-B (August 2009).
REN is a United States-developed yardstick, but 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.
In Canada it is called a Load Number (LN); which cannot exceed 100. (The LN of each device represents the percentage of total load allowed).
In the United States 1 REN was formerly equivalent to a 6930Ω resistor in series with an 8 µF (microfarad) capacitor. The modern ANSI/TIA-968-B specification (August 2009) defines 1 REN as an impedance of 7000Ω at 20Hz (type A), or 8000Ω from 15Hz to 68Hz (type B).
In Europe 1 REN used to be equivalent to an 1800Ω resistor in series with a 1 µF capacitor. The latest ETSI specification (2003-09) calls for 1 REN to be greater than 16 kΩ at 25 Hz and 50 Hz.
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 telephone. Note that the REN of modern telephone equipment may be significantly lower than 1: as a rough guide, externally-powered digital-ring phones may have a REN as low as 0.1, while modern analog-ring phones (where the ringer is powered from the phone line) typically have a REN around 0.8.
Maximum REN loading
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.
Effects of overload
If the total allowable REN load is exceeded, the phone circuit may fail to ring (or otherwise malfunction. See call waiting and caller ID). In extreme cases, the telephone service provider may temporarily disconnect an overloaded line to reduce load.
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