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The ringer equivalence number (REN) is a telecommunications measure that represents the electrical loading effect of a telephone ringer on a telephone line. In the United States, the REN was first defined by U.S. Code of Regulations, Title 47, Part 68, based on the load that a standard Bell System model 500 telephone represented, and was later determined in accordance with ANSI/TIA-968-B (August 2009).
Although the REN was developed in the United States, analogous measurement systems exist internationally. In some countries, particularly in Commonwealth nations, the REN is also known as the ringer approximated loading number (RAL).
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. The REN of modern telephone equipment may be significantly lower than 1. For example, externally powered digital-ring telephones may have a REN as low as 0.1, while modern analog-ring telephones, in which the ringer is powered from the telephone line, typically have a REN of approximately 0.8.
In the United States, the FCC Part 68 specification defined 1 REN as equivalent to a 6930 Ω 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 ringer), or 8000Ω from 15 Hz to 68 Hz (type B ringer).
The total REN load on a subscriber line is the sum of the REN loads of all devices connected to the line; this number expresses the overall loading effect of the subscriber equipment on the central office ringing current source. Subscriber telephone lines are usually limited to support a load of 5 REN or less.
If the total allowable ringer load is exceeded, the phone circuit may fail to ring or otherwise malfunction. For example, Call waiting, caller ID and ADSL services are often affected by high ringer load. 20th century equipment tends to contribute to a larger REN than new equipment.
Some analog telephone adapters for Internet telephony require analog telephones with low REN, for example, the AT&T 210 is a basic phone which does not require an external electrical connection and has a REN of 0.9B.
In Canada it is called a Load Number (LN); which must not exceed 100. The LN of each device represents the percentage of total load allowed.
In Europe 1 REN used to be equivalent to an 1800 Ω 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.