Ringer equivalence number

In telecommunication, a ringer equivalency 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 the "Code of Federal Regulations", Title 47, part 68.

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 the United States 1 REN is equivalent to a 6930Ω resistor in series with an 8 µF (microfarad) capacitor. 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.

Explanation

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 0.2, 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.

Call waiting and caller ID are often affected by high REN. Older (pre 2000) equipment contributes larger REN than new equipment.

DSL/high speed modem service requires all other equipment in the system to have an added "filter". Each filter adds to the REN total.

A DSL modem along with 3 older phones and the required filters could easily exceed a REN of 4 or 5. This could cause the failure of the phones to ring, call waiting or caller ID, to become "spotty" or inoperable. Even the modem could fail to work properly. Unfortunately, there is little that telephone repair can do to correct the problem. The only current fix appears to be the purchase of high quality modern equipment. "Bargain" equipment, although new, often has exceedingly high REN.

(Source: partly from Federal Standard 1037C, ANSI/TIA-968-A, ETSI TS 103 021)