Repetitive Electrical Impulse Noise (REIN) is when electrical "noise" from an external source interferes with the ADSL (or other electronic) signal. Usually the presence of other power equipment is not an issue, however on occasions an electrical item can introduce additional noise in the same band of frequencies utilised by DSL.
The result of REIN on a broadband service is to increase noise on the line, and hence to reduce the maximum speed at which the line can run - in other words, broadband runs slower than it should.
Typical causes of REIN include ignition systems in central heating boilers, thermostats, motors, pumps and fans, and heavy electrical equipment such as relays and contactors. A faulty piece of equipment (causing arcing and sparks) can generate electrical noise. Another cause is poor earthing.
REIN interference can be difficult to solve. BT have specialist REIN engineers with equipment that can help locate the source of the interference. Sometimes an obvious effect of REIN is that a broadband line runs much better (with a higher signal to noise ratio and/or faster speed) at a weekend when industrial equipment is idle.