Abstract
This article discusses the sabotage of loops of intruder alarm systems. Although loop alarm systems are now gradually being replaced by digital alarm systems, they are still significantly present in practice. This paper describes two experimentally verified techniques for sabotaging balanced loops. The first technique is based on the jump replacement of the balancing resistor by a fake resistor. The second technique is based on inserting a series-parallel combination of two rheostats into the loop. By alternately changing the resistance of these rheostats, a state is reached where the balancing resistor is shorted by the parallel rheostat and replaced by the series rheostat. Sabotage devices for both attacks are technically simple and inexpensive, so they can be made and used by an amateur. Owners of loop alarm systems should become find out about this threat.