Differential loop

A differential loop is a special form of the closed- circuit current loop in alarm systems .

While with the simple closed-circuit current loop only the presence of the closed-circuit current is monitored, the differential loop also evaluates the level of the current. However, this depends on the voltage depending on the component. The current is normally determined by a terminating resistor at the end of the detector group (loop), the nature of the line (length, cross-section, material) and transition resistances at connection and (series) switching contacts. If the value changes up or down because a detector responds or a line is interrupted, a corresponding message is output.

The resistance in the junction box furthest away from the line or inside a detector forms one of the four resistances of the measuring bridge . If the voltage falls below or exceeds a certain threshold value , this is evaluated as an alarm or sabotage .

As a result of this procedure, it is not easily possible for an attacker to sabotage the line by disconnecting it or by short circuiting, because the former would lead to an interruption in the current and the latter to an increase in current.

Another effect is that detector with normally open contacts (NO = normally open) and detectors can be used with normally closed contacts (NC = normally closed) in a loop. The interconnection of the resistors and the trigger criteria, the manufacturer of the headquarters of the burglar alarm system (EMA) and the hold-up alarm system (I & HAS) and the specification of the VdS depending GmbH.