"Indusi" is an acronym derived from "Induktive Signalsicherung", or Inductive Signal Protection. The official term is PZB, for Punktförmige Zugbeeinflussung, "spot-wise train control", as opposed to Linienzugbeeinflussung (LZB), linear train control.
The Indusi was introduced in 1934, most signals still were wire-operated semaphores, so the trackside magnets do not need power supply. The idea is to prevent running a red signal under almost any circumstance.
The communication takes place by magnets that are mounted to the right of the right rail. A similar magnet is mounted to the locomotive. The locomotive's magnet continuously emits magnetic fields with frequencies of 500, 1000 and 2000 Hz, respectively.
The trackside magnet contains a passive resonance circuit which is tuned to one of these frequencies and may either be switched on (active) or off (inactive).
If the switch is closed, the resonance circuit is shorted, so it cannot resonate and hence the magnet is inactive. If the switch is open, and the magnet is passed by the locomotive's magnet, it will begin resonating at its proper frequency, thereby extracting energy from the locomotive's magnet and reducing the voltage in the respective resonance circuit of the loco's magnet by some 80-90%.
Since the trackside magnet gets its energy from the loco's magnet by induction, the system in principle does not need a trackside power supply and thus works perfectly even with wire-operated semaphore signals. In case that the switch is operated by a solenoid (i.e. it is an electro-magnetic relay) connected e.g. to a colour light signal, a power failure will leave the switch open, so the magnet will be active.
The theory of operation is (theoretically) quite simple (If it would practically be simple, it wouldn't be German...)
If the signals show clear (with line speed), the magnets are inactive and thus have no influence on the train. If a signal shows stop, then all magnets are active.
At the distant signal, there is a 1000 Hz magnet. If that is detected, the driver must acknowledge that he has understood the distant signal by pressing an attention button within 4 seconds, or the brakes will be applied. Subsequently he must reduce the train's speed to a certain level within a certain distance, see also details below.
If that speed limit is exceeded, the brakes will be applied.
Next comes a 500 Hz magnet, where the speed is checked again against an even lower limit. All this is to ensure that, even if the driver acknowledges the distant at caution, but does not brake sufficiently or does not brake at all, the main signal is not passed at danger.
At the main signal there is a 2000 Hz magnet. This magnet will always cause an emergency braking when detected active by the loco, and so the train will come to a full stop within the safety overlap after the main signal. (There is always a short distance between the main signal and a danger point, which may be e.g. a point, a crossing or a buffer.)
A 1000 Hz magnet is placed with a semaphore or colour light distant signal, with an Hl or
combination (Ks) signal which functions as distant or combined
signal, with a level crossing signal, and with a stand-alone distant signal post plate if
that announces a colour light signal for Falschfahrbetrieb (wrong
Note that in the remainder of the Indusi pages I will refer to Hl and Ks signals serving as distant signals as 'distant signals'.
A 500 Hz magnet is placed between the distant and the main signal, usually at some 250 m before the main signal or 450 m before the danger point, and
a 2000 Hz magnet is placed at the main signal.
|The 1000 Hz magnet is active when:|
|»||The distant signal shows Vr 0 or Ks 2: expect stop|
|»||The distant signal shows Vr 2 or Ks 1+Zs 3v with a speed of up to 60 km/h|
|»||The level crossing signal shows Bü 0: stop before level crossing|
|»||Always at a stand-alone distant signal post plate|
|home||signalling rules & practice|
|bi-directional traffic||Indusi part 2|