The Indusi can also be employed to enforce speed restrictions, although it was not originally designed for this matter.
The principle of operation is this: A device detects the approach of the train and activates a track magnet (say, a 2000 Hz magnet). A timer is set, and after a fixed time the magnet becomes inactive again. If the train runs at or below the permitted speed, it will reach the 2000 Hz magnet after it has been deactivated, so no action is induced. If however the train would be too fast, it would reach the magnet while still active, and so the train will do an emergency braking. This is called Geschwindigkeitsüberwachung (Speed Supervision) or GÜ for short.
Another way of enforcing speed limits is by using a track magnet which is always active by itself (a 1000 Hz magnet would limit the speed to 85 km/h, a subsequent 500 Hz magnet would limit the speed to 45 km/h).
Depending on the situation, either of the magnets can be used.
The speed restriction determines the Meßstrecke (Measurement Length). At this length before the track magnet, the switch-on magnet is located. This magnet detects the passing of the loco’s Indusi magnet and activates the track magnet. At DB lines, about three meters after the track magnet there is the switch-off magnet, which deactivates the equipment. At DR lines, the switch-off magnet is located at about seven meters before the switch-on magnet, so the equipment is always powered on but reset immediately prior to activation, so it is in a defined state, should the previous train have left it activated.
Suppose a speed limit of 80 km/h is to be enforced. To provide for a certain tolerance, the speed is checked against a limit of 95 km/h. At the announcement signal, there is a 1000 Hz magnet. If the train’s speed is at or below 95 km/h, the 1000 Hz magnet is already turned off, so the train is not influenced. If the speed is above 95 km/h, the Indusi is triggered. The driver has to acknowledge the warning light and subsequently must lower the speed to 95 km/h (I am assuming the loco is equipped with DB’s Indusi I60, set to train type "O").
Since that Indusi (again, train type "O" assumed) enforces a maximal speed of 95 km/h if triggered, it would be useless to enforcing speed limits of 100 km/h or higher. To that end, a 2000 Hz magnet is placed at a given distance before the speed commencement signal or the point where the lower speed must be met. The distance is chosen such, that, should an emergency braking be triggered, the speed is below the limit when the train passes the speed commencement signal.
|limit||check speed||magnet type||magnet location|
|100||120||2000||485 m before commencement signal|
|110||130||2000||405 m before commencement signal|
|120||135||2000||355 m before commencement signal|
|130||140||2000||315 m before commencement signal|
|140||150||2000||215 m before commencement signal|
So far, I’ve talked about enforcing limits of 80 km/h or higher. So what about limits between 10 km/h and 70 km/h? That can be done by:
In this case, standalone track magnets without additional equipment are used, i.e. the magnet used is active at all times.
At the speed announcement signal, a 1000 Hz magnet is placed. The driver has to acknowledge the warning and must reduce the speed to 95, 75, or 60 km/h depending on the train type.
With speed limits of 10 km/h through 40 km/h, at 150 m before the speed commencement signal, a 500 Hz magnet will be placed, enforcing a speed limit of 65, 50, or 40 km/h.
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