Signal relay

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A signal relay is a relay that must meet the requirements in the area of train protection so that it can be used for safety-relevant circuits in the area of ​​railway safety technology. The term “signal” is not to be understood here in the IT sense, but is derived from the main area of ​​application of signal relays: railway safety technology (also: signal technology, train safety technology).

The main area of ​​application is the relay interlocking, but signal relays are also used in other interlocking technologies.


Signal relay "K50" from Siemens (type C)

General security requirements

Basically, relays are very well suited for safety-relevant circuits, because - in contrast to electronics - they only have a few types of failure and therefore the safety verification can be carried out completely at component and circuit level. In order for this to be possible, however, signal relays require special properties that go beyond the properties of conventional relays.

Safety-relevant properties of conventional relays:

  • High response threshold
  • Preferred failure direction (monostable relay)
  • Self-cleaning contacts

Additional properties of signal relays:

  • Large contact distances
  • Double break in contact
  • Safe opening, even when other contacts are welded
  • Forced guidance of the contacts

The most important feature of signal relays is the positive guidance of the contacts . This means that all contacts must be mechanically rigidly connected to one another. This is the only way to infer the position of all other contacts in terms of circuitry by checking one contact.

Special security requirements according to classification

Comparison between signal relay type C (front, neutral relay N3 from WSSB ) and type N (rear, neutral DC relay НМШ for interlockings of type EZM (G) )

When designing safe relay circuits, two different types of signal relays are used, which are divided into two classes according to the UIC definition: Type N (not controlled) and Type C (controlled).

With type N relays, also known colloquially as "gravity relays", the relay armature will always drop when the voltage on the coil is switched off. In addition, a material combination (e.g. carbon / silver) is used for the contacts that makes welding impossible. Type N relays are - due to the design requirements - larger and heavier, but due to the excluded failures, the circuits are simpler.

With type C relays, the relay armature dropping is not guaranteed. This can e.g. B. remain attracted by residual magnetism and breaking of a spring even when the voltage is switched off. In order to prevent dangerous states from occurring, the switch-off of the relay must be checked, for which the forced guidance of the contacts is indispensable. In addition, contact welds are not physically excluded. Type C relays are smaller and lighter, but the necessary circuit tests, in particular waste tests, make the circuits more complex.


Monostable signal relay type II (»normal relay «) from WSSB


Monostable signal relays only have one stable - the released - position. Since this is the lower-energy state, the dropped position is used as the preferred failure direction in the circuit design, since it is much more likely that a relay dropped out incorrectly than incorrectly picked up.


Bistable signal relays have two stable positions, which are usually achieved by a coil system each. Bistable relays are used to save states - even in the event of a power failure or any other interruption in the coil supply line. In addition, they are more energy-efficient, because after changing the position, the power supply to the coil that was last attracted is usually interrupted by a contact from the relay itself. Bistable relays are more complex to construct.

Backup relay

The stable positions of the backup relay are created by mechanical supports. Support relays are historically the oldest bistable relays. The fine mechanical effort for the support mechanism is disadvantageous.

Support relay from VES

Toggle relay

With toggle relays, the stable states are established by spring force. Since a safe condition cannot be guaranteed if these are broken, toggle relays may only be used with special circuit-related measures for safety-relevant circuits.

Latching relay

The latching relay is the most modern bistable signal relay. The stable states are created in different ways: By spring force and magnetism. The magnetically attracted position is considered the preferred failure direction.

Special types

Block relay

The block relay works on the principle of the stepping mechanism and is used for classic line block tasks. It can correspond with alternating current block fields and with other devices that work on the same principle, such as VES block magnets.

Motor relay

The motor relay is based on the asynchronous motor principle. It works both very frequency and phase selective and is therefore largely insensitive to external and interference voltages. The motor relay is mainly used as a track relay for track circuits and as a block relay in the automatic section block. Motor relays are available as two- and three-position relays. A two-position relay has two defined end positions, de-energized and energized in an end position usually turned to the right. The three-position relay has an additional end position in a counter-clockwise position; it occurs after the control phase has been shifted by 180 °, usually through polarity reversal. This means that three pieces of information can be transmitted over two cable cores.

Circuit diagram

Contrary to the usual representation in electrical engineering, the " arrow short circuit " is used for electrical circuits in railway safety technology.

Specialist literature

  • H.-J. Arnold among others: Railway safety technology. transpress Verlag, Berlin 1980, DNB 810486628 .
  • Klaus Fischer among others: Securing rail and road traffic - technical basics. transpress Verlag, Berlin 1983, DNB 850307201 .
  • Wolfgang Fenner , Peter Naumann, Jochen Trinckauf : Railway safety technology. Publics Corporate Publishing, Erlangen 2003, ISBN 3-89578-177-0 .
  • Werner Köhler: Relay. Basics, designs and circuit technology. Franzis-Verlag, Munich 1978, ISBN 3-7723-1602-6 .
  • Ulrich Maschek: Securing rail traffic Chapter 3.6 Relay information processing. Springer Vieweg, Wiesbaden 2012, ISBN 978-3-8348-1020-5 .