Railway signalling relays are large considering the mostly small voltages (less than 120 V) and currents (perhaps 100 mA) that they switch. Contacts are widely spaced to prevent flashovers and short circuits over a lifetime that may exceed fifty years. BR930 series plug-in relays<ref> http://www.morssmitt.com/railway/qstylests.htm </ref> are widely used on railways following British practice. These are 120 mm high, 180 mm deep and 56 mm wide and weigh about 1400 g, and can have up to 16 separate contacts, for example, 12 make and 4 break contacts. Many of these relays come in 12V, 24V and 50V versions.
The BR Q-type relay are available in a number of different configurations:
- QN1 Neutral
- QL1 Latched - see above
- QNA1 AC-immune
- QBA1 Biased AC-immune - see above
- QNN1 Twin Neutral 2x4-4 or 2x6-2
- QBCA1 Contactor for high current applications such as point motors. Also DC biased and AC immune. <ref> http://www.morssmitt.com/railway/signalling-infrastructure/signalling-relays/q-style-br930-signalling-relays/qbca1-ac-immune-dc-biased-contactor-relay/ </ref>
- QTD4 - Slow to release timer <ref> http://www.morssmitt.com/railway/signalling-infrastructure/signalling-relays/q-style-br930-signalling-relays/qtd4-slow-to-release-delay-units/ </ref>
- QTD5 - Slow to pick up timer <ref> http://www.morssmitt.com/railway/signalling-infrastructure/signalling-relays/q-style-br930-signalling-relays/qtd5-slow-to-operate-relays/ </ref>
Since rail signal circuits must be highly reliable, special techniques are used to detect and prevent failures in the relay system. To protect against false feeds, double switching relay contacts are often used on both the positive and negative side of a circuit, so that two false feeds are needed to cause a false signal. Not all relay circuits can be proved so there is reliance on construction features such as carbon to silver contacts to resist lightning induced contact welding and to provide AC immunity.
Opto-isolators are also used in some instances with railway signalling, especially where only a single contact is to be switched.
Signalling relays, typical circuits, drawing symbols, abbreviations & nomenclature, etc. come in a number of schools, including the United States, France, Germany, and the United Kingdom.
Intro History Basic design and operation Types Pole and throw Applications Relay application considerations Protective relays Railway signalling See also References External links
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