- Double switching
Double switching is the practice of using a multipole switch to close or open both the positive and negative sides of a DC electrical circuit, or both the hot and neutral sides of an AC circuit. This technique is used to prevent shock hazard in electric devices connected with unpolarised AC power plugs and sockets. Double switching is a crucial safety engineering practice in railway signalling, wherein it is used to ensure that a single false feed of current to a relay is unlikely to cause a wrong side failure. It is an example of using redundancy to increase safety and reduce the likelihood of failure, analogous to double insulation. Double switching increases the cost and complexity of systems in which it is employed, for example by extra relay contacts and extra relays, so the technique is applied selectively where it can provide a cost-effective safety improvement.
Landslip and Washaway Detectors
A landslip or washaway detector is buried in the earth embankment, and opens a circuit should a landslide occur. It is not possible to guarantee that the wet earth of the embankment will not complete the circuit which is supposed to break. If the circuit is double cut with positive and negative wires, any wet conductive earth is likely to blow a fuse on the one hand, and short the detecting relay on the other hand, either of which is almost certain to apply the correct warning signal.
The Clapham Junction rail crash of 1988 was caused in part by the lack of double switching (known as "double cutting" in the British Railway industry). The signal relay in question was switched only on the hot side, while the return current came back on an unswitched wire. A loose wire bypassed the contacts by which the train detection relays switched the signal, allowing the signal to show green when in fact there was a stationary train ahead. 35 people were killed in the resultant collision.
United Flight 811
A similar accident on the United Airlines Flight 811 was caused in part by a single-switched safety circuit for the baggage door mechanism. Failure of the wiring insulation in that circuit allowed the baggage door to be unlocked by a false feed, leading to a catastrophic de-pressurisation, and the deaths of nine passengers.
Some tri-colour Light Emitting Diodes for railway use were wired with four wires, one for each of the three colours, and a common wire for the return. Due to water ingress and other problems, the lamp units were displaying false greens. The solution was to change to wiring with six wires with separate positive and negative wires to the LEDs of each colour.
Faulty attitude indicator
Big airplanes have three independent attitude indicators, one for the pilot, one for the co-pilot, and a third one to resolve disputes between the first two. A Peruvian airplane apparently had a faulty wire in one of the indicators. The indicators for the pilot and co-pilot were switched to common mode, so they both displayed the same wrong attitude indications. In the dark, it was not possible to tell the true horizon in any way other than the attitude indicator, and the plane crashed into the sea.
Around 1994, new standards for the electrical couplings between carriages of United Kingdom passenger trains introduced the requirement for separate earth wires for critical functions such as brakes and doors. Common earths can cause interference between circuits that are otherwise independent, with unpredictable effects.
A similar crosstalk problem occurred when using Phantom circuits to increase the number of telegraph or telephone circuits.
- Redundancy (engineering)
- Double insulation
Wikimedia Foundation. 2010.
Look at other dictionaries:
Double Switch — may refer to: Double switch (baseball) Double switch (basketball) Double Switch (video game) Double switching, using a multipole switch to close or open two sides of a circuit This disambiguation page lists articles associated with the same title … Wikipedia
Double, Double, Toil and Trouble — DVD cover Directed by Stuart Margolin … Wikipedia
Double knitting — is a form of knitting in which two fabrics are knit simultaneously with two yarns on one pair of needles. The fabrics may be inseparable, as in interlock knitted fabrics, or they can simply be two unconnected fabrics. In principle, an arbitrary… … Wikipedia
Double bass — Contrabass redirects here. For other uses, see Contrabass (disambiguation). Not to be confused with Acoustic bass guitar. For the technique used in percussion, see Double bass drum. Double Bass Side and front views of a modern double bass with a… … Wikipedia
Double Glazing & Conservatory Ombudsman Scheme — The DGCOS, or Double Glazing Conservatory Ombudsman Scheme, was established in early 2010 to provide an independent regulatory body for UK’s double glazing and conservatory industry. The DGCOS was launched in May 2010 with the backing of… … Wikipedia
Double agent — For other uses, see Double agent (disambiguation). A double agent, commonly abbreviated referral of double secret agent, is a counterintelligence term used to designate an employee of a secret service or organization, whose primary aim is to spy… … Wikipedia
Double Talk — For the language game, double talk, see Gibberish (language game). Not to be confused with doublespeak. Double Talk Also known as Celebrity Double Talk Genre Game show Created by Bob Stewart … Wikipedia
double cross — 1. tv. to betray someone. (Originally a more complicated switching of sides in a conspiracy wherein the double crosser sides with the victim of the conspiracy against the original conspirator.) □ Don’t even think about double crossing me! □ Frank … Dictionary of American slang and colloquial expressions
Electric double-layer capacitor — Maxwell Technologies MC and BC series supercapacitors (up to 3000 farad capacitance) An electric double layer capacitor (EDLC), also known as supercapacitor, supercondenser, electrochemical double layer capacitor, or ultracapacitor, is an… … Wikipedia
Immunoglobulin class switching — Mechanism of class switch recombination that allows isotype switching in activated B cells Immunoglobulin class switching (or isotype switching or isotypic commutation or class switch recombination(CSR)) is a biological mechanism that changes a B … Wikipedia