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I have been designing a system whose purpose is to provide high reliability to a set of outlets (8), it takes to inputs of diffrent phases mains current, now the purpose of the system is that if the input we currently are using dies (Goes permanently to 0v) we would switch to the second one without or with minimal power loss to the outlets (enough to maintain the device running), now electrical engineering is not my area of expertise (at least at this level) and i cant figure out how to do this, i have tought about using a TRIAC with a zero-cross circuit but i can't know how the outputs will be affected and if it will serve the purpose, could anyone give me an idea about if im going about this the right way or if there is a better way to do this.

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    \$\begingroup\$ If EE is not your area of expertise, why are you designing this system? \$\endgroup\$ – Andy aka May 15 '15 at 16:20
  • \$\begingroup\$ Im a single developer doing all of the system, the rest (code and others) was what i was here to do, but it just fell on me to do this \$\endgroup\$ – INdek May 15 '15 at 16:26
  • \$\begingroup\$ What type of device? Does thew device have a power supply to convert the AC input into DC internally? Is the device an AC induction motor? \$\endgroup\$ – bigjosh May 15 '15 at 16:29
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    \$\begingroup\$ Have a word with your boss because it sounds like you might be underqualified to design this section. Safety of you and what you design is of a concern. \$\endgroup\$ – Andy aka May 15 '15 at 16:29
  • \$\begingroup\$ @bigjosh the typical use case will be computer servers and switches both of witch run DC internaly \$\endgroup\$ – INdek May 15 '15 at 16:30
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If you really have to design it yourself (and not recommended), one approach you can use is relays, that would be safer and easier to get right. But relays will take a little time to switch so the output of the device when switching over will drop, now if the mains devices connected to it are things like laptops where they have power supplies with a fair amount of capacitance or batteries involved, these will tolerate a momentary loss of mains power for perhaps 100 miliseconds. If your output devices can't tolerate a momentary loss of power then don't go down this route.

You need a sensor (or two) to detect the loss of mains input power, you can do this with a 240 volt relay (coil energised by 240 volts) and the relay contacts triggering a little circuit to give a logic level input into a microcontroller. So as the mains power fails you get a change in logic level to the microcontroller which you can detect in software and trigger the relays to switch power to the other mains input.

Triacs: will give a faster switch over time and better in applications where the devices being powered can't tolerate a momentary loss of power, but triacs need more sophisticated circuitry to drive them: you have to apply pulses to the gate connection. And preferably those pulses need to be synchronised to the mains waveform when the signal crosses through zero (called zero voltage switching) so as to reduce the amount of radio frequency interference generated. It's all getting a bit messy.

SSR - think you mean SCR - silicon controlled rectifier. These devices are only unidirectional, so not suitable in your application.

If you can buy something off-the-shelf, I'd recommend that.

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Sounds like you are trying to build an Automatic Power Switch. Since increasing reliability is the motivation here, it might make sense to purchase a commercial product that has already been field tested. It is likely that a commercial solution will not only be more reliable than a homebrew one, it will also likely be cheaper as well once you factor in all the time and effort.

Take a look at some of the models from Tripp-Lite, which are used in data centers...

http://www.tripplite.com/product/automatic-transfer-switch-pdus/942

Automatic Transfer Switch PDUs Provide redundant power for single-corded network devices with PDUs with Automatic Transfer Switching (ATS)

Dual ATS input cords support connection to separate primary and secondary power sources If the primary power source becomes unstable or fails altogether, the ATS will switch to the secondary power source until the primary input is restored and stable ATS functionality is supported by any two compatible AC power sources, regardless of phase, to support a variety of advanced redundant power networking applications Models with the “NET” suffix offer switched PDU outlets and built-in network interface for remote monitoring and control

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