Automatically change between 2 different AC power sources to power 1 single phase AC output up to 40A.
 
When the Primary input power supply is lost (for whatever reason) the transfer switch will fall back to the Secondary input power supply within 12 milliseconds. Then if the Primary power source returns this device will switch back to that Primary supply.
 
This unit can be used to create a UPS system or can be used in stand alone solar systems with a generator, grid and/or an inverter. They are often used in caravans and boats to automatically switch between shore power and on-board power supplies.
 
The 40A Transfer Switch and why you might want one
 
Once you comprehend what the transfer switch can do and how it automatically makes your solar powered lifestyle that much easier you'll understand why it is a very cost effective device that quite literally sells itself.
 
Essentially it will switch between two AC power sources up to a limit of 40A at 230/240V. It has two AC inputs and one AC output, this output typlically goes to the main loads in a dwelling, RV, caravan or marine vessel.
 
Here's where it gets interesting; one of the AC inputs is called the "Primary" and the other is the "Secondary". The Primary input is dominant, meaning that the device will always use that Primary power AC source if it is available. However, if the Primary source ever disappears or fails then the device will look to the Secondary input to get its power supply. This preferential switching feature alone can be used in a wide range of different scenarios. 
 
Here are 4 examples:
 
The Grid as Back-Up: 
In your suburban home you could have your stand alone inverter supply fed into the (dominant) Primary input and have the mains grid power going into the Secondary input. So if your batteries ever ran down too low that your stand alone inverter switched off then the transfer switch would quickly switch over to the grid supply as a back up source for your home. Most of the time your home would run from the stand alone inverter but if it ever needed servicing or the batteries ran down then the grid would be your back up supply, keeping everyone happy. Once the inverter supply is restored the device will quickly use it to power your dwelling as it's always the dominant power source.
 
Remote Area Power Supply: 
In your stand alone off grid home in the middle of nowhere, you could have your generator AC output wired into the Primary input of the transfer switch with your generator switched off most of the time. Your stand alone inverters power output would be wired into the Secondary input to run the house most of the time. However, when the solar charge controller or your inverter monitors that the batteries are getting too low it can close a relay to help start your (electric autostart) generator. As soon as the generator AC supply is up to speed and stable the device will switch over and use that Primary power source while your off grid inverter goes into standby sleep mode. The battery charger also gets its power exclusively from the generator and once the batteries have reached float state (full) the charge controller will release its relay hold on the generator and it will turn off the generator. The transfer switch will sense the loss of its Primary power source and switch back to the Inverter supply on the Secondary input that is now running on fully charged batteries. Look Ma' no hands!!
 
Caravan, Marine & RV: 
On a boat or in a caravan you could have the "Shore Power" (external water proof male power socket) wired into the Primary input of the transfer switch and have your stand alone inverter as the Secondary source. When you pull up to the mooring or the caravan park your inverter is already running your fridge and other devices from it's batteries; but as soon as you plug in the shore-power mains power cord (or a generator) the transfer switch will swap you over to run from the grid supply as it is the Primary (dominant) source for the device. When it's time to leave you simply unplug and it switches back over to the inverter that is wired into the Secondary input, happy days.
 
UPS or Battery Back Up: 
In your suburban home or business you could have some solar panels or a battery charger, charging up batteries. At the transfer switch you could set the grid supply as the Primary power source and the battery powered stand alone inverter as your Secondary input power source (the inverter would normally be asleep in standby mode, using virtually no battery power). Most days the batteries would be charged from a relatively small solar system or charger while the house or business would run from the grid as most dwellings do. But if there was a natural disaster related power failure or black out then the transfer switch would instantly swap you over to your stand alone inverter (Secondary power supply) and you could continue to function. Once the grid power was restored it would automatically take over again as it is the Primary source.
 
 
Some key points:
 
The transfer switch is an AC (Alternating Current) device and it should only be installed by an electrician.
 
When the device switches over it does it very quickly at 12 milliseconds, but more importantly it makes sure to switch over at the zero crossing point. This means that it switches at a position in the AC sine wave where the voltage is momentarily at zero. This is an important feature as it means that devices like computers (even ones without internal batteries) and other sensitive electronics won't experience any losses or abrupt changes in their power supply.
 
It is not recommended to employ this device to switch between supplies while you are running large inductive loads, like large electric motors such as power tools or large AC pumps.
 
Water tight entry gland/s should be used to keep the internal components dry.
 
This transfer switch frees you up by automating the change over between dual power sources.