What is the component called that allows switching between two power sources so that only one of them can be used at a time?
A transfer switch allows you to switch between two or more power sources.
An Interlock is a device that prevents a breaker or switch from being toggled. In the situation of two power sources, an interlock is used to allow only one power source to be on at a time.
Unless it is totally safe for the two supplies to be connected together, to avoid risks of a faulty switch it is usual to use a 'change-over panel', containing two contactors, where either one being closed opens a switch to prevent the other closing electrically (electrical interlocking), and often a lever or system of levers between the two also peventing both being closed at the same time (mechanical interlocking)
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3So an interlock or transfer switch, just like the previous answer given almost TWO YEARS AGO. – Speedy Petey Aug 3 '16 at 16:16