What a transfer switch does and why it exists

A transfer switch decides where the house gets its power. It's a device that, in Wikipedia's words, "switches a load between two sources."¹ One source is the grid. The other is your backup, such as a generator or a battery system.

The key rule is that the grid and the backup source are kept isolated from each other.

Why the grid has to be cut off

The transfer switch article says the generator has to be isolated from the distribution system. One reason is safety, "as utility workers expect the lines to be dead."¹

Line crews work on wires they think are off. A live feed coming back from somebody's house, with no warning to anyone up on the pole, breaks that assumption. Bad.

Never use an outlet to do this job

Plugging a generator or a battery into a wall outlet to power the house is exactly the unsafe version of this. It skips the part that separates the grid. Don't do it, and don't let anyone talk you into it as a shortcut. A transfer switch installed by a licensed electrician is the right way. Read more on the electrician page.

Manual and automatic

Some switches are thrown by hand. Others work on their own.

With a standby generator, an automatic switch "senses the power loss" and starts the generator.² It then "transfers the electrical load to the generator."²

Battery systems have their own built-in versions of this logic. The manual for your model says how it behaves. Your installer can tell you which kind you have and what it does when the grid returns.

A manual switch needs somebody at home to throw it. Automatic ones don't. Simple. That matters for a household that travels, or that has a medical device which can't wait. Talk it through before you choose.

Break-before-make

The usual type is called open transition, or break-before-make.¹ Such a switch "breaks contact with one source of power" first.¹ Only then does it connect the other. That means a brief gap with no power at all. Clocks may blink and a computer without its own battery may restart. It's the cost of a clean separation.

Closed-transition switches also exist. They overlap the two sources for a very short time. The source says that overlap should be less than 100 milliseconds.¹ Ask your electrician which type you'd get.

If a device of yours can't stand even a short gap, note that on your loads list. Your electrician can then plan around it.

Short version

Two sources, one switch, kept isolated from each other. A licensed electrician installs the switch. It's set up for your home and checked before anyone relies on it. If a plan you're shown leaves this part out, stop and ask why before you sign anything.

What it feeds

The switch can feed the whole house or a backup loads panel. Which one is a design choice, explained on the whole-home versus partial page.