Learn English – work in/at/on something

X at Y means X is a place, and Y is close enough to X that if we wanted to find Y, we'd first have to find X. A place is large amount of space where things can be built or where things can happen.

X in Y means X surrounds Y, either physically or logically.

Buildings surround you, so when you enter a building, you are said to be in it, and can say you work in a building.

Businesses sometimes aren't just a building or room in a building, they have a campus, etc. Or maybe they have multiple buildings. So then they can be elevated to "place status" and therefore you use at.

I work in a kindergarten.

You're saying there's a room or building labeled "kindergarten" and that's where you work. If it's part of a school or greater institution, this is proper to say.

I work at a kindergarten.

You're saying there's a place called a kindergarten and you work there. This gives the impression it's a separate building, with it's own parking lot, etc.

This is a great question!

The first and third options - "in a power plant" and "at a power plant" - are both correct for this specific example of work, so long as you mean "doing some job inside an existing power plant".

The second option "on the power plant" would mean something different - e.g. that you are an engineer or labourer involved with the construction of a new power plant. For instance:

"I'm a design engineer, and I am working on the power plant == "I am working on the design of the power plant."

However, in other cases, the "on" preposition will be appropriate. The general principle is that:

• "at" or "in" are generally used when the worksite is indoors (e.g. an office, factory, shop, etc), while
• "on" is typically used when the worksite is an outdoor structure (and the work is done largely outside).

The following examples illustrate.

Examples where "in" or "at" are (largely) interchangeable

• "I work in a bank." or "I work at the bank."
• "I work in a factory." or "I work at a factory."
• "I work in a grocery store." or "I work at the grocery store."
• "I work in a power plant." or "I work at a power plant."

Note that in these examples, the "at" preposition will generally be more appropriate where the workplace is a specific workplace (preceded by "the").

Examples where "on" is essential

• "I work on an oil rig."
• "I work on a ship."
• "I work on the rail tracks."

Examples where "on" or "at" are largely interchangeable

• "I work on a construction site." or "I work at a construction site."
• "I work on the wharves." or "I work at the wharves."

Examples where "at" is essential

• "I work at the outdoor vegetable markets."