Learn French: En cas d'effraction, il y a un boîtier qui prévient la police. - In case of a break-in, there is a control box that warns the police.

When using en cas de (in case of / in the event of), you generally drop the definite article (le, la, les) before the noun that follows. You are talking about the general concept of a break-in rather than one specific event, so it is simply en cas d'effraction.

Could I use "au cas où" instead of "en cas de" here?

Not directly with a noun. En cas de is a preposition and must be followed by a noun or pronoun, like effraction. If you wanted to use au cas où (in case), you would need a full clause with a verb in the conditional mood, such as: au cas où il y aurait une effraction (in case there should be a break-in).

I thought "prévenir" meant "to prevent". Why is it translated as "warns"?

Prévenir is a tricky false friend! While it can sometimes mean "to prevent" (like a risk or a disease), its most common everyday meaning is "to warn" or "to notify" someone. If you want to say "to prevent" an action from happening in the first place, you would use the verb empêcher.

What is the difference between "un boîtier" and "une boîte"?

While une boîte is a generic word for a box (like a cardboard box, a tin, or a storage box), un boîtier refers specifically to a hard, protective casing, usually for electronics or mechanical parts. Think of a control box, a computer case, or an alarm panel.