What does FD30 mean?

FD30 means the door was tested to hold back fire for 30 minutes. FD60 is 60. F for fire, D for door, then the minutes.

Add an S, as in FD30S, and it also has smoke seals. Most flat entrance doors and corridor doors want that S, because in a real fire the smoke gets to people before the flames do.

You can't tell by looking. The proof is a label or a plug in the top edge or the hinge side, sometimes a stamp in the glass. Older doors were tested to BS 476 part 22 and newer ones to BS EN 1634-1, and a door tested to either one can be perfectly good.

A thin panelled door with no label, the sort a lot of Victorian conversions have, is a different matter. We write down what it is and why we can't call it a fire door.