Sunday, July 26, 2020

Are Your Latch On?

The other week I found myself locked out of our shed and subsequently learned more than I ever expected to about Yale locks, or night latches as I now know they're called.

The image at the top is a pretty standard night latch. It is opened from the outside with a key and from the inside with the handle. The latch (the gold tongue on the top left image) is sprung, which means that simply closing the door will push the latch onto the striker plate and into the box (both top right), locking it.  A deadlock which stops the latch from moving can be applied from the inside using the button (or, more correctly, the snib).

Night latches are an old technology, insecure, and make it easy to accidentally lock yourself out. The snib helps with the last of these by being able to hold the latch back inside the body of the lock. This means that even if the door closes, the latch can't engage and the door remains unlocked.

Which is nice to know, but my problem was that I couldn't get in. I tried a bunch of things, searched the web and tried some more things, thought about trying to drill the lock ... and then called a locksmith.

When he turned up he unlocked the door in 10 minutes with some airbags that forced the door slowly open while he aggressively turned the key back and forth. You gotta know where to tap.

Once in, we could tell that the door had been closed with the snib in the deadlock position. The manufacturer's intention is that the deadlock is used when the door is shut but there is nothing to stop it being used with the door open. Even so, that usually won't be a problem because the deadlocked latch will bump against the strike plate instead of sliding in, and the door will stay open.

However, if the door doesn't fit particularly well, and if it's slammed shut, the latch could be forced home and then the lock won't open even with the key. There's a reverse scenario too, where the latch is snibbed inside the body of the lock and the door is pulled to without locking, leaving a gaping security hole.

We usually use the snib to keep the door on the latch when we're going in and out of the shed but I have no way of knowing how many times we might have been at risk of either of those unwanted outcomes.

As testers, this feels like familiar territory doesn't it? We have some functionality which is ...
  • ... intended to be used a particular way
  • ... and effective when used in the intended way
  • ... but possible to be used in an unintended way
  • ... and generally harmless when used in the unintended way
  • ... until sometimes when it turns out to be harmful.

More familiar territory, thinking of options and their trade-offs. Here's some:
  • Change the lock for something more modern. OK, but costs money and short-term hassle.
  • Stop using the snib at all. OK, cheap but means we need to carry a key all the time.
  • Don't make a mistake with the snib. OK, cheap and easy, but we are human.

Right now, I've gone with the last one by (familiar territory again) taking inspiration from Jerry Weinberg:

Image: Yale

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