Door Locks - Why Credit Cards Are Not Keys
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Door Locks - Why Credit Cards Are Not Keys

In this article, basic mechanics of modern doors and locks are discussed in order to prove that credit cards are not effective tools for opening a locked door.

We all have seen those movies and TV shows in which some private detective or a random bad guy gains entrance through a locked door by means of a credit card. Now, while I’m all for finding some use for plastic cards that doesn’t include spending money I don’t have, I am here to argue that you cannot use one to compromise a modern, properly functioning lock in a properly installed door. Some of you may have enough faith in my level of expertise at this point to stop reading right now and either sleep better at night or pitch the MasterCard from your burglar bag, depending on which side of the law you reside. For all the rest of you, and those that just love reading my articles, I will support my conclusion.

Take a look at any modern, exterior door frame. When the door closes, its motion is limited by a raised rail portion of the frame. This rail is aptly called a “stop”, and, if you look closely, you will see that the stop is not simple a piece of wood nailed onto the side of the frame. Granted, a stop can (and used to be) nailed in place, but many decades ago, modern milling machines allowed the stop to be milled so that it is an integral part of the frame. What this means is that a card, or for that matter, a knife blade, or any other flat object cannot be slipped between the stop and the frame. There simply is no seam through which said object can slip. In my many demolition projects, I have had occasion to try to pry off a stop. I can tell you that the job involves a big hammer, a sharp chisel, and a lot of noise.

That said, I accept that somewhere, a door may have a nailed-on stop. Further, I suppose a very dedicated burglar could quietly whittle away with a box knife until they had removed enough of the stop material to gain access to the gap between the door and the frame. Assuming the lucky or industrious burglar found themselves with this situation, they still have one or two more problems to overcome before walking into the grand foyer.

Examine the latch mechanism of your open exterior door. You will see the sloped latch which pokes out from the edge of the door. Go ahead and push it in; it should move freely in and out of the door. Of course, this movement is necessary for the door to close without a turn of the knob or lever. Now, notice the funny little D-shaped pin that rests on the flat portion of the sloped latch? That pin is called a “latch plunger”. Did you ever wonder why that was there? Go ahead and push the latch plunger in about halfway. Now, try to push the sloped plunger. I would laugh if I was there, because you cannot push the sloped latch. (Ha ha) It is locked in the extended position. Go ahead and let the latch plunger return and the sloped latch operates as before (it is normal to try this a few times until you embarrass yourself with your play).

So now, let’s think about how the latch engages the striker plate. Since you’re examining the door anyway, notice that, as it closes, the striker plate lets the latch engage the door frame, but the latch plunger is kept compressed into the door. Remembering your little test and my little joke, you can conclude that the latch is now locked in the extended position. If you doubt it, try to use a coat hanger to pull the latch open while the door is closed. If everything is operating as designed, you will not be able to get the latch to retract into the door without turning the knob or lever. If things are not operating as designed, you may want to check out my Factoidz article entitled Doors – Keep Them Shut.

So, you can now see that, even with access to the gap between the door and the frame, a properly adjusted door and lock simply cannot be compromised with a card, even without a stop.

Apart from the cool mechanics of the latch plunger, most minimally, security-minded homes are equipped with a deadbolt lock. These locks have a bolt that extends into the casing and cannot be retracted without physically turning the lock. If you test this by attempting to shut your door with the bolt extended, you will be looking up door casing repair in Factoidz. If you don’t have a deadbolt, get one. If you have basic handyman skills, you can install it yourself and the peace of mind it brings is even worth the cost of a locksmith. In any case, I declare the credit card key myth as busted. Now, you can scoff at those unrealistic TV shows and sleep better knowing credit cards may get you into trouble, but they won’t get anyone into your home. – D.B. Sweet © 2010

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