Let’s talk about the “drop spec”

The Handheld Nautiz X2 rugged PDA.

Can you trust the MIL-STD-810G test?

Now let’s assume that a device has passed the MIL-STD-810G four-foot drop test and four feet is really all you need. Or is it? What if it falls off a porch, or slips out of your hands when you’re on a roof? Will your device survive?

The answer is, as is the case with so many things in life, it depends. Most things are designed to be able to handle more than the stress that comes with normal operation. That’s because of the engineering safety factor, the ratio between expected stress and absolute stress that something can handle before it breaks.

Safety factors vary. If stress is predictable, the engineering safety factor may be low, like 1.5 or 2. If stress is unpredictable and may vary greatly, the safety factor may be much higher, like 5-10 or even 20.

Interestingly, the MIL-STS-810G drop test, as commonly applied to rugged mobile computers, usually tests with no safety factor at all. Devices that are likely to fall from four feet are tested to see if they can survive falls from four feet. That’s probably okay.

But what if a handheld is tested for a four foot drop, but since it can be used as a phone it may drop from six feet? Shouldn’t it be tested for falls from six feet? Even that would only mean a safety factor of 1.5 above the expected likely fall from four feet.

But the height of a fall isn’t the only thing that matters. A fall isn’t an absolute, easily quantifiable event. Every fall is different. That’s because a device may fall onto any number of surfaces, from a grassy meadow that cushions the fall, to a field of sharp rocks that makes it much worse.

It also greatly matters how a device makes contact. If it falls flat on its back onto a flat surface that is not too hard, it’ll likely survive substantial drops. But if it falls onto an edge or a corner, the impact force will be much higher. And if a vulnerable component like a screen hits a sharp rock, all bets are off.