circuit breaker/fuse necessary for IoT home monitor?

Question

In most 12v CCTV power supplies, each power cable is behind a glass fuse. I haven't seen fuses or circuit breakers used when distributing 5v power used in IoT projects. Is there a reason overcurrent protection is not needed in these projects?

edit: I'm thinking specifically about fire safety. e.g. the risk of a short circuit overheating 22awg wire run through the house?

Answer 1

Is there a reason overcurrent protection is not needed in these projects?

You're comparing apples and oranges.

The two systems are completely different and have different concerns.

Case in point:

• You want to burgle somewhere with CCTV installed. You can creep up on one camera unobserved.
• You know a little about electronics and figure if you cut the feed to the camera you can short out the power and blow the whole system, thus letting you wander in unobserved.

But there's fuses on each feed, so all you do is disable the one camera and everything else is fine, catching you red handed.

Go directly to jail. Do not pass go. Do not collect £200.

Now... do you really care that much if all your home temperature sensors lose power because of a fault in one? Probably not. And the chances of there being a fault other than from human negligence is slim. Sure, you could add fuses, but why add the expense? It's really not needed for such a non-critical system.

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edit: I'm thinking specifically about fire safety. e.g. the risk of a short circuit overheating 22awg wire run through the house?

22AWG wire has a resistance of about 53Ω per km. Say you have a loop of cable 20m long (10m there and 10m back again) and it shorted out at the far end. That's round about 1Ω.

At 5V you'd be drawing 5A of current.

If you have a really powerful power supply then it would try and supply that, and the wire would warm up - giving off about 25W of heat over the entire length. You might feel a bit of warmth from it.

Of course, most people would be using a smaller power supply than that for simple IoT things, so the power supply would just go "No. You're not having that!" and would die, or its internal fuse would blow. Or it would drop the voltage and get hot itself.

The chance of being able to start a fire from 5V over 22AWG from a normal power supply is pretty much zero. You're more likely going to start a fire from using a low quality power supply that itself melts and catches fire (i.e., a counterfeit Chinese phone charger).

With a very powerful power supply (tens of amps or more) and a short circuit close to the source (so low resistance) the wire will melt. Just that. It'll melt. Once melted there's no circuit, so nothing else happens. There's your fuse right there.

Once a wire has melted it doesn't continue to burn. And you don't get arcs at low voltages. It takes a lot of work to start a fire from low voltages. A lot of work and a lot of current.