Instructables

Is there a way to prevent blowing a home fuse with another fuse?

This is probably the dumbest question I've asked, and I know that me asking it and why I'm asking it is going to scare so many decent people. But I can ensure you that I'm taking appropriate safety measures when I do these kind of things.

My questions is: I can either buy or easily build some sort of portable breaker box? I would like this box to be a sort of "test platform" of sorts, so I can make things that will plug into a 110 outlet (say a hotwire cutter) and if (for some reason) I do have a short somewhere, I will trip the test platform breaker, and not the fuse in my apartment. 

Reason I ask is I work graves, and it hard for me to meet up with my landlord. meaning, if I blow a fuse (which my apartment still uses) I will have to go 10+ hours on average before I can get it replaced.

Thanks in advance! 

kelseymh4 months ago

This is an excellent safety feature, provided you ensure that the breaker on your portable box is ****LOWER**** rated than the outlet/house circuit you're connecting to. For example, suppose that your wall outlet is rated for 10 amps, and it is part of (with multiple outlets) a 25 amp circuit at the panel. Then your portable box should have NO MORE than a 10-amp fuse/breaker built in, and maybe no more than 8 amp.

With that in mind, what you propose is easy to do, and is exactly how most "surge protector" power strips are built. If you look, you should find a little button (looks like a momentary contact switch) at one end. That's the breaker reset button.

For your project, just do the same. Use a plastic dual-gang (square) outlet box with a cover; install a dual outlet on one side, wired to the end of your extension cord (which itself should be rated *HIGHER* than the current limit). On the other side, install a mini circuit breaker in series with the hot wire, _before_ the outlets.

Leviton sells a screw-in "fuse" style breaker with a push-button on the top. You could use a light-bulb receptacle and screw that "fuse" in place. Mouser has a bunch of push-button style designed for mounting on circuit boards.

DoctorWoo (author)  kelseymh4 months ago

I'll have to touch base with my landlord or my apartment complex's maintance man. I'm not 100% what my apartment is rated for. I want to say your example is oddly spot on. That is, that my apartment is close to 25 amps.

As for the surge protector comment, I think I may actually have a surge protector that I use for workbench that has a fuse to it. I always forget to check for those!

As for the leviton, I'm actually using one on my hotwire cutter. I'm pretty sure its wired correctly, so it will trip prior to anything outside the setup. But, again, I would like to make sure I have all my safty ducks in a row when testing stuff like this. I may take a look at those mouser push-button breakers, though.

Many thanks!

mrandle3 months ago

Maybe a GFI outlet would work? Any one want to weigh in on it.

DoctorWoo (author)  mrandle3 months ago

Thats my thought as well. I know their there to break before a breaker to ensure no one gets shocked. I do now have access to the fuse panel (per permission from my land lord) so I might just [saftly] try it, and if I break the fuse, I can easily replace it with a breaker fuse.

steveastrouk4 months ago

You really need to use super-fast breakers if you're doing that. There are ones you can get for protecting industrial gear that cut faster than a fuse can, on modest overloads.

Chances are your apartment is protected by breakers and not fuses, whatever you use has to be MUCH faster than the apartment protection.

Remember a fuse doesn't LIMIT current, it only stops it if more than a certain amount flows, so a dead short actually pulls several 100 amps. The fuse ruptures in <20mSec and HAS to disrupt several 100 amps, not the fuse rating.

House breakers have to potentially disrupt 10,000Amps.

Its a pretty intelligent question !!!!

DoctorWoo (author)  steveastrouk4 months ago

Ah, ok! I did some looking at the mouser breakers, and found something like that. I'll have to see how hard it would be to get one.

And sadly, my apartment is fused, not set with breakers. I knew that when I moved in, and I didn't think anything like this would crop up when I moved in, so it was never a real issue for me.

As for the fuse.breaker difference, I did not know this. I know there was a difference with how they trip, and how they handle the access amps when tripped.

My issue now though is that my entire apartment is set for 15amps (like I said, REALLY old building) so I don't know if I can set up anything like I outlined. I would assume I could use a 10amp breaker, and that will do the deed. But, again, better safe then sorry situation.

What you have to guarantee is that the 10A breaker trips before the 15A. The current rating of the breaker doesn't tell you. You need to check the " I^2t "

kelseymh4 months ago

By the way, I'd like to second Steve's comment below. This is *NOT* a "dumb question." There are a lot of "safety and health" things which are silly overkill, but high voltage is not one of them. You seem to already know a fair bit about what you're doing (given both your question and your follow-ups below), I suspect you'll end up with a well-designed and safe system when you're done.

DoctorWoo (author)  kelseymh4 months ago

Thank you for the vote of confidence! I actually should resend my "stupid question" comment after looking around at the mouser-style breakers mentioned above. There are more then enough people out there that are trying to *remove* breakers from devices that should have them