Ski Builders Forum

120v 15A circuit. Is a 15A fuse too large to provide any protection withing the circuit? Or should it be sized slightly smaller (do they make a 14A fuse?) then the breaker so that the fuse blows first?


Obviously I can;t go too small, or the blanket will always blow the fuse...

NO fuse. Nothing in your pid / heat blanket circuit needs to be fused. Especially not the blankets. Your breaker will provide all the protection necessary.

I say 'yes' to fuses. You should at least put a fuse to your controller. Check the spec sheets to see the ratings for your controller and size the fuse accordingly. Putting fuses to your blankets is not a bad idea either. You should size your fuse so you only use 75% of its current rating. For a heating circuit, you want a fast acting fuse.

The circuit breaker in your house panel is there to protect the wiring from melting and burning your house down. It does very little in protecting the equipment that is plugged in.

-kelvin

kelvin wrote: The circuit breaker in your house panel is there to protect the wiring from melting and burning your house down. It does very little in protecting the equipment that is plugged in.

-kelvin

Exactly why I used 3 fuses, 1 to the PID itself, one on the DC out to the SSR and one out to the blanket. The way I look at it is that the SSR is now protected, the PID is protected if the SSR has some sort of meltdown and tries to feed 120v back to the PID, and the PID is protected from drawing too much current.

fusing your p.i.d is reasonable.

fusing your blankets is a complete waste.

Why?

If the blanket shorts, ANY weak link withing the wiring can potentially fail. I'd rather have a fuse right there, as opposed to waiting for the short to be sensed back at my electrical panel...

Sorry, I was being to general and making a few assumptions.

First off fusing your pid controller is not a bad idea, as it's power useage is significantly lower than the amperage rating of the circuit and the electronics could be damaged by a small amount of overcurrent. Probably less than an amp fuse is desireable.

Fusing your blankets just isn't necessary because the situation that would cause your breaker to trip won't do any damage to your blanket if sized properly and a fuse won't necessarily protect against a short adequately anyway.
I was assuming that your breaker is sized correctly for your blanket. Your breaker should be sized per the NEC at 125% of your load. So what size blanket are you using?

I am running 20 amps of heating blankets and the hydraulic arms which draw 3 amps while running, on a 30 amp breaker so I don't feel the need to fuse my blankets. If the breaker was a 60 amp then the correct solution would be to downsize the breaker.

Conventional wisdom states that the most common type of fault in a power system (by far) is a single-phase-to-ground fault. On solidly-grounded power systems, the available ground-fault current level can be significant. In some situations, ground fault current levels that are even higher than the maximum three-phase fault current level are theoretically possible. However, many ground faults produce only relatively low levels of fault current due to impedance in the fault path (due to arcing or to some othersource of impedance from phase to ground). While such faults can cause significant equipment and facility damage if not cleared from the system quickly, phase overcurrent protective devices may not respond quickly to the lower fault levels—if they detect the fault at all. For example, an 800 A ground fault might simply appear as an unbalanced load to a 4000 A fuse or circuit breaker not equipped with ground-fault protection

http://ecatalog.squared.com/pubs/Circui ... DB0601.pdf

granted they are talking about much bigger circuits but the theory is the same.

Weak link? There shouldn't be any. Your wiring should be done properly so that the breaker is able to do it's job, which is to protect the circuit from overcurrent.

Do what you want to do, I just don't think it's necessary at all to fuse your blankets. It surely won't hurt anything, unless the fuses burn up while you're cooking a pair of skis and you don't notice.

doughboyshredder wrote: Weak link? There shouldn't be any. Your wiring should be done properly so that the breaker is able to do it's job, which is to protect the circuit from overcurrent.

Do what you want to do, I just don't think it's necessary at all to fuse your blankets. It surely won't hurt anything, unless the fuses burn up while you're cooking a pair of skis and you don't notice.

Agreed on most points. There shouldn't be a week link, but there could be, and it could be the shop's wiring. I know mine is good, but I was building this for someone else. Everything inside the control box is 12 gauge wiring and rated for 20amps (except the 14 gauge to the PID, but that draws <1A) The blanket is drawing 5 amps, round hat to 6 with the PID. Do they make a 7.5 amp breaker? I've never seen one...

If the fuse to the blanket burns up then the PID will never be able to reach the set temperature. If the breaker on the circuit goes then the PID won't even turn on. Either way you actually have to look at the thing to see what it is doing.

Thanks though, I was looking for the 125% number, I've used it in the past but could not remember when I made the thread.

BTW, why is there large capacity fuses withing the disconnect switch of large industrial tools? Shouldn't the breaker in the main panel protect all the downstream equipment?

cause the NEC requires it?

no actually, it is because there may be a supply of hundreds of amps that all of the equipment is on, and each piece of equipment has to be isolated.

I guess in the situation you are describing it does make sense to fuse the blanket. I guess a 6.25 amp fast blow fuse would be sufficient.