Please Don’t Use AC Fuses in a DC System!

It is hard to believe and almost sad that folks are still trying to get away with using AC rated fuses in the DC side of an off grid power system. I cannot stress how important this topic is.


dc fuse samplesAlternating current (AC) and direct current (DC) are two different animals.

120240 volt AC current 60hz (in North America) and 230 volt AC current 50hz (the rest of the world) changes polarity 60 (or 50) times per second. It changes from positive 120/240 or 230 volts to negative 120/240 or 230 volts constantly. But in order to do that it must cross zero volts many times per second. If you start an arc with AC current it will die or extinguish itself when the voltage crosses zero volts.

AC current passes zero volts

That is why you can only make a tiny spark with AC current. When you open an AC circuit with a light switch for example there is only a tiny spark or arc to deal with which an AC switch can handle easily and over and over. When an AC fuse blows there is only a tiny arc and the fuse only has to open the circuit a few millimeters. Then the circuit is broken and all is good.

Direct current on the other hand is constant. When you open a DC circuit the current just keeps on coming. arc welding uses dc to melt metals together as AC current would not work.Low voltage DC will make smaller arcs while higher voltage DC can make massive arcs.

An easy illustration is the stick welder (electric welder / arc welder). They always use DC because they need to make a an arc to melt the electrode (welding rod) and the metals they are welding together. The result is extreme heat and the merging or melting of metals. The crazy part is arc welding is done with only 28-40 volts DC. If you have ever watched arc welding, the welder can make an arc an inch long or more and this is with relatively low DC voltage.

It is not uncommon now for battery banks to be 48 volts nominal which is really more like 58-60 volts when the batteries are charging. It is also not uncommon to see voltages up to 250 volts DC in an off grid solar system and 600 volts DC in a grid tie system. Now you can imagine how big of an arc 600 volts DC can produce.

AC fuses will usually survive in a DC system as long as the fuse does not blow. When the fuse blows, an open circuit and an arc is created. Since the AC fuse only opens the circuit a few milimeters one of two things will happen:

  1. The fuse will arc and actually fuse itself back together. This is very dangerous because now the circuit is reconnected even though something that was wrong blew the fuse in the first place.
  2. The fuse will arc for a longer period of time and melt the fuse and cause anything close to catch fire.

Both scenarios are very dangerous and not worth risking even if you are to cheap to spend the extra money for a UL or CSA rated DC fuse.

That is why we have decided to have a DC fuse super sale for the month of August. Please take advantage and protect your system and family from destruction.

Thanks for reading and keep following your dream to self sustainability…


{ 1 comment… read it below or add one }

David C Prasso March 23, 2016 at 9:21 pm

Hey Jody,

Thanks, I appreciated readingredients on DC vs. AC fuses, the welding example and the arc hazard…

I could not find the 400 amp DC fuse to order on line (maybe because I have a mobile phone and no desktop?)…if you send it, we can get you a money order in USD to the return address, ok?

Peace of Christ and happy Easter (!)


Leave a Comment

Previous post:

Next post: