The Difference Between AC and DC Fuses

August 17, 2015 · 4 comments

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It saddens me that folks are risking their lives, family member’s lives and personal property by trying to save a dollar by using off-the-shelf AC (alternating current) fuses (or circuit breakers and switches) in their 12, 24 or even 48 volt battery systems, not to mention their 120, 150, 200 or even 600 volt DC (direct current) solar arrays.

USING AN AC FUSE IN A DC SYSTEM IS EXTREMELY DANGEROUS!

DO NOT USE AN AC FUSE IN A DC SYSTEM OR YOU WILL BURN YOUR HOUSE DOWN!

ac and dc fuse assortment

In fact we will have some fun later in this article and burn or blow up some AC fuses by using them with DC current. Stick around, it will be entertaining if nothing else.


AC (alternating) current does exactly what is says, it alternates or changes from positive to negative 60 times per second (60 hertz) in North America and 50 times per second (50 hertz) in the rest of the world.

To switch from positive to negative and negative to positive it has to cross over zero volts each time. If you were to open a circuit (turn a light switch) at exactly zero volts, there would be no arc (or spark).

alteranting current passes zero voltss 50 or 60 times per second.

If you were to open a light switch when the voltage is at 120 volts positive there will be an arc but only until the voltage reaches zero which happens in the fraction of a second. Now the circuit is open and the arc is gone.

If you turn off a switch (open circuit) at -120 volts there will be an arc (spark) for a fraction of a second until the voltage reaches zero.

Once the voltage reaches zero the arc is extinguished all by itself.


DC (direct current) is a different animal. It just keeps on coming like a freight train.

Open a low voltage DC circuit (like when you blow a fuse) and there will be a small arc.

Open a high voltage DC circuit (like when you blow a fuse) and there will be a huge (long) and super hot arc (spark). This is where the problem lies.

Usually as long as your AC fuse doesn’t blow, things will be fine,

BUT WHEN AN AC FUSE BLOWS IN A DC SYSTEM, IT CREATES AN OPEN CIRCUIT AND A HOT AND DANGEROUS ARC.

The higher the voltage in your DC system the larger and hotter the arc will be when the circuit is opened. Sometimes there is nothing you can do such as when a cable from your battery to your inverter gets lose or the terminal gets loose. An arc will form and extreme heat will be produced. Hopefully in this circumstance, known as an “arc fault”, someone will notice it.

By the way if there is an arc fault on the wiring going from your charge controller to your solar array, the same can happen unless you have a charge controller that is designed to handle arc faults. The “Classic” made by Midnite Solar does exactly that. If it senses an arc fault it will open the circuit and sound an alarm to let you know your system needs some work. Absolutely genius idea!

Robin and boB have been the key players in the renewable energy business.

Robin Gudgel of Midnite Solar (and his brother boB) have always been the “first” for just about everything in the solar industry. They made the first DC breaker panel and DC disconnect, the first rugged and reliable MPPT charge controller (the MX60), the first 150 volt DC rated circuit breakers, the first 300 volt DC circuit breakers, the first 600 volt DC circuit breakers, the first user friendly small UL/CSA listed breaker panels, the first use friendly pre-wired DC circuit breaker panels (e-Panels) and now the “Classic” charge controller which has at least 10 firsts! There is no one like the Gudgels from Midnite Solar. They are the pioneers of the renewable energy industry.


Back to our topic of the difference between AC and DC fuses.

You only need to open an AC circuit (when a fuse blows) a few millimeters at 120, 230 or 240 volts as the arc will be gone when the voltage crosses the zero point. (Remember our diagram above?)

When an AC fuse blows, it will only open the circuit a few millimeters as that is all that is necessary. If you use an AC fuse with DC current and the circuit is opened (fuse blows), one of three things will happen:

  1. The gap made by the open fuse will be too small and the DC current will jump across, get hot and melt (fuse) the metals back together. This is very dangerous as the condition that caused the fuse to blow or breaker to trip is still there. Now the wiring will heat up and eventually catch fire.
  2. The gap made by the open fuse will be too small and the DC current will arc continuously until it has melted everything around it and/or caught your home on fire.
  3. The gap will arc and melt until the gap opens far enough to extinguish the arc (spark). Although this is physically possible, I have never seen it.

DC CIRCUITS MUST BE OPENED FAST AND FAR ENOUGH TO EXTINGUISH (GET RID OF) THE ARC!

THIS IS EXACTLY WHAT A DC FUSE DOES! IT OPENS THE CIRCUIT FAST AND FURIOUS TO PROTECT YOUR VALUABLES!


An arc welder uses high amperage low voltage DC to weld metals together.To illustrate the arcing ability of DC current let’s look at the common arc welder used by many to weld metals together.

Could you weld with AC current?

From what we have learned already, the answer is no. If an arc welder (the human welder) tried to make an arc with the typical AC current from household electricity he would only be able to make a tiny one and only for a fraction of a second. Not much good for welding!

There is one exception to the rule: there is a form of tig welding used to weld aluminum and magnesium with a square wave AC. Check it out here.

square waveWhen magnesium or aluminum are exposed to air, oxide forms on these metals that must be removed. The positive cycle of the square wave removes the oxide and the negative cycle of the square wave welds the metals. We are not welding experts but were reminded by a reader that this type of welding is done with AC although it is not a typical AC wave form.

The positive and negative cycles of the wave form resemble DC in that they are steady and straight compared to a typical pure sine wave which is more rounded and peaks at a certain point.

Could you weld with DC current?

Absolutely, and that is how is it done. And surprisingly it only takes about 28 to 40 volts DC to arc weld (stick weld). In fact in an emergency you can connect two 12 volt batteries in series (for 24 volts DC)and use a high quality set of booster cables for an emergency arc welder. We have done it here at Solar Homestead when we have had breakdowns where CAA or AAA doesn’t go.

A big difference between AC and DC is stick welding.


THE RULES FOR AC AND DC FUSES:

1. No matter what fuse you are using and no matter what the application you MUST USE A UL OR CSA APPROVED FUSE FOR THE PURPOSE IT WAS INTENDED.

These DC rated fuses cannot be used in a home or cottage.

These DC rated fuses are not approved for homes and cottages.

For example: You might find a UL listed DC fuse but it was designed for a mobile application such as a car, truck or boat.

This type of fuse cannot be used in an off grid home or cottage.

2. Use AC fuses for alternating current

3. Use DC fuses for direct current

4. In a DC application, make sure you get the correct amperage fuse for your project and do not exceed the maximum voltage rating of the fuse.

5. In an AC application make sure you get the correct amperage fuse for your application and do not exceed the maximum voltage rating of the fuse.

How do you know what size fuse should be used in your DC (or AC) system?

Most people don’t know but a fuses job is to protect the conductor (wire/cable) downstream of the fuse. It is not necessarily there to protect your inverter, charge controller or other DC appliances, it’s primary purpose is to protect the current carrying conductors from overheating and catching fire.

Of course if there is a short circuit in your inverter, the fuse will blow but the damage has already been done to the inverter. The fuse blows to prevent the wiring from catastrophic fire, damage to property and prevent injury or death.

That being said, your fuse will be sized according to the size of your wiring or cabling.

Look at the chart below to see what size fuse is required to protect each gauge of wire:

wiring and fuse sizes

First only look at the left two columns of the chart if you are using copper wire as you probably are. I kept the aluminum side of the chart as sometimes we get a good deal on aluminum wire that we cannot resist. Go with the 90 degree C wire for most of your amp calculations as those are the ones used with solar such as USE-2 (underground service entrance) and most of the other ratings in the 90 degree C would apply to PV wire and battery / inverter cable like DLO (diesel locomotive cable).

When you find an amperage rating on the chart you NEED to find a fuse or breaker that is either the same size as the current rating or smaller.

If you use a higher amperage fuse than the conductor can handle, the conductor will overheat and fail before the fuse blows. That is not good.

Example#1 Let’s imagine you have an inverter that requires 4/0 battery cable. If you look at the 90 degrees C rating of 4/0 (0000) cable you would need a fuse or breaker that will trip or blow at 260 amps or less. The fuse would be a 200 amp class T fuse (as they are readily available) to be on the safe side. It is more difficult to find a 250 amp Class T fuse. A 250 amp DC breaker would be the recommended breaker as they are readily available. Remember using a smaller fuse will not be problem as it will protect your wire by blowing before your wire over heats. However if you are actually using the maximum rating or the wire you may need to size your fuse as close to the maximum rating of the wire as possible.

Usually in a solar application most conductors or over-sized as to reduce the losses associated with resistance.

Example #2 Let’s suppose you are using the typical 10 awg pv wire used with most MC4 cable. While it is rated to handle 40 amps we usually would protect it with a 15 amp fuse (or breaker) as most solar modules say to protect with 15 amp series fuse (or breaker).

I know it sounds weird to most electricians to put  15 amp breaker an a #10 wire but we are using the #10 to keep the voltage drop as low as possible as we don’t want to lose any most power than necessary due to the resistance of the wire. Solar installers always use larger conductors (wire) than a typical electrician as they are trying to keep the voltage drop as low as possible.

The rule is protect your wire with the (90 degrees C rating) listed amperage fuse above or smaller.

While you could theoretically protect a #10 wire with a 40 amp fuse, most of the larger solar modules (over 200 watts) you are using will likely say 15 amp series fusing max.

15 amp series fuse

If you use too small of a fuse the worst thing that can happen is the fuse will blow.

If you use too high amperage of a fuse the worst that can happen is a fire.

I would rather blow a fuse.

Leave a Comment

{ 4 comments… read them below or add one }

Diogenes of Sinope September 7, 2015 at 1:53 am

Actually, you CAN weld with AC. That’s how aluminum is welded — when aluminum melts, it immediately forms an oxide layer on the surface, which has to be “blown away” by rapidly switching polarity. Of course, it’s not the same sine wave as the mains current, there’s a lot of complicated stuff going on in a GTAW or GMAW machine equipped to use AC, but it’s done quite frequently.

Reply

Jody Graham September 10, 2015 at 11:23 am

Hi,
Thank you for your comment and info. I have updated the article to reflect what you have told us. Although welding aluminum or magnesium is not done with a typical clean sine wave it is still considered AC current. Read what we have written above and if you think we still need to make corrections please let us know. Take care…Jody

Reply

Ben August 17, 2015 at 7:57 pm

Hiya
Thanks for your info and posts etc. Been reading some. Not this one yet. But I thought it might be interesting for you and other people out there to SEE what can happen.
The installer of my system has made some tests, here a video of it.
http://s4solar.co.nz/information/video/dangerous_substandard_pv_systems/

Cheers
Ben

Reply

Jody Graham September 10, 2015 at 11:25 am

Hi Ben,
Thank you so much for your comment and video. It definitely shows that AC components do not belong in a DC system. Take care and keep up the good work…Jody

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