Always wear eye protection when working with electrical circuits and batteries.
What is the difference between an A.C. rated and a D.C. rated fuse or circuit breaker?
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Safety Devices Fuses and Circuit Breakers
The most important parts of any alternative energy system are the safety devices.
Whether you use fuses or circuit breakers, only a fool would go without short circuit and
over current protection.
In a d.c. (direct current) system it is important to use fuses or circuit breakers that are
rated for direct current use.
Please click here to find out why: A.C. or D.C. - What is the difference?
What is the difference between over-current and short circuit conditions?
Over-current protection is a fuse or circuit breaker that is placed between the load (such as
an inverter, light, fan, telemetry equipment or pump) and the battery.
In case the load draws more amperage than it is rated at, this will open the circuit and shut down
If a load over-amps itself (draws more amperage than it is rated for) it can become damaged or
start a fire.
Short circuit protection is a fuse or circuit breaker at the positive output of the battery.
In case of a short circuit (a dead short between positive and negative) the device will open the
circuit on the battery positive and take the battery(s) out of the system.
A short circuit on a battery bank can cause an explosion or fire.
Fuses and circuit breakers
What is the difference?
A fuse is a device that has a fusible (meltable) conductor (or link) between the ends.
When the amperage exceeds the rating of the fuse, the fusible material melts and opens the
In an alternative energy or recreational vehicle system there are only three types of fuses to be considered.
The smallest would be an automotive fuse.
These will be found having a glass (or ceramic) cylinder with metal endcaps or a plastic body
with connecting tabs such as an ATC fuse. These would be a good choice for lights and other small loads.
The next would be a Class "R" time delay fuse. This is a cylinder fuse that is called a dual-
element fuse. The amp load can exceed the name plate rating for up to several seconds before
the fuse blows. The higher the amp draw past the rating, the quicker it will blow. These are used
with motors so the fuse will delay while the motor is starting - a motor can draw two to four times
the rated amperage to get it started.
The picture above shows a three pole Class R block on one of our smaller pv systems.
One fuse is between the photovoltaic modules and the charge controller.
One is between the charge controller and the batteries.
And one is between the batteries (two six volt Trojan T105's in series for 12 volts) and the
As the battery bank is small, I felt that the Class R fuse would have a sufficient arc interrupt rating.
Notice the groove on the end of each fuse.
This is a real "R" (rejection) block with a knife blade on one end of each pole.
This is to prevent non-R (time delay) fuses such as Class M fuses from being used.
These fuses can be a bear to pull out of the block, so each one has a wire tie hanging on it so I can
pull them out without using a screwdriver or pliers to remove them.
The most inportant, to me, is the Class T fuse. This is a very fast acting fuse with a high arc interrupt rating that
should be put on the positive side of the battery bank. Everything should
pass through this fuse. A Class T fuse has the meltable (fusible) link but it also has a filler that
melts when the fuse blows and flows between the end caps (on fuses under 100 amps) or tabs
(on fuses of 100 amps or more). The filler helps to prevent an arc jumping inside the fuse body.
When properly sized and installed, in a dead short it will clear the short and remove the batteries from
A fuse has no moving parts and is pretty much unaffected by temperature variations.
As a rule, fuses have a higher Arc Interrupt Rating than most circuit breakers.
Some fuses have a time delay before blowing.
What is Arc Interrupt Rating? Please click on this link to find out:
A fuse can only provide protection once.
They can not be used as a disconnect unless used in an expensive "bolt" pull box.
Class T fuses do not come cheap, but when a battery bank self-destructs due to inadequate
fusing - the cost no longer matters.
Above is a 300 amp Class T fuse in one of our 12 volt pv systems.
It is in the Positive cable coming off the the battery bank.
This system has eight Trojan T-105 six volt batteries wired in series/parallel.
The fuse is easy to access and has a clear, or at least it was clear when I installed it years ago,
cover to protect the metal parts.
This is a Class T fuse.
100 amp and larger have holed tabs on each end like the one shown.
Fuses under 100 amps are cylindrical like the Class R fuses and fit in a similar shaped fuse block.
They are either silver washed or tin plated for corrosion protection.
A circuit breaker is a mechanical device that opens a circuit the amp draw passing through the breaker exceeds is rating.
It will trip (open the circuit) in one of two most common ways, depending on its design.
A thermal circuit breaker heats up when the amp draw exceeds is amp rating and then trips.
A magnetic circuit breaker generates a magnetic field as the amperage increases to the point of the contacts tripping apart.
A thermal circuit breaker is affected by ambient temperature, the higher the housing temperature is, the lower the trip amperage.
A magnetic or magnetic/hydraulic circuit breaker is relatively unaffected by temperature.
A circuit breaker can be used more than once. When it trips you can fix the problem and then reset the breaker.
Some circuit breakers can also be used as a disconnect switch.
Please note that the larger circuit breakers have limited life cycles.
This means that they can only trip so many times under load as well as being manually switched under load
before they need to be replaced.
Many circuit breakers have a short time-delay rating.
Circuit breakers cost more upfront than a fuse of the same amp rating.
In most cases, a circuit breaker of a given amp rating will have a lower Arc Interrupt Rating than
that of the same amp rated fuse.
These are two CF 125 volt d.c. rated 60 amp circuit breakers.
As you can see, they have been back mounted to a two by four foot piece of plywood that has been
mounted to the wall.
This system uses two Morningstar TriStar-60 charge controllers.
Each controller has a circuit breaker on the incoming power, from the solar panels, and outgoing
to the battery bank (by way of a power distribution block and Class T fuse).
Besides circuit protection, this allows the charge controllers to be disconnected from the system
if the need for trouble shooting arises without having to disconnect any cables.
Alternating Current and Direct Current - what is the difference?
Alternating current, the stuff that comes from the utility company, operates at 60 cycles
per second in this part of the world.
The flow of current changes directions sixty times a second.
This means that one-hundred and twenty times a second, current is traveling in neither
direction, no potential.
Direct current, the stuff that comes from photovoltaic modules and batteries travels in
the one direction.
When you shut off a switch handling 120 volts a.c. there is little, if any, arc produced
between the two contacts.
In a 12, 24 (and so on) volt direct current switch, the contacts must come apart fast enough
and be in the optimal position to break the potential arc.
As a kid I remember the wall switches in older houses. They would make a loud snap when
the lever was moved. Some of these switches would handle either a.c. or d.c.
There are a few wall switches made today for a.c. or d.c., these have a strong spring and
are loud when operated - some of these have the letter "T" on them for tungsten rating.
All of this comes into play in regards to over-current/short circuit safety devices.
When the element in an a.c. fuse melts, the arc is fairly easy to interrupt - this also applies
to circuit breakers in an alternating current system.
When a direct current rated fuse blows it is designed to clear the arc and open the circuit.
If you use an a.c. only rated fuse in a d.c. system, it can blow and still have an arc passing
through the fuse body. This can result in a catastrophic failure of your batteries and is
a very dangerous condition.
The same is true of circuit breakers.
Basically all circuit breakers operate on the same mechanical principle, the contacts are
moved apart by heat and or magnetics and the circuit is open.
In a d.c. rated circuit breaker the contacts (paddles) are shaped differently (some with
tungsten mating surfaces) and swing apart faster and further than in an equivalent a.c. breaker.
Some d.c. rated circuit breakers have contacts that travel a longer distance so that the pivot
point of the swing actually gets in the way of a potential arc.
You may have heard the expression "the fuses were jumped" or "the breakers were jumped",
this means that an arc is continually passing through the safety device after it has blown
or tripped. In this condition there is no protection provided.
A quick example.
I am looking at a Littelfuse (yes, that is the correct spelling) JLLN 400 Class T fuse.
It is rated at 125 volts d.c. with an Arc Interrupt Rating (current limiting) of 20,000 amps.
It is also rated at 300 volts a.c. with an Arc Interrupt Rating (current limiting) of 200,000 amps.
As you can see, it is a lot more difficult to break (or clear) a d.c. short circuit arc than
an a.c. short circuit.
As a side note, some of the Ferraz Shawmut Class T fuses have a 160 volt d.c. rating and an
arc interrupt rating of 50,000 amps d.c.
Arc Interrupt Rating (A.I.R.) and Arc Interrupt Current (A.I.C.)
These two terms are pretty much interchangeable.
These ratings give us an idea of how many amps a safety device can clear in a short circuit.
Lets say you have a small sealed 12 volt battery rated at 10 amp hours.
In a dead short circuit, this battery might unload one-hundred amps or more in fractions of a
A short circuit protection device would not have to have a high A.I.R. or A.I.C. to clear the short
and isolate the battery.
What if you have eight six-volt golf cart batteries wired in series or series/parallel.
In a short circuit this combination could produce thousands of amps in very short period of
The short circuit device would have to be able to clear a high amperage arc to remove the
batteries from the system.
If the protection device does not have a high enough rating, cables and connections can become
vaporized and the batteries can explode from the intense heat generated.
When the fuse or circuit breaker is not rated high enough to handle the battery bank, the arc can
continue through the device (jumping it) like it was a piece of solid metal.
Please keep in mind: the higher the system voltage of your battery bank, and amp rating, the more difficult it is
to clear a short circuit.
Please click onto the image to go back to fuse ratings.
This Littelfuse 300 amp Class T fuse shows two Arc Interrupt Ratings.
On the left it shows 200KA (200,000 amp) rating at 300 volts A.C.
On the right it shows 20KA (20,000 amp) rating at 125 volts D.C.
Yes, there is a difference.
As a side note: some Ferraz Shamut A300 fuses are rated at 150 volts d.c.
and have a 50,000 amp A.I.R. rating.
Isn't a circuit breaker good enough on its own?
This question comes up more often than you may think.
When it comes to protecting a battery bank, in most cases it isn't.
As a rule, Class T fuses have a higher Arc Interrupt Rating than most d.c. rated
Here are some examples of d.c. rated circuit breakers along with their d.c. voltage and
This is an Airpax 209 series CF (captive lug on top and bottom) circuit breaker.
It is rated for direct current use up to 125 volts and has an A.I.R. rating of 5,000 amps.