Fuses (for Beginners), AC fuse in DC circuits

In practical life as electrical engineers and electricians we, some times, inspect control circuits and distribution panels, and realize that AC fuses are used in DC circuits! It’s not that simple! Using AC fuse in DC circuit is a safety issue. Let’s find out why!

What is a Fuse?

A Fuse is an electrical safety and protection device that protects electrical circuits from over current  and short circuits.

Fuse is a simple but important protection component. Many electrical devices use fuse as a protection device.

Fuses are one time use protection device, its filament link melts down in case of fault. This link is made of copper, Zinc, Silver, aluminum, or any other alloy having a predictable trip current.

AC Fuse vs DC Fuse

Never use AC fuse in DC circuit! the difference between AC and DC fuses comes from the difference between AC and DC current.

AC current vs DC current

AC current
AC current

Before diving into the reason of why you should never use AC fuse in DC circuits!, lets discuss a little about the main difference between AC and DC current.

AC or alternating current, changes its direction and magnitude 50 or 60 times per second, this is why its called alternating, AC current crosses the zero value 50/60 times per second.

On the other hand DC current is a constant in both direction an magnitude, and doesn’t go zero until the source of power is lost.

AC Current Crossing to zero helps well

As mentioned above, AC current crosses zero 50/60 times per second, this zero value helps well in AC fuse designing, How is that? To better understand this we will take it from the fuse principle of protection.

Fuse principle of protection

A fuse consists of a metal filament. In normal conditions, a small or limited amount of current passes through the filament, smaller than the fuse rating.

  • When an overload or short circuit occurs, A large amount of current flows through the filament, it heats up and melts.
  • Due to melting of filament a gape creates among the fuse elements and the flow of current stop.

At the moment of fuse blowing the current keeps passing through air between the fuse element blown parts producing an electric arc.

When DC or AC current passes through an electrical conductor of any size it produces temperature rise depending on conductor size and current value.

In case of the current passing through the AC fuse or DC fuse exceed the rating value of the fuse, the temperature rises more and more, and the fuse element will no longer be able to withstand and then blows.

What will cause the arc to extinguish?

In AC fuse case: As AC current goes zero several times each second, this zero value helps the arc to be extinguished.

In DC fuse case: The current never goes zero, so it is more difficult to extinguish DC arc than AC one, For that reason DC fuses are designed to be longer than AC ones, this extra length helps in DC arc extinguishing.

The main question is,

Could AC fuses be used in DC circuits?

The answer is clearly, Never do that!

AC fuse in DC circuits is dangerous! Using AC fuse in DC circuits will cause the arc not to be extinguished safely and may cause fire situation.

Different Types of Fuses

Medium Voltage AC FUse Link
Medium Voltage AC FUse Link

Two main types of systems are in use nowadays. i.e. AC and DC. Therefore, types of fuses are also of two types. i.e. AC Fuse and DC Fuses.

AC fuses

Ac fuses consist of Low and High voltage fuses. These are further divided in such a manner.

Low Voltage fuses

This type of fuses is used in low voltage distribution networks. The different types of low voltage fuses are.

  • Switch Fuse
  • Cartridge fuse
  • Rewirable fuse
  • Drop out fuse.
  • Sticker fuse.

Medium and High voltage fuses

This type of fuses is used in high voltage AC transmission lines. It consists of :

  • Expulsion fuse.
  • High Rapture current (HRC)FUSE.

DC Fuses

Dc fuses are manufactured for Direct current equipment. Different types of DC fuses are :

  • Cartridge fuse.
  • Automotive fuse.
  • Resettable fuse.
  • Semiconductor, and over-voltage suppression fuses.

Do Fuse Have a Positive and a Negative End?

AC Fuse In a Circuit
fuse in a circuit

Fuses have no polarity and no positive or negative side. They are just a wire or a link in the circuit, wires have no polarity, right?

In a typical circuit, a fuse is a conductor as a result, it has no sense of direction, polarity, or orientation.

Due to the high melting point of the fuse, regardless of the current direction, the Fuse will shut off the current route.

Do All Fuses Do the Same Thing?

Yep, all fuses do the same protection task, overcurrent and short circuit protection. Although not all fuses operate the same way. The operation curve of the fuses is not the same. Besides, you should check the current and voltage rating of each fuse before using it to protect a circuit.

For instance, cars have DC fuses, each one has its own current rating to protect whatever it should protect.

A fuse serves as a sacrificial device, according to Little fuse’s “Fuseology.” A conductive strip of material within a fuse, commonly known as its “breaking capability” melts if the Fuse is overloaded.

When a system overloads or shorts, the Fuse protects the more extensive system by breaking the circuit or “opening”.

Can Fuse Protect Against High Voltage?

low voltage fuse
low voltage fuse

Fuses provide no protection against voltage spikes or drops, fuses are current sensitive devices that acts depending on current rating. However fuses are used in low and high Voltage circuits.

A high-voltage fuse must have creepage and clearance distances between its terminals that exceed safety requirements, in addition to assuring that the Fuse will break a high-voltage circuit regardless of how quickly or slow it explodes.

If you use a low voltage fuse in a high voltage rating the fuse may fail to suppress the arc, the fuse may not clear the overcurrent safely.

Does Voltage Affect a Fuse?

Yes, Of course, voltage affects a fuse, and the specified Fuse’s voltage rating must be more than or equal to the circuit voltage. Fuses are connected in series in the circuit so, the voltage rating is only relevant while the Fuse is attempting to open.

Fuse elements must have the ability to open fast and extinguish the arc once melted, as well as prevent the open-circuit voltage in the system from striking across the open fuse element.

Fuse links are not voltage-sensitive devices, but it’s crucial to remember that a fuse link’s ability to function appropriately during fault situations is determined by the system voltage.

As a result, they must not be used in circuits with voltages higher than their rated voltage. They can, however, be successfully employed in circuits with lower voltage levels. This answers the important question, Why Do Fuses Have Voltage Ratings?

Fuses have a voltage rating because, the rating is set by its capacity to open a circuit in the event of an overcurrent scenario.

Arc suppression will be compromised if a fuse with a voltage rating lower than the circuit voltage.

When supplied from three-phase, securely grounded, 480/277V circuits with a single-phase line-to-neutral voltage of 277V, 300V rated fuses can be used to safeguard single-phase line-to-neutral loads.

This is legal since a 300V fuse will not have to interrupt a voltage more than its 300V rating in this application.

Read also my article, What is Overcurrent Protection.

Is It Normal for A Fuse to Get Hot?

hot spot effect on 11 KV cutout fuse
hot spot effect on 11 KV cutout fuse

No, it’s not normal for any fuse to get hot. You shouldn’t ignore this if your fuses getting hot.

A current much below the fuse rating might create enough heat to melt an inline fuse holder if there is substantial resistance in the contact between the Fuse and the fuse holder.

While the initial resistance may be more minor, and the first heating may not be sufficient to reach the fuse melting point, the heat might produce oxidation of the metal connections, increasing contact resistance.

What Causes A Fuse to Get Hot?

  • Overloading the fuse causes it to get hot, if the fuse curve is a slow one, it may get hot for a while before it opens the circuit.
  • Bad contacting between the fuse holder and the circuit causes hot spots. This issue is repeated in my workplace 11KV overhead power line.
  • Corroded connections, wiring, or a device that is beginning to malfunction and impeding normal current flow are all causes of high resistance that can cause heating fuses.

What Are the Standard Fuse Sizes?

In the 2017 NEC, the paragraph was turned into new Table 240.6(A), Fuse and fixed trip circuit breakers are available in the following sizes: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 amps, instead of these Additional standard fuse sizes are 601 and 10, 6, 3, 1 amp.

Should Fuse Be on Positive or Negative?

The requisite protection would be provided by a fuse alone on the positive terminal enough to protect your electrical circuit. A fuse should be placed as near to the source as practicable.

One reason is that fuse isolators are widely utilized in AC applications where both the Live and Neutral must be disconnected. Even so, a fuse on the live and a link on the neutral can be used.

What causes a fuse to blow?

A short circuit an over current should always trip a breaker or blow a fuse, if and only if its value is greater than the fuse rating.

If you’re wondering what went wrong and how a short circuit “blow a fuse,” check out the list below.

The Wrong Type of Fuse Was Installed

Fuses are available in a variety of forms, sizes, and combinations. Many appear identical yet have extremely diverse purposes if you or someone else inserted the incorrect rating fuse in a fuse holder, so it can be the cause of below the fuse when the device operates because the device has drawn a higher current than the rated value of fuse.

Damaged or Outdated Electrical Outlets

Any defective wiring or linked elements run the danger of causing a power failure (surge) that blows a fuse.

So, once again, the issue is not that the fuse failed to function correctly but rather that there was malfunctioning equipment.

An Overloaded Circuit

You should be able to identify the perpetrator in this scenario by checking for an overloaded outlet or an individual appliance.

Consider a power strip with a plug-in in every outlet, especially if the devices connected are high-power rating.

A Short Circuit

A sort of electrical defect is a short circuit. Faults arise when an electrical current deviates from its intended course (circuit) owing to a lack of resistance (e.g., from insulation or a circuit breaker).

As a result, the connection between the two conductors delivering electrical power to the circuit is compromised, and the impact of the “short” or “fault” excessive current flows into the power source.

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