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The fuses
The fuse is a small device used to protect an installation against possible electrical overload: in fact, each electrical installation is divided into sector and each sector has a fuse whose intensity corresponds to that the line to support.
It is said that there is overload when the electrical intensity is higher than the installation can withstand. The fuse, meanwhile, is composed of a small lead wire and integrated into the input circuit of the sector.
Lead is a metal that heats up quickly in case of electric current and if the current is too strong, the lead automatically melts and cuts off the current in the sector concerned.
The electrical circuit is cut, there is no danger for people, for installation or for property. The fuse is housed in the electrical panel of a home and each electrical panel has many fuses protecting each different power line: bathroom, lamps, washing machine and electric heating But it can happen that we find fuses in some electrical devices too.
Fuse and circuit protection
Over currents
Over-current is a condition that exists in an electrical circuit when the normal load current is exceeded. The two basic forms of overcurrent are overloads and short circuits. The primary role of fuses and circuit breakers in a circuit is to protect personnel and equipment when dangerous overcurrents occur.
Short circuit
A short circuit is an overcurrent condition, whereby an abnormal, low resistance circuit path appears in the circuit. This low resistance path bypasses normal load and can generate very high currents (up to 1000 times normal current under certain conditions).
When a short circuit occurs, an abnormal low resistance path is created, which causes the circuit current to increase as the circuit resistance is reduced. The current of a short circuit can exceed 1000 times the normal current of the circuit.
Overload
An overload is a state of overcurrent, where the current exceeds the normal full load capacity of the circuit, but where no fault condition (short circuit) is present. A momentary overload condition (also known as “peak” currents) can also occur when a circuit is first initialized due to the loading of a capacitor or the starting of a motor.
What is the normal operating current of the circuit?
In order to choose the correct amperage for the fuse, you must first know the full constant current load of the circuit at an ambient temperature of 20 ° C (68 ° F). Once the current value is determined, the fuse rating must be chosen so that it represents 135% of this value (rounded to the next standard value).
For example, if the normal constant current is 10 A, a 15 A fuse should be selected [10 Amps x 135% = 13.5 Amps, rounded to the next standard value, 15 A].
It is important to note that if the fuse is to be used in an environment with very high or very low ambient temperatures, the current rating of the fuse will need to be considerably higher or lower.
What is the normal operating current of the circuit?
In order to choose the correct amperage for the fuse, you must first know the full constant current load of the circuit at an ambient temperature of 20 ° C (68 ° F). Once the current value is determined, the fuse rating must be chosen so that it represents 135% of this value (rounded to the next standard value).
For example, if the normal constant current is 10 A, a 15 A fuse should be selected [10 Amps x 135% = 13.5 Amps, rounded to the next standard value, 15 A].
It is important to note that if the fuse is to be used in an environment with very high or very low ambient temperatures, the current rating of the fuse will need to be considerably higher or lower.
What is the operating voltage?
The rule of thumb is that the rated voltage of the fuse should always be higher than the rated voltage of the circuit it protects. For example, if the circuit voltage is 24 V, the rated voltage of the fuse must be greater than 24 V (yes … it can be 250 V … as long as it is higher than the circuit voltage) .
3. Is the circuit AC or DC?
There are two types of circuits: AC (alternating current) and DC (direct current). Alternating current is what you typically find in your home that comes from the power grid. Alternating current is created primarily by mobile machinery such as generators and delivered through the power grid.
Direct current is typically used in electronics and automotive applications. Direct current is usually created by a chemical reaction (like batteries and solar cells in an alternator) or converted to alternating current by an AC to DC power supply.
With alternating current, current and voltage oscillate constantly. This oscillation helps the fuse to drain quickly. The direct current does not oscillate so the fuse must find other means to empty during start-up.
Because of these differences, some fuses are designed specifically for use in direct current applications (such as automotive fuses). Some AC fuses can be used in DC applications, but voltage derating is possible in these cases.
What is the operating temperature?
Room temperature is a fancy way of talking about the outside air surrounding the fuse. Normally, fuses are tested under “laboratory conditions” by safety organizations such as UL and CSA. Laboratory conditions are almost always 20 ° C or 77 ° F. Unfortunately, real world conditions are often not what you find in a laboratory.
Fuses are heat sensitive devices which means that heat (from overcurrent) can cause the inside of the fuse to melt. The more heat there is, the faster the fuse element will melt, and vice versa.
If the fuse is subjected to a temperature higher than 20 ° C, then the fuse amperage must be increased to compensate for the higher temperature (to avoid “nuisance tripping”).
Likewise, if the fuse is used at a lower temperature, then the amperage of the fuse will need to be reduced (otherwise it may never open).
The basic rule is that for each 20 ° C higher or lower, the fuse rating must be changed up or down by 10 to 15%. Here is an example of fuse recalibration in case of high ambient temperature:
Normal full load current: 1 A Normal fuse rating: 1.5 A (135% of full load current rounded up) Ambient temperature: 65 ° C Recalibration: 2 A (130% of normal caliber)
Is the protection device used for short circuit protection, overload protection, or both?
If the device is to be used as a short-circuit protection, the fuse or circuit breaker must quickly interrupt the fault (usually in less than 4 milliseconds) in order to provide maximum protection to the equipment and personnel.
If the fuse or circuit breaker is only intended for overload protection, then it can be much slower to respond to overcurrent: seconds or even minutes rather than milliseconds.
All fuses offer some form of short circuit and overload protection, but many circuit breakers ONLY offer overload protection and therefore cannot protect against dangerous short circuits.
What are the physical size limits?
Often the fuse or circuit breaker must be installed in a location with size limitations. It is for this reason that fuse and circuit breaker manufacturers have created a wide selection of components in different sizes. However, the engineer usually has to make compromises.
Generally speaking, the smaller the fuse, the less current or capacity it can withstand. For example, a miniature fuse can be limited to 15A while a larger 1/4 ” x 1 1/4 ” glass tube fuse can withstand up to 40A.
Also, although the fuse may be smaller, the corresponding fuse holder may be significantly larger which further complicates matters.
Does the fuse have to be field replaceable?
Fuses are designed to open a circuit when an overcurrent occurs, whether it is a short circuit or an overload. The engineer must decide whether the fuse should be field replaceable or not.
The main reason for making the fuse field replaceable is simply the convenience for the end user so that their equipment can work again. There are two main reasons for not making the fuse field replaceable: 1.
Including a fuse holder can cost the manufacturer much more than soldering the fuse directly into or onto the circuit board. 2. The manufacturer may not want the end customer to access the interior of the device to replace the fuse due to liability issues.
This is especially true when the short circuit is the cause of the problem. 3. The manufacturer may have built in “planned obsolescence” of some parts and want to replace the entire PCB rather than replacing the fuse.
Is reset a problem?
Disposable fuses and resettable fuses are available to the engineer. Both types of fuses provide overload protection. Resettable fuses are limited to circuit applications that provide 14 A (at 12 V) and less current at higher voltages. Circuit breakers can also provide reset and can range from 1A to 300A.
Single-use fuses are as the name suggests. Once called upon to act, the inner link melts and the fuse must be replaced. Even if the fuse is replaced, there may still be a short or overload in the circuit, which may cause the new fuse to open.
Before replacing the open fuse with a new fuse, care should be taken to correct the problem that caused the first fuse to open. Conversely, when a fuse is intended for use in extreme low temperature conditions, the fuse rating must be lower than that used under normal conditions.
Here is an example of fuse recalibration in case of low ambient temperature: Normal full load current: 1 A Normal fuse rating: 1.5 A (135% of full load current rounded up) Ambient temperature: -15 ° C
How will the fuse be installed?
One of the most important aspects to consider is the installation of the fuse in the circuit. There are several options:
Direct soldering:
In this method, the fuse is soldered directly into or onto the printed circuit board (PCB). The downside is the lack of spare parts in the field, as mentioned in the section above, but this installation method can significantly reduce the cost.
Fuse Sockets:
Fuse sockets are relatively inexpensive and allow field replacement. Fuse sockets are usually installed on a printed circuit board, so to replace the fuse, the end user will need to open the equipment.
Additionally, removing a fuse from a fuse socket without disconnecting the power source may result in electric shock when touching the fuse. Fuse sockets are available for all “tube” fuses as well as micro fuses. Fuse sockets are usually limited to 20A normal current (also available in 30A).
Fuse sockets are generally not rated or recognized by safety organisations.
Panel Mount Fuse Holders:
Panel mount fuse holders allow easy end user access to replace the fuse in the field. The panel mounted fuse holder is impact resistant, meaning the fuse is safely removed when the fuse holder cap is removed, preventing any possibility of electric shock.
Fuse holders are generally tested and approved by safety organizations such as UL and CSA. Fuse holders are generally offered up to 30 A.
Fuse block:
Fuse blocks are like fuse holders, but they should not be installed on the circuit board. Fuses mounted in fuse blocks are usually accessible only by opening the equipment, which can result in electric shock if the equipment is not disconnected from the power source.
Fuse blocks are one of the few methods of installing high amperage fuses.
In-Line Fuse
Holder: In-line fuse holders are typically used as an assembly of a wire harness or when no surface is available for another type of fuse installation. In-line fuse holders are typically offered up to 100A in low voltage applications and up to 30A in higher voltage applications.
What are the cost considerations?
Costs may vary depending on the size, performance and type of fuse installation. Generally speaking, the larger a fuse; the more expensive it will be (due to the higher manufacturing costs). The performance characteristics of a fuse also have a significant impact on cost.
A 10 Amp low voltage automotive fuse can cost a fraction of the cost of a 10 Amp 500V super high speed ceramic tube fuse. Safety agency approval adds to the cost of the fuse. . One of the most expensive parts of a fuse is the fuse holder. A typical panel-mounted fuse holder can cost several times the price of a fuse.
