EducationFuse definition - Fuse types - Theory

Fuse definition – Fuse types – Theory

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Fuse definition

What is a fuse

An electric fuse has become easily the most vulnerable section of an electric circuit. They interleave or switch off whenever the voltage flowing through them in any given time is high. Hence, they belong to the overload safety category. Electrical fuses cut off the stream of power to sockets and lighting. When the fault has been fixed, the fuse has to be changed or replaced.

Here is the principal means to guarantee the security of electric circuits inside homes. Until the’90s, fuses would be the most frequently experienced. Afterwards they were replaced with self-acting circuit breakers. Nowadays, various forms of circuit breakers are most frequently employed.

Non-replaceable current limiting fuse using a tall breaking evaluation and special measurements or mounting recommendations in order a replacement can’t be installed. The concept is to split fuses to two classes: non-replaceable as well as replaceable. The fuse definition must capture the essence of nonreplaceability along with the characteristics inherent just in a nonreplaceable fuse. The term limiting current with a high breaking rating is incorrect and second from place as it doesn’t have anything to do with non-replaceability. In addition, it is pointless to discuss specific size or installment recommendations, as what will be a particular (one of a sort, in certain species, unrepeatable) size or a uni-atomic installment recommendation include? And lastly, interchangeability isn’t about not having the ability to mount a replacement, for instance a fuse connection from a different maker.

Fuse definition – that expression is used interchangeably with fuse connection. It’s a protective component which, as a consequence of melting of one or more especially constructed and chosen current-carrying components (so-called fuses), breaks the circuit to which it’s been integrated, disconnecting the current when it exceeds a predetermined value within a predetermined time. The criteria specify by this expression an apparatus comprising a fuse link and also components to opening up a circuit to turn away a current exceeding a predetermined price. The titles fuse and fuse connection are so used interchangeably, particularly in the electric shop, but it isn’t appropriate to do this in dictionaries and prior to specifying both conditions. It ends up that a fuse is also a component. Containing several components. Those who can not deal with pronouns compose by turning the current after it’s crossed. Until today, electricians believed a fusible link was utilized to switch off the current. This turns out to not be accurate, because along with the fuse – that isn’t understood what it’s for – you will find a few components to start the circuit to switch off the current.

Semiconductor protection fuse A fuse which works extremely quickly to protect semiconductor elements (diodes, thyristors, triacs). Sometimes referred to as a rectifier fuse or ultra quick fuse. Even a semiconductor fuse is a fuse which somebody – or incorrectly – employed to guard a semiconductor device, i.e. a fuse using undefined properties.

Time-delay fuse A fuse that may continue to keep an overcurrent using a predetermined value for a minimal given time before opening the circuit (burning off ). This fuse definition applies to all fuses, so it is too broad and does not clarify anything. What is more, it is incorrect to use future tense.

Short-circuit current

The figure below tells regarding the so called extension times, i.e. that time to arc ignition, also states nothing concerning the phenomena during a short circuit when exceptionally substantial currents are flowing. Essentially, it may be stated it isn’t just the high voltage which boosts arc formation and upkeep. The next very important positive factor is that the current capacity of this circuit in which the fuse is set.

fuse fault current

The further short-circuit current that could flow (and certainly will flow), the tougher it is to extinguish the arc. The data from Figure 1 is unworthy under short circuit conditions. In the event the short-circuit current is extremely big, the arc lasts for a comparatively long time, to say the very least. To describe fuses in this regard, one speaks about the breaking capability. Breaking capability is only the expected price of this short-circuit current where a fuse will break the circuit. The question: what’s the breaking capacity of hot and many generally utilized time-delay (T) and quick (F) fuses?

Well, here is a dreadful surprise – both possess pitifully low breaking capacity: 35A in AC and 20A in DC. Meanwhile, in electricity grid circuits, the anticipated short-circuit currents are higher. It follows that into electricity grid circuits, those fuses can’t efficiently and immediately disrupt the current flowing via the consequent arc. What exactly? This implies, by way of instance, the favorite fuses of types F and T are completely not able to shield against a short circuit of semiconductor parts – namely triacs, thyristors, diodes, functioning in network circuits.

Designation of fuses

Each Fuse has published markers. They specify the parameters. What parameters? We’ll clarify!

Operation class – includes 2 components marked with letters. The first component refers to the activity class, and the next component Refers to protected items.

Action classes:

  • full range – g,
  • part range – a.

Examples of protected object categories:

  • protection of general purpose device – G,
  • protection of cables and wires – L,
  • protection of motors – M,
  • protection of transformers -Tr.

Other designations are:

  • ip – impact short-circuit current,
  • lk – continuous fault current,
  • lth – thermal short circuit current.

Fuse symbol

Fuse – The circuit is Broken up by melting Following a Predetermined time for a Specified current

fuse symbl

Single-pole fuse disconnector – This type of disconnector includes a fuse connection.

Single-pole fuse disconnector symbol

Thermal trip – Thermal trip offers overload protection to the load. Used like in motor circuit breakers

thermal trip symbol

Lock with mechanical Interlock (change ) – General emblem for lock security devices

lock fuse symbol

Fuse types


fuse types

Fuses, known as fuse connections, and colloquially”plugs” have an extremely simple construction. Within the ceramic or soapstone sleeve there’s a fuse, i.e. a metallic plate using only defined current strength that joins two connections of this fuse. The bushing is full of quartz sand to extinguish the arc once the fuse burns . After the fuse is blown off, it’s typically suggested by the color-coded indicator eyelet popping away.

The ease of this fuse structure translates to high reliability of performance. The fuse only burns when current flows through the fuse compared to the triggering current of the fuse, the quicker the overload or short circuit surpasses the triggering current of the fuse. The timing of the answer into an overload or short circuit is determined by the time-current characteristics of the fuse excursion. Be aware that fuses as well as their standards and packaging indicate their conventional rated current. The genuine trip current of fuses is 1.6 to 2.1 times the rated current (for average overcurrent circuit breakers, 1.13 to 1.45 times).

Characteristics of fuses

Commonly sold fuses have characteristics:
– (accessible D, D0, WT/NH types) – full-range fastener recommended for general usage in many circuits, such as lighting and sockets at residential property. Not suggested for electrical motor circuits pacious lighting setups that have many lamps at which large inrush currents happen.

gG/gL – (accessible D, D0, CH, WT/NH types) – complete range, delayed, slower in action compared to the quick type. It’s suggested in areas where momentary high starting currents happen, which might burn fuses of quickly form inserts, in addition to from superordinate switchgears to keep selectivity of security switching off.

aM – (accessible for CH and WT types) – non invasive, partial-range, i.e. protects devices largely against short-circuit. These fuses do not reply to little overload and inrush currents, therefore they need to work with extra engine circuit breakers or thermal relays for contactors.

gTr – (accessible WT/NH types) – transformer, mainly to protect 15/0.4kV transformers. They shield them about the lower voltage side from overload. They’re supplemented using fuses mounted onto the moderate voltage side (e.g. WBGN), shielding the transformer against short circuits.

Utra Quick, aR, gR – (accessible types D, D0, CH, WT/NH) – quite quick fuse connections to overcurrent protection of semiconductor circuits, mainly in power electronic equipment.

gPV – (accessible types CH, WT/NH) – quite quick DC fuse connections for voltages 1000-1500V, for overcurrent protection of chains of photovoltaic modules, and particularly when over two chains are linked in parallel. Inserts are utilized on both rods”+” and”-“.

Overvoltage fuse

In The so called low-voltage area (voltage under 1000V), the protectors are broken up into 4 classes indicated with letters from their alphabet: A, B, D and C

Generally speaking, a protector Of a specified class protects against overvoltages of a particular voltage level and at a particular site. Let us begin with a concise discussion of those.

Class A protector Isn’t Used in home setups, it’s utilized to protect overhead electricity lines.

Class B arresters (kind 1) shield against high voltage surges such as those due to a direct lightning strike on the electricity line.

Class C surge protectors (form 2) shield against overvoltages with marginally lower overvoltage from the electricity line.

Class C and B protectors Are normally installed in household switchgear.

A class D shield (form 3) can be utilized for immediate protection of selected gear sensitive to voltage surges. It’s mounted at the switchboard or Behind the socket in the box, or right onto your device.

Each protector limits the surge of electrical potential only to a certain level. The closer to the power system the higher class of equipment should be used. For example, a protector:

  • Class A will reduce the voltage level to 6kV,
  • Class B will reduce voltage level to 2.5kV,
  • Class C will reduce voltage level to 1.5kV,
  • Class D will reduce the voltage level to 0.8kV.

Protectors Of specific classes must therefore be utilised in cascade, slowly cutting down the voltage degree. And therefore it’s in practice. When there’s but 1 switchgear at the house, either C and B class guards should be utilised inside (you can find also two-in-one B+C protectors).

If the construction has many flooring, then class B circuit protections Should be utilised at the home switchgear, also class C arresters from the switchgears on each floor.

Under sockets, even when the device is still sensitive to apparatus, we all could Also utilize class D surge arresters.

Class A protectors Aren’t available anyway, so we are doomed Into the winner or disfavor of their energy business.

Because This is a string on home switchgear, at the next Article I will concentrate on class B and class C protections (in additional nomenclature – class I and II).

The symbols used when speaking to your surge protector (arrester) are as follows.

surge protector symbol

A standard B or C (or B+C) type surge protector consists of two components:

– Surge protector base

– Replaceable insert, which contains the protection element

surge protector photo

The Foundation of this shield is mounted onto a DIN rail (TS35). It’s two terminals. The top terminal joins the stage (L) or neutral (N) cable, and where a lot of electrical capacity may appear.

To the reduced one we Join the PE cable, which is joined to the security strip of the supply board.

In the Event of Protectors utilized in three-phase circuits, the top terminals are obviously correspondingly more.

The protective conductor Ought to possess a minimal cross-section of 4mm², though it doesn’t hurt . In the end, there’s a possibility that a rather large current will flow through this cable and we don’t need to have the cable to burn .

Underneath the PE terminal you will find 3 hooks. Ordinarily, the collection comprises a plug that’s inserted inside this area and allowing you to attach the cables (from the presented guardian, throughout the renovation phase, the plug moved out to the world without telling anybody ).

Regardless, thanks to Those terminals, we’ve got the chance of remote telling in The event of damaging your add or yanking it away. This sign may be Connected such as to the enter of the alert (Interface on the right). In Cases like This, the control panel will soon be advised of this Insert harm by breaking up the electric circuit between the red and Green cables.

surge protection connection

The insert contains all the key elements that make the protector functional:

  • Class B (type I) – the basic element is the spark gap,
  • Class C (type II) – the basic element is the varistor.

The General principle is that on a single (upper) side of this protector we connect phase wires and perhaps neutral cable, and about the opposite hand the protective breaker.

After the voltage at the circuit is fine, the resistance between the Wires attached in the top along with also the protective conductor is extremely large, approximately many GΩ (Giga Ohm). Because of the, no current flows through the protector.

When There’s a voltage surge from the circuit, current begins Flowing throughout the protector to the ground. The potency of this current is based on the origin of the surge along with the chosen class of arrester.

In class B arrester the Major component is (or should be) spark gap. I said previously that through normal operation the resistance of the arrester is quite significant. In the event of a spark gap, then this resistance is colossal, since the spark gap is a break in the circuit. When immediate lightning strikes a part of the electric installation, the resistance of the discharge gap falls to nearly zero, because of the electrical arc. Because of the appearance of a rather high electric potential, an electrical arc creates from the spark gap between formerly broken parts. An electrical arc is, let us say in easier terms, a more lasting electrical discharge (spark).

Due to the between, for Instance, a phase conductor where There’s a really large voltage leap along with the protective breaker a short-circuit happens along with a current of top intensity flows right to the floor, bypassing the home electric system. Following the release will be extinguished, the spark difference yields to regular (i.e. the fracture from the circuit).

A class C arrester includes a varistor within it. A varistor is a special Resistor which has an extremely large resistance at low electric capacity. If There’s a voltage spike at the circuit because of indirect release, Its resistance decreases quickly, causing current to flow into ground And a similar situation like a spark gap.

The distinction between class B, and class C is the latter can catch voltage spikes of reduced potential compared to direct lightning strike. The drawback is the varistors wear out fairly fast.

The important problem in surge protectors, whatever the class utilized, will be to possess grounding system using quite great parameters, i.e. having really low electric resistance. If the resistance is too large, the surge current (due to lightning) can flow through the electric system rather than the protector, leaving behind burned gear plugged into the sockets.

Residual current fuses

Suppose An appliance or lamp fails, leading to current flowing through the enclosure into your grounding system.

If It’s a short Circuit, i.e. that a rather large current will leak, and after a short while the overcurrent circuit breaker may trip, which can ascertain that the cables will clot. The single question is if the device will endure such a huge current flowing through it.

But what if matters Are not so”great”?

If the end result of this Harm is flowing into the grounding system current using a modest value of the seriousness of eg 0.5A? Subsequently the current may flow for days / months, and you’ll find that some thing is happening just on the invoice.

The next”channel” Through which electrons could escape is a individual. In the end, an electrical shock isn’t anything more than an electrical current flowing through someone.

Residual current circuit Breaker can be used to test – appearing from the electric circuits inside the house – what’s the distinction between the current flowing from itand also the return current (incoming). If everything is okay, the distinction will be 0.000A (I left a little simplification here, except for the very first part it’s sufficient ).

If the gap is too Big (value is based upon the breaker, however in homes it’s generally 30mA or 0.03A), the breaker blows the voltage into the devices attached to it.

It’s very often stated that the residual current circuit breaker is Used to protect against electrical shock. That is obviously true, however…

It’s Hard to disagree with the statement that if a current Of a particular intensity flows via a individual into the ground means This current won’t go through the cable to the Residual current circuit breaker and the latter will probably react by Cutting the voltage. Nonetheless, there are two instances when The circuit breaker won’t function and the individual current will be razed. I’ll concentrate on them at another section, however in the meantime, Let us proceed to another matter.

Like every electrical equipment, therefore that the residual current circuit breaker includes specific parameters describing its own characteristics. I will concentrate on the most crucial. Their comprehension is sufficient to Pick the Ideal Kind of circuit breaker:

Differential current IΔn – A simple parameter that states what’s the limiting value of this gap between the current that popped from the breaker and the current that returned into the breaker. In flats and homes, the most commonly used circuit breaker using IΔn = 30mA. As a reminderI would add this is actually the contractual limitation of current which may flow through the entire guy, and that does not pose a danger to life.

In exceptional conditions, used circuit breakers with a residual current of 10 mA.

Additionally, there are less sensitive residual current circuit breakers, which find the remaining current simply to the amount of 500mA, however they’re utilized to protect equipment in industrial plants or apply to people.

Rated current In – the maximum current that can flow through the breaker over a long period of time.

Rated voltage Un – the voltage for which the breaker is designed. For home use, there are two options to choose from:

230V – a two-pole breaker that has connectors for the phase wire and neutral wire

400V – 4-pole switch with connections for 3 phase wires and neutral

Number of poles – as mentioned above the circuit breaker can be:

2-pole – 2P

4-pole – 4P

The type of tripping – in this respect we distinguish three types of circuit breakers with designations:

AC – the most common (and cheapest), detects AC differential current – you will 99% need this breaker.

A – detects alternating and alternating differential current in the form of e.g. single lead rectified sine wave.

B – detects AC, AC and DC differential current.

The most essential element within the circuit breaker would be that the so-called Ferranti transformer. Within this piece I won’t go over the principle of its performance. The time will soon come when I’ll write about the operation of transformers and also the occurrence of induction of electromotive force generally. Ordinarily, for today you may safely forego the situation.

For the time being, the most essential issue is that you understand that this sort of transformer is present at the remaining current device and it doesn’t interfere in any manner with the electric circuit, i.e. that the way of discovering changes in current is noninvasive.

Ok, of the more tangible elements, each residual current circuit breaker has:

1. an on-off button.

2. status indicator.

3. a button to test the circuit breaker

4. Terminals for connecting supply and drain wires.

5. Terminals for mounting on a DIN rail (TS35)

6. rating plate with a mini schematic diagram of the “Test” button functioning.

residual circuit breaker construction

The nameplate is that your information regarding the parameters of this circuit breaker published on front. Along with the parameters, quite often there’s also a miniature diagram suggesting what happens once you press on the “Test” button.

Thus, We got two remaining current circuit breakers Whose main parameters are:

  • Both will trip at a residual current of 30mA (0.03A),
  • Both have a rated current of 25A,
  • Both operate on AC voltage,
  • Rated voltage 230V.

In addition, there are additional parameters that are less important and are printed on the Legrand circuit breaker, while the Hager circuit breaker has put the other parameters on the circuit breaker data sheet, including:

  • Minimum operating temperature of the breaker -25ºC,
  • Short-circuit capacity of 500A (i.e. the maximum current that can flow through the breaker during a short circuit),
  • fuse symbol with the number 10 000 means that the breaker can withstand a short circuit current of 10000A provided it is protected with an additional fuse.
residual circuit breaker description

You will find such or similar print on your switch. I additionally marked two elements (a and b) to make it easier to identify what I am actually writing about.

residual circuit breaker schematic
  1. four terminals of the switch marked with numbers from 1 to 4 (2 at the bottom 2 at the top, just like in real life)
  2. symbol of manual push-button (in this case it is the “Test” button) – i.e. a symbol resembling the letter “T” turned by 90 degrees
  3. the terminal pair connectors and the “Test” button connector – connected by a dashed line. And this dotted line is interesting here. Its presence means that the elements it connects interact with each other. Actuation of one element influences the other. I will talk more about this below.
  4. The ring, ellipse means that there is a transformer in this place (here Ferranti transformer)
  5. the rectangle I marked with an “a” is the current limiting element, probably a resistor.
  6. the rectangle I marked with a “b” is the trip coil of the circuit breaker.
  7. Don’t forget the black as pepper dots here, which indicate the connection of two wires / a branch circuit.

Connect the phase cord (L) to terminal 1 and the cable (N) to terminal two. Provided that the change is away (OFF place, green shade ), the electrical potential isn’t passed and finishes at those terminals.

Should you lift the change key (ON place, red shade ) You join 3 parts concurrently:

  • Terminal 1 with terminal 3,
  • Terminal 2 with terminal 4,
  • Test button with neutral.

Simultaneous switching of 3 buttons results, obviously, by the building of this component, and also on the diagram it’s represented by linking the buttons using a hidden line. Turning on a single will turn on others (analogically for turning ).

Aside from the fact that every thing functions nothing special. The essential point is that current flows throughout the phase cable via the switch into the load, and current of equal intensity”yields” into the change during the neutral cable. In case the gap between the current around the phase cable and the current on the neutral cable is less than 30mA, then you’re good.

testing circuit breaker

Pressing That the”Test” button will automatically link with the phase wire to the neutral cable. As a result of existence of resistor”a”, this doesn’t lead to a short circuit and a restricted current flows in the wire. But, it’s big enough to trigger the circuit breaker to trip.

The Travel of the electron at the diagram above begins at terminal 1. It then flows through a location that’s surrounded by a ring (the divider). From that point on the transformer anticipates exactly the exact identical number of electrons to flow through the neutral conductor (just”another way”). Nothing like this can happen because electrons don’t flow into terminal 3 but”flip” at which the circuit has been shut i.e. in the path of pressed”Test” button and from that point through the neutral wire to terminal two. Within this entire performance the transformer (ring) was bypassed.

To Amount up, from the point of view of this transformer the current flowing Through the phase cable isn’t the same as the current flowing through. The cable.

testing circuit breaker

When the current flowing “illegally” outside the transformer is exceeded, the transformer interacts with coil b, which disconnects all 3 switches, opens the circuit, and cuts off the current flow.

Thermal fuse

A thermal fuse is really a new sort of electrical overheating safety component. This type of component is generally installed in heat-prone appliances. When the electric appliance fails and creates warmth, once the temperature exceeds the unnatural temperatures, the thermal fuse will then fuse to reduce the electricity source to protect against the electric appliance from resulting in a fire.

The thermal fuse is just like the fuse we’re knowledgeable about. It generally only functions as a highly effective path from the circuit. If it doesn’t exceed its rated value during usage, it won’t fuse and won’t have any influence on this circuit. It’ll fuse and cut the electricity circuit only if the electric appliance fails to create strange temperatures. This differs in the fused fuse, that can be blown off by the heat created if the current exceeds the rated current in the circuit.

thermal fuse

Ordinarily, a fuse consists of 3 components: 1 is the compacted part, that’s the center of the fuse, and that blows off the current when it’s blown. The melt of the exact identical kind and specification of this fuse should have exactly the exact identical substance, the exact identical geometric dimensions, along with also the resistance value. It ought to be as little as possible and constant. The absolute most essential issue is to get exactly the identical fusing characteristics. Household fuses are ordinarily made from lead-antimony metals.

The next is that the electrode component, generally two. It’s an significant part the connection between the melt and the circuit. It has to have good electric conductivity, shouldn’t create clear installation touch resistance; next is that the mount area the melt of this fuse is usually soft and slender, the role of the mount would be to correct the melt and create the 3 components a rigid complete for effortless setup and usage, it has to have good mechanical power, insulation, heat resistance and fire resistance, and ought not to be broken, wrinkled, burnt, or short-circuited through usage.

thermal fuse structure

The thermal fuse can be divided into:

According to the material: it can be divided into the metal shell, plastic shell, oxide film shell

According to temperature: it can be divided into 73 degrees 99 degrees 77 degrees 94 degrees 113 degrees 121 degrees 133 degrees 142 degrees 157 degrees 172 degrees 192 degrees…

thermal fuse types

Thermal fuse gets the characteristics of accurate melting temperatures, high withstand voltage, small dimensions and very low price. The thermal fuse casing is indicated using the rated temperature value along with the rated current value, it isn’t tough to spot, and it’s extremely easy to use. It may be widely utilised in electric equipment, electrical heating and functional electric appliances such as overheating protection. Thermal fuse mainly includes the following parameters:

Rated Temperature: Sometimes known as the working temperature or fusing temperature, it describes the temperature where the temperature climbs to the fusing temperatures at a rate of 1°C per minute beneath no-load conditions.

Fusing precision: describes the gap between the actual fusing temperatures of the thermal fuse as well as the rated temperature.

Rated Current and rated voltage: Typically, the nominal current and voltage of both thermal fuse have a particular margin, typically 5A and 250V.

Thermal Fuse is a one-time-use protection component. Its usage impacts not only is contingent upon the performance of the component itself but more importantly, how best to choose and set up the thermal fuse properly. The thermal fuse is usually linked in series in the circuit if it’s used. Thus, when deciding upon a thermal fuse, its rated current must be higher than the current employed at the circuit. Never enable the current during the thermal fuse to surpass the designated rated current. Prior to picking the rated temperature of the thermal fuse, you need to know and assess the temperature gap between the temperatures to be shielded and the place at which the planting fuse is set up.

In Addition, the duration of the time and also the accessibility of Venting are also strongly linked to the choice of the graded Temperature of the thermal fuse.


Michal Pukala
Electronics and Telecommunications engineer with Electro-energetics Master degree graduation. Lightning designer experienced engineer. Currently working in IT industry.