European Electrical Fuses and Circuit Breakers

Fuses are older safety devices that are still used in some homes, but have largely been replaced by circuit breakers. A fuse is a small, cylindrical device that contains a wire or filament that melts when the electrical current exceeds a certain level. When the fuse melts, it breaks the circuit and stops the flow of electricity. Fuses come in different ratings, which indicate the maximum current that can safely flow through them. If a fuse blows, it must be replaced with a new one of the same rating.

Circuit breakers, on the other hand, are more modern safety devices that use an electromechanical switch to break the circuit when the current exceeds a certain level. Unlike fuses, circuit breakers can be reset after they trip, making them more convenient to use. Circuit breakers come in different sizes and ratings, which indicate the maximum current that can safely flow through them. If a circuit breaker trips, it can be reset by flipping the switch back to the "on" position.

In general, circuit breakers are considered to be more reliable and convenient than fuses, but both devices serve the same basic purpose of protecting home electrical wiring from damage caused by overloading or short circuits. It is important to make sure that the fuses or circuit breakers used in your home are rated appropriately for the electrical load they are protecting.

Low voltage fuses according to IEC 60269 (formerly IEC 269, equivalent to EN 60269 and VDE 0636) are used in distribution networks, industry, and by end-users, for example in fuse boxes. The typical rated voltage is 230/400 V AC. For industrial plants, there are designs available for up to 1000 V DC or AC voltage.

There are various types of fuses (such as screw-in fuses, NH fuses, and cylindrical fuses), which are produced in different operating classes (trip characteristics).

Time-current diagram for operational class gG (gL), example

Fuses, like other types of protection devices, are characterized by their trigger characteristics. Along with the rated current and breaking capacity, this is an important parameter.

The trip characteristic describes the range of trigger times for specific relative overcurrents with respect to the rated current, in a time-current diagram. The tolerances for the same characteristic are relatively large. For example, at 1.5 times the rated current, the trigger time is approximately one hour, while at 15 times the rated current (short circuit), it is less than 50 ms.

It is characteristic for all time-current diagrams of protection devices that the tolerance range is larger for low overcurrents than for relatively high overcurrents. If tight tripping tolerances are required (for example, to protect a small transformer against overload), a fuse element is often unsuitable. Alternatively, temperature fuses or bimetal overcurrent switches are used.

Slow-blow D-type fuses were introduced around 1930. To distinguish them from conventional fast-blow fuses, they were marked with a stylized snail symbol, or the letter T enclosed in a circle for Switzerland. In 1967/68, the distinction between slow-blow and fast (normal) fuses for circuit protection fuses was abandoned, and the uniform operating class gL (later gG) was introduced. The gL (gG) characteristic is slow-fast, meaning that it is slow at low short-circuit currents and fast at high currents. The snail symbol marking was retained for gL D-fuses for decades.

As a rule of thumb for fuses with the gG (gL) operating class, when the current exceeds four times the rated current (or five times for gL), the fuse will trip within five seconds, and when it exceeds nine times the rated current, the trip time will be 0.2 seconds.

The operating class of a low voltage fuse is designated by two letters, where the first letter indicates the functional class and the second letter indicates the protected object. The functional class of a fuse indicates its ability to conduct certain currents without damage and to be able to interrupt overcurrents above a certain range.

There are two functional classes:

With regard to protected objects, a distinction is made between:

Combined, this results in the following common operating classes:

European and US-American fuses differ in terms of their nominal current definition and triggering characteristics.

Closely related to the triggering characteristic is the selectivity of an electrical distribution system: in the event of a short circuit or overload, only the fuse of the affected circuit should trigger, but not any higher-ranking fuses that also protect other circuits. Therefore, the fuses must be coordinated with each other in terms of their response behavior.

In the case of a short circuit or high inrush current, the let-through energy I²t (integral of the squared current over time, also known as melting integral or current integral) is important. When multiplied by the fuse's ohmic resistance, it describes the energy value that just barely does not cause the fuse to trip: the power dissipation (Joule heating) at the fuse element depends on the square of the current and leads to a certain temperature that triggers the fuse within a certain time. The let-through energy should never be fully utilized in the dimensioning of fuses, as they change thermally over many such switching cycles and may trigger prematurely.

A screw-type fuse holder for a D-fuse consists of a fixed fuse base with the fitting element (fitting screw) and a removable screw cap with a window. The fuse insert (melting insert, fuse cartridge, fuse) has a colored operating state indicator (identification marker, also switching state indicator or interruption detector) that is located behind the window of the screw cap when the fuse is screwed in, and a foot contact that is paired with the diameter of the fitting insert. The fitting inserts are often color-coded and are identical to the color of the identification element of the fuse (see table below). The inside diameter of the insulated head of the fitting screw limits the diameter and thus the rated current of the fuse sizes that can be used. The screw is to be tightened securely with a special tool that engages in two grooves on the cylinder mantle of the insulating body and must be chosen appropriately for the load capacity of the installed line.

The fuse insert is the reactive, replaceable part of a fuse.

Screw fuses have foot contacts with diameter gradations depending on the rated current. The base of the fuse holder contains a corresponding colored fitting element (fitting screw, fitting insert) that prevents fuses with a higher rated current than intended from being used. Traditionally, there is an exception for Diazed DII fuses, which allows a 10 A fuse to be equipped with a 6 A fitting screw. The special type is designated as 10A/6F, 10/6A, or 10R/6.

In the middle of the head contact of the fuse insert is a colored metal plate, the identification marker, as a switching state indicator. It is underlaid with a spring and held by a wire with high resistance, which is attached to the foot contact of the fuse insert. After the melting of the melting conductor, the holding wire of the identification marker also melts, causing the identification marker to be ejected. A glass panel in the screw cap prevents the identification marker from falling out and allows for visual inspection of the tripped fuse.

Identification markers and fitting inserts are color-coded depending on the rated current. When developing D-type fuses in 1906, the colors of the Germania postage stamp set from 1900 were chosen as a mnemonic device. These and later postage stamps had the following colors: 5-pfennig stamp green, 10-pfennig stamp red, 15-pfennig stamp gray, 20-pfennig stamp blue, 25-pfennig stamp yellow.

The main difference between D-type and D0-type fuses, in addition to their different dimensions, is the permissible operating voltage: while D-type fuses are suitable for a voltage of up to 500 V, special types up to 750 V (both AC and DC), the D0 system is only intended for a voltage of up to 400 V AC and 250 V DC.

Today, screw fuses of the gG operating class (formerly gL until 1998) are used as line protection fuses, for example, to protect lines to distributors. Screw fuses are occasionally still used in conjunction with motor protective switches to protect motors when machines with particularly high starting currents are operated.

Screw fuses (D, D0) may only be operated under load under the following conditions:

• Only by trained personnel
  • AC voltage over 400 V, rated current maximum 16 A
  • DC voltage 25-60 V, rated current maximum 6 A
  • DC voltage 60-120 V, rated current maximum 2 A
  • DC voltage 120-750 V, rated current maximum 1 A
• Also by laypersons
  • AC voltage max. 400 V, rated current up to 63 A
  • DC voltage max. 25 V

The D-System (also DIAZED; diametrically graduated two-part Edison fuse plug) was developed by Siemens-Schuckertwerke, initially in today's size DⅡ. DIAZED is a brand, therefore the neutral standard designation is D-System or D-fuse. It replaced the previously common one-piece fuse plugs, which are still used today in the USA as "plug fuses". What was new about this system was the separation of the screw cap and the fuse insert ("cartridge"). D-fuses are available in five sizes. The designation consists of the letter D and a Roman numeral. Slow-blow types are also designated as DT.

¹Thread of the screw cap: E = Edison thread, G = pipe thread, straight, R = pipe thread, external thread conical

The NDz fuses (less commonly called ND or DⅠ) with a smaller diameter were introduced at the end of the 1920s and are also referred to as "saving cartridges" because they can be installed in DⅡ sockets with a reducing sleeve. Today, they are hardly used in old installations, although the short DⅠ design with screw cap thread SE 21 is widespread in Switzerland. The most common Diazed fuse is probably size DⅡ. It can also be fitted with a retaining clip in DⅢ sockets. The sizes DⅢ, DⅣ and DⅤ are still used in older mains distribution boards today. The sizes DⅣ and DⅤ have not been installed in new installations for decades, as NH fuses are better suited for such high currents and for operation under load. The sizes DⅡ and DⅢ are also available in normal or extended versions for higher rated voltages. Typical examples are 690 V three-phase alternating current in industry and power plants, as well as for railway power systems and trolleybuses up to 750 V or up to 1200 V.

¹Thread of the screw cap: E = Edison thread

The D0 system (also NEOZED; new type of DIAZED fuse, neo: "new") was introduced in 1967 by Siemens and Lindner as an advancement of the previously dominant D system (DIAZED), and has replaced it in new installations as far as fuse protection is still used. The advantages over the D system are smaller dimensions and lower power loss (less heat development) at the same rated current. NEOZED is a trade mark, therefore the neutral standard designation is D0 system or D0 fuse (pronounced D zero). D0 fuses are produced in three sizes.

The designation of a size consists of "D0" and another Arabic numeral:

¹Thread of the screw cap: E = Edison thread, M = metric thread

D01 fuses also fit in DL sockets and can be used in D02 screw sockets with a special holding spring. The D03 design is very rarely used because NH fuses have proven to be more reliable for high-rated currents. D03 fuses are no longer allowed to be installed in new systems.

For D and D0 fuses, there are sockets for screw mounting, for DIN rail mounting, and for busbar mounting ("rider socket"). In addition, for D0 fuses, there are fuse switch disconnectors as fuse sockets with an integrated switch disconnector. Before each fuse change, the socket must be switched off by a flap located in front of the fuses. This voltage- and load-free change increases operational safety and safety for the user, as the user cannot come into contact with live components in any case. In newer versions of these switch disconnectors, the fuse cartridges are no longer screwed but contacted by spring force.

As a replacement for the D-System, the space-saving DL-System for 380V AC was introduced in the GDR. The design is similar to that of D01 fuses, but is designed for up to 20A. For older systems with legal protection, DL fuses are still manufactured with the operating class gG and a rated voltage of 400V AC.

D01 fuses (NEOZED) up to 16A also fit into DL sockets, but not vice versa.

¹Thread of the screw cap: E = Edison thread

Low voltage high-performance fuses, also known as NH fuses, are also referred to as knife fuses, sword fuses, or (in connection with house connection boxes) as tank fuses. The characteristic feature is the significantly larger size compared to screw fuses, as well as the massive contact blades at both ends for guiding and separating larger currents. Common versions of NH fuses enable safe disconnection of short-circuit fault currents up to 120 kA (rated breaking capacity), with the standardized rated current being up to 1,250 A (rated current). Outside the standard, fuses with a rated current of up to 1,600 A are available. NH fuses have an indicator that displays a defective fuse. Depending on the version, it is designed as a flap indicator mounted on the end face (top) or as a center indicator visible from the front with the fuse inserted. NH fuses with two indicator (combination indicators) are also available. NH fuses are available with different triggering characteristics, which are described in the operating class section.

NH fuses are manufactured in various sizes for different rated current ranges. Size 0 is no longer permitted in new installations.

NH fuses are used in the high current range of low voltage networks and are widely used in industrial plants. They are also used in public power grids, for example in transformer stations, main distributions, or in the meter cabinet of buildings and as a metering fuse.

In the pre-meter area of customer systems, the TAB 2007 (Technical Connection Conditions of Energy Network Operators) require a disconnecting device per meter. Quote:

"A disconnecting device is a device for disconnecting the customer system from the distribution network, which can also be operated by the customer (electrical layman) (e.g. SMB)."

Selective circuit breakers or Neozed load disconnect switches, for example, meet this requirement, but NH fuses do not. Therefore, NH fuses are only used as metering fuses in new installations if another disconnecting device that can be operated by laypersons (e.g. in the form of a metering backup with a Neozed load disconnect switch) is provided.

In the UK, consumer units in older installations are equipped with fuse holders that can be fitted with closed fuse links or semi-open, rewireable fuses.

In this system, produced by companies such as Wylex, the user can replace the fuse wire in the fuse element. Loose fuse wire can be purchased in supermarkets, petrol stations, and hardware stores. The Rewireable Fuse Carrier is specified in the British Standard BS 3036 and can be fitted with fuse wire rated for currents of 5 A, 15 A, 20 A, or 30 A.

According to BS 7671, the rated current of such fuses must not exceed 0.725 times the continuous rated current of the circuit. Circuit breakers can be used as a replacement for these fuses.

Possible hazards of this system include untrained individuals working on electrical systems, intentionally or accidentally over-fusing circuits, and the use of unsuitable, conductive "fuse material" such as coins, nails, hairpins, wire remnants, or paperclips. The type of fuse material used cannot be determined without removing the fuse. In addition, the breaking capacity of rewireable fuses is much lower than that of sand-filled fuses, which can cause arc faults in adjacent installations.

Circuit breakers, just like fuse links or power breakers, can automatically disconnect a circuit in case of overload or short-circuit. For Germany, the following rules apply for new installations (according to the Technical Connection Requirements in conjunction with DIN 18015-1):

• In residential circuit distribution boards, only circuit breakers that can be operated by non-professionals may be used for lighting and socket circuits. Fuse links are only permitted for fixed devices (such as water heaters) or as a primary protection for sub-distribution boards.
• Selective circuit breakers (SLS) are used for protection in the pre-metering area. NH fuses are only permitted in this application area if another "non-professional disconnecting option for the customer's system" is provided, such as a Neozed isolator switch for post-meters.

In residential or office spaces, circuit breakers with a B-characteristic are typically used. The C-characteristic is used for protection of lines and devices for consumers with high inrush currents, since B-characteristic can cause false triggering during start-up. When protecting circuits with electronic devices (electronic ballasts, switching power supplies) with circuit breakers, special attention must be paid to their high inrush currents.

Circuit breakers with B-characteristic are available for the following rated currents in accordance with the Renard series: 6, 10, 13, 16, 20, 25, 32, 35, 40, 50 and 63 amperes. Other values may also be available depending on the manufacturer. Type C, D, K, and Z circuit breakers are available in a wider variety of types with values up to under 1 A. In residential spaces in Germany, individual circuits are typically protected with B-16 circuit breakers (16 A).

The H-characteristic has been used for household circuits since the 1950s to achieve reliable rapid tripping in the presence of high-impedance networks or ground faults during short circuits. However, under current network conditions, the sensitive short-circuit tripping can be undesirably triggered, affecting devices with switching power supplies (such as computers, televisions) or motors (such as vacuum cleaners). In such cases, it is recommended to replace H-circuit breakers with B-circuit breakers. A H10 circuit breaker can usually be replaced by a B13 circuit breaker, since they have the same overload characteristic.

Circuit breakers are classified not only by their rated current and design, but also by their tripping characteristic. The currently standardized characteristic types are B, C, D, E, K, and Z, which are highlighted in the table. The two values for overcurrent tripping denote the non-tripping current (small test current) and the tripping current (large test current). The maximum tripping time applies to the tripping current. Some manufacturers specify narrower tolerances for the tripping currents for overcurrent and short-circuit protection.

Circuit breakers must be able to switch off high short-circuit currents. The switching capacity, referred to as the rated short-circuit breaking capacity I_cn, is normatively classified as follows:

In addition, there are requirements for short-circuit current limitation. In Germany, according to the technical connection conditions for circuit breakers up to 32 A, only energy limitation class 3 (selectivity class 3, "high requirements") is valid, which has the highest short-circuit current limitation according to VDE 0641.

In the event of a short circuit, the current (prospective short-circuit current) is very high, determined only by the network impedance (internal resistance). The circuit breaker limits the short-circuit current to a lower value due to its design. A high energy limitation ensures high selectivity with upstream fuse links and protects the system from electromagnetic effects.

Circuit breakers have a plastic housing. Older versions were cylindrical and were used instead of the previously common screw-in fuses in Edison screw threads or screwed in with a thin metal strip. Modern circuit breakers have rectangular housings and can be mounted closely together on a mounting rail (DIN rail).

Single-pole circuit breakers are usually one module (1 TE) wide today. The width of one module is 18 mm. According to the DIN 43880:1988-12 standard, the installation width of the devices should be between 17.5 and 18.0 mm. Two-pole versions are produced with widths of 2 TE, 1.5 TE, or 1 TE. Three- and four-pole circuit breakers are correspondingly wider. There are also circuit breakers with a width of 1.5 TE per pole. These are usually designed for rated currents from 80 A to 125 A and/or with very high breaking capacity. A selective circuit breaker is 1.5 TE wide, older types are 2 TE. They are mounted on a busbar with a center-to-center distance of 40 mm. Alternatively, the selective circuit breakers are also mounted on normal DIN rails, but they do not fit in conventional small distribution boards.

If a circuit breaker is also to switch the neutral conductor, special circuit breakers must be used, since the contact for the neutral conductor must open laggingly and close leadingly. This ensures that the phase is never switched through without the neutral conductor.

1. Switch lever for manual on/off operation. Also includes the visual indication of the switch state.
2. Trip mechanism for releasing the circuit breaker under fault conditions.
3. Switch contact for making or breaking the electrical connection.
4. Terminal connectors for electrical connection.
5. Bimetal strip for thermally triggered overload protection.
6. Calibration screw used by the manufacturer to set the thermal tripping behavior (part of the characteristic curve).
7. Electromagnetic tripping coil for high currents, typically short-circuit currents.
8. Arc-extinguishing chamber for extinguishing the arc during interruption of a short-circuit current. The arc moves from the opening switch contact (3) to the area of the arc-extinguishing chamber, where it is extinguished by splitting and cooling.

The shutdown mechanism can be triggered in four ways:

• Shutdown due to overload
  • If the predetermined nominal value of the current flowing through the circuit breaker is significantly exceeded for a longer period of time, shutdown occurs. The time until the shutdown depends on the strength of the overcurrent; it is shorter for high overcurrents than for slight deviations from the nominal current. A bimetal is used to trigger the shutdown mechanism (thermal shutdown) by bending when heated by the current flowing through it.
• Electromagnetic shutdown due to short circuit
  • If a short circuit occurs in a system, shutdown occurs within a few milliseconds due to an electromagnet that is powered by the current flow.
• Manual shutdown
  • Circuits can be manually shut down at the circuit breaker for maintenance or temporary shutdown. For this purpose, a toggle switch or release button is located on the front.
• Shutdown by additional modules
  • For most circuit breakers from reputable manufacturers, there are attachable auxiliary switches, under-voltage and over-current release devices, residual current devices (RCDs), arc fault detection devices (AFDDs), and motor drives (automatic reclosers) that can be used to operate the circuit breaker. The additional modules are attached to the right or left of the circuit breaker or wired accordingly in the distribution board, depending on the circuit breaker.

An important feature of circuit breakers is the non-influencable tripping mechanism. It ensures that in case of a short circuit, immediate tripping occurs even if the operating handle is being manipulated or held in the "on" position.

After an overload trip, the bimetallic strip must first cool down before a reset can be performed. The manual reset required for restarting alerts the user to a potential problem and prevents automatic resetting (fail-safe). This prevents uncontrolled restarts of overloaded equipment or the uncontrolled re-energizing of defective devices/installations.