Fuses and inserts

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Fuses & Inserts

Fuses are a large group of devices, without which the use of any electrical installation would pose a great risk to both people and property, as well as the installation itself. In terms of typology, electrical fuses should be included in the group of electrical switches, which also includes other disconnectors, switches and disconnects.

Fuses and fuse links - protection against short circuits and overloads

Overloads, shorts and overvoltages are states in which normal operation of electrical equipment is not possible. Fortunately, each installation can be protected against the effects of these abnormalities by using appropriate electrical apparatus. Fuses belong to the group of devices protecting against the effects of overloads (i.e. short circuits and overloads).

Fuse circuits, i.e. where fuses are used

The place where fuses and fuses are installed is the electrical switchgear. Today, the protection of the home installation is usually realized by overcurrent circuit breakers, but in older installations you can still meet with fuses. Their function is to break the continuity of fused circuits (e.g. protecting plug sockets or lighting in the home), in the event that an overcurrent occurs, which results in the flow of higher than rated current for a time exceeding the nominal time of the fuse.

What is a fuse link?

How does a fuse link work? A fusible link fuse consists of a receptacle permanently installed in the switchgear and a replaceable fuse cartridge mounted in the receptacle. When the fuse is in the socket and the installation is operating normally, the power supply is connected through the fuse to the fuse circuit of the home installation. In this case, the device draws power from the mains. However, when there is a short circuit or overload in the circuit, the fuse element in the cartridge (usually a silver-plated wire) starts to melt. When the wire melts, the circuit is interrupted and the dangerous phenomenon will not lead to undesirable consequences such as destruction of the device or fire. In a way, fuses even provide protection against electric shock. However, they do not serve this purpose (here you need an apparatus with much greater sensitivity to much smaller deviations from the permissible level of current), but can only prevent an electric shock by disconnecting the voltage and inactivating the circuit.

Fuses can be subdivided in many ways. Although they occur in high voltage networks, as well as SN, dealing most often with LV installations, it is worth knowing that the most common distinction used by manufacturers in this area is the division into installation fuses (that is, fuses with a cartridge and socket) and the so-called power fuses, used in industry, with fuse bases and knife-type fuse links, the so-called NH.

Installation fuses

Installation fuses consist of a socket and a fuse element. The fuse link has usually a ceramic body (nowadays fuses with plastic bodies are also available) and is filled with silicon gas to extinguish the arc on the fuse element.

Installation fuses are also called D-type fuses because of the parameterization - all D-type fuses have a rated breaking capacity of 50 kA for AC and 8 kA for DC. There are five types of fuse links (DI to DV) and the grouping of a particular fuse link depends on its current rating.

Time-current characteristics. Inserts gG, gL, gM and others

Fuse-links have different time-current characteristic curves which are identified by two letters:

  1. the first one (small) defines the full-range or part-range breaking capacity of the insert:
  • a - part-range characteristic (protection only against short-circuit effects),

  • g - full range characteristic (protection against the effects of short circuits and overloads),

  1. second letter (capital) indicates the category of use of the cylinder, which means that it is intended to protect specific devices:
  • G or L - wires and cables,

  • M - electric motors,

  • Tr - transformers,

  • B - mining equipment,

  • R - semiconductors.

On the market there are fuses with different properties and purpose, identified by symbols: gG, gL, aM, gR and others.

Colour coding of fuse links

The marking of fuse links does not only reflect their characteristics and intended use, but also other parameters. Since it is permissible for unqualified persons to replace worn out fuse links, for safety reasons the color fuse link markings have been adopted, which indicate the rated current of the fuse link. For the most common fuse links used in domestic installations these are:

  • pink (for rated current equal to 2A),

  • brown (for 4A)

  • green (for 6A),

  • red (for 10A),

  • grey (for 16A),

  • blue (for 20A),

  • yellow (for 25A),

  • black (for 35A),

  • white (for 50 A),

  • copper (for 63 A).

Power fuses

Power fuses with high breaking capacity are used in industry mainly to protect electrical machines and equipment and lines against overloads and short circuits in circuits with high short-circuit and operating currents.

Power fuse consists of 1- or 3-pole fuse base and 1 or 3 fuse links, depending on whether the circuit is single or three-phase. The base is a part made of insulating material, which is used to fit the jaw clamps and the screws for attaching the conductors. In the jaw clamps fuse links are installed, most often in a standardized system NH (German Niederspannungs-Hochleistungs-Sicherungen) defining low-voltage fuses with high short-circuit breaking capacity. NH fuses are popularly called knife fuses because of the characteristic appearance of the contacts and their placement in the jaw terminals of the bases.