Under an electrical fire is understood in electrical installations unacceptably high temperatures, the different degrees of impact, such as a fire or a smoldering, often in combination with a power failure, can have. In virtually all cases, so that a damage in the area of electrical installation is connected. There are different trigger sources of cable fires.
Loose terminal connection
Electrical cables are installed in electrical installations in individual sections which are joined together. These electrical connections are carried out in various ways. Examples are terminals in terminal boxes, as they are used for permanent connection of permanently installed electrical equipment or in the area of distribution. Also contacts in connectors such as connector or industrial connector according to IEC 60309 are connected in many cases with the cable via a clamp.
Proper terminal connection must have a conductor cross- section corresponding to the cable cross-section to avoid an unacceptably high current density in the terminal area. According to an improper installation, aging or shock can clamp connection, especially when they are executed by means of a screw, loosen over time. This results in the area of the clamping bolt to a high current density, it is equivalent to an unacceptably high contact resistance. In the terminal may cause arcing, which can continue to increase the temperature on contact interruptions, temperatures above 1000 ° C are not uncommon. This results in the area of the metal clamp as those used for insulating plastic materials to thermal damage. Depending on the materials, a pyrolysis occur, thereby resulting from verschmorrende plastics combustible gases and vapors which can be ignited by the hot, often glowing clamping point or arc.
For this reason, electrical terminal points shall be designed to prevent a relaxation of the electrical connection. These designs can be constructive measures in the terminal, for example, the execution of a spring -loaded terminal instead of a screw. On terminals with operating regular high current load, such as in the range of main lines, Schraubklemmverbindungen are having to tighten certain torque with a torque wrench by multiple clamps and the need for the individual bolts, guaranteed. Cable -side measures are the use of cable lugs, with strands uses of ferrules. Strands, which are clamped in screw terminals, should not be tinned with solder because solder is flowing property and the screw itself mechanically relaxed and thus solves.
Since the electric current remains by a loose terminal point generally below the permissible exposure action values in the circuit is at all times the regular electrical loads, there is no tripping of overcurrent protective devices. The smoldering fire can thus be overlooked when only small effects under certain circumstances for a long time. In part, forming Cable fires caused by the additional incorporation of fault arc protection devices ( AFDDs ) can be identified and further damages such as fires be prevented from building through automatic shutdown of the plant.
A cable break caused by mechanical stress or deterioration of an electrical cable, for example by exceeding the maximum allowable bend radius of a cable. A wire break is identified by a localized reduction of the wire cross-section. In this area there is high current densities and hence to arcing faults, with the associated heat load. The thermal effects are the same as in loose terminal connections. As a possible remedy can also be used in case of cable break fault arc protection devices ( AFDDs ).
The thermal overload a cable is characterized in that a cable in the cable cross-section is generally dimensioned too small and it comes over the entire length of conductor to an unacceptably high heating and heat loss. When properly executed electrical installations this cause can be ruled out, because if an overload protection device, such as circuit breaker or fuse trips and the power interrupts. Cable cross-sections and the permissible continuous current load depend inter alia on the conductor material, conductor cross-section, insulation material and ambient temperature used and are defined in standard works.
In special cases where a sufficiently large conductor cross section due to technical reasons is not possible, the loss of heat must be removed continuously by additional cooling devices on the cable. Examples of this are the high voltage cables and oil cables used in high-voltage area with an additional external water cooling dar.
As a major difference to the defective clamp connections or cable breaks the thermal overload of a cable shall always distributed over the entire cable length.
Outside influences can lead to an electrical fire, for example by direct lightning or installation failure in the transformer station, which can have an over-voltage with series of cable fires. In these cases, appropriate measures such as lightning protection and surge protection devices are needed.
Excessive overheating in electrical installations can be determined prior to the occurrence of cable fires using thermal imaging cameras. In the thermal image areas with high temperatures are easily recognizable, which can be a remedy even before the occurrence of a loss. Since thermal imaging cameras determine the surface temperatures without contact, so periodic monitoring measurements can be made without interrupting the power supply in Hochspannunsganlagen.