Cable fire
In electrical installations, a cable fire is understood to be an impermissibly high level of heating, which can have various effects such as a fire or a smoldering fire , often in combination with a power failure . In practically all cases, this is associated with damage to the electrical installation. There are different sources of cable fires.
causes
Loose clamp connection
Electrical lines are laid in electrical installations in individual sections that are connected to one another. These electrical connections are implemented in different ways. Examples are terminals in junction boxes, such as those used for the permanent connection of permanently installed electrical devices or in the area of sub- distribution. Contacts in connectors such as appliance plugs or industrial plugs in accordance with the IEC 60309 standard are in many cases connected to the line via a clamp.
A proper clamping connection must have a sufficiently large conductor cross-section in order to avoid an impermissibly high current density in the clamping area. As a result of improper installation, aging or vibrations, terminal connections, especially if they are made using a screw terminal, can loosen over time. This leads to a high current density in the area of the clamping screw; this is synonymous with an impermissibly high contact resistance . In the area of the terminal, there can be arcing faults , even if the contact is interrupted , which can cause the temperature to rise further; Temperatures above 1000 ° C are not uncommon. As a result, thermal damage occurs in the vicinity of the metallic clamp, as with the plastic materials used for insulation. Depending on the material, pyrolysis can occur, with scorching plastics producing flammable gases and vapors, which can ignite at the hot, often glowing terminal point or the arc fault.
For this reason, electrical terminal points must be designed in such a way that loosening of the electrical connection is prevented. These designs can be structural measures in the area of the terminal, for example the design as a spring-loaded terminal instead of a screw terminal. In the case of terminals with operationally high current load, such as in the area of main lines, screw clamp connections are secured by multiple clamps and the obligation to tighten the individual screws with a certain torque with a torque wrench . Measures on the cable side are the use of cable lugs , in the case of strands, the use of ferrules . Litz wires that are clamped in screw terminals must never be tinned with tin solder , as tin solder has flowing properties and the screw connection itself is mechanically relaxed and thus loosens.
Since the electrical current through a loose terminal point usually remains below the permissible tripping values, the regular electrical consumer is in the circuit at all times, there is no tripping of overcurrent protection devices . The smoldering fire can therefore go unnoticed for a long time with only minor effects. Cable fires that are forming can sometimes be detected by installing additional arc fault protection devices (AFDDs) and further damage such as fires to the building can be prevented by automatically switching off the system.
Cable break
A cable break occurs as a result of mechanical stress or wear and tear on an electrical cable, for example when a cable falls below the minimum permissible bending radius. A cable break is characterized by a locally limited reduction in the conductor cross-section. In this area, there are high current densities and, as a result, arcing faults, with the associated heat load. The thermal effects are identical to those of loose clamp connections. Arc fault protection devices (AFDDs) can also be used as a possible remedy in the event of a cable break.
overload
The thermal overload of an electrical line is characterized by the fact that the cross-section of a line is fundamentally too small and an impermissibly high level of heating or heat loss occurs over the entire length of the line. With properly executed electrical installations, this cause can be ruled out, since in the event of an overload, the protective device such as the miniature circuit breaker or fuse is triggered and the power supply is interrupted. Conductor cross-sections and their permissible continuous current load depend, among other things, on the conductor material used, conductor cross-section, insulation material and ambient temperature and are specified in standards.
In special cases where a sufficiently dimensioned conductor cross-section is not possible for technical reasons, the heat loss must be continuously removed by additional cooling devices on the line. Examples of this are the high- voltage cables and oil cables used in the high-voltage area with additional external water cooling.
The main difference to defective clamp connections or cable breaks is the thermal overload of a line, which is always distributed over the entire length of the line.
External influences
External influences can also lead to a cable fire, for example due to a direct lightning strike or installation errors in the transformer station , which can lead to overvoltage and cable fires. In these cases, suitable measures such as lightning protection devices and overvoltage protection devices are necessary.
recognition
Inadmissibly high temperatures in electrical installations can be detected with thermal imaging cameras before cable fires occur . Areas with high temperatures are easily recognizable in the thermal image, so that faults can be rectified before damage occurs. Since thermal imaging cameras determine the surface temperatures without contact, periodic monitoring measurements can also be carried out in high-voltage systems without interrupting the power supply.
Significant cable fire
In the early morning of April 7, 2017, a cable fire broke out on the east side of Linz Central Station , which shut down the signal box and thus the most important railway line in Austria as well as three other routes for about 12 hours and left tens of thousands of passengers stuck in trains.
Individual evidence
- ^ A b John J. Shea: Conditions for Series Arcing Phenomena in PVC Wiring. (No longer available online.) 2005, archived from the original on January 30, 2013 ; Retrieved January 18, 2014 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ^ Understanding the Combination AFCI Expansion. (PDF; 3.5 MB) Retrieved May 25, 2012 .
- ↑ Continuous current carrying capacity for electrical cables according to DIN 48201
- ↑ Central station is at a standstill: tens of thousands are waiting orf.at, April 7, 2017, accessed April 7, 2017.
- ^ Ö1-Radio, ORF, Mittagsjournal April 7, 2017, 12 noon 25th - ÖBB spokesman announces resumption of train traffic "in the late afternoon".
