How arc fault detection devices minimise electrical fire threats
This article discusses a break-through in electrical protection and identifies critical application areas where as arc fault detection devices enhance safety.
An arc is defined as a discharge of live electricity through insulation material accompanied by a partial volatilisation of the electrode materials. Under normal circumstances, one cathode and one anode are separated by a thin space of air through which an electric arc is formed. The temperature at the center of the arc can vary between 5000°C and 15 000°C. When highly ionised and pressurised gas is produced in the area of the arc, the result is a release of hot gas and metal projectiles in all directions within the confined area of the arc.
Triggers that lead to electric arcs
Overvoltage
An electric arc can be considered a high current discharge. It is possible to create an arc between two electrodes by initiating a low-current discharge, and by gradually fueling its growth. Gases in general are good insulators. The difference in potential alone between the electrodes does not allow the passage of current.
However, when the applied voltage exceeds a critical value called breakdown voltage, a discharge begins between the electrodes. If the source does not limit the current, this discharge degenerates into an irreversible arc. This is the case with damaged cable insulation. Under the effect of this breakdown voltage, conditions will favour the creation of an electric arc in the phase and in the neutral (see Fig. 1).
Fig. 1: Arc fault cycle begins as low current discharge and grow in intensity over time.
Improper contact
If two contacts where a current normally travels are separated, conduction is maintained by an electric discharge which initiates within the inter-electrode space. When the two electrodes are brought into contact, they do not occupy the whole of the facing surfaces.
Only the rough edges and irregularities on the surface act as support zones. At the moment of the separation of the contacts, the entire current (I) passes from one electrode to the other across a tiny surface, which is always less than 1 mm² thick. The resistance (R) through the contact then increases, and the dissipated energy (R*I²) leads to a considerable increase in the local temperature. The boiling point of the metal is then reached and a metallic molten bridge is formed between the two contacts.
The elongation of the bridge by the separation of the contacts causes it to rupture, which ejects molten metal in the form of microdroplets at a rate of 100 to 300 ms. An arc of metallic vapours is created. This is the case with bad contacts usually found in damaged switches and power strips (see Fig. 3).
The science behind arc formation
- Two types of arc fault-related event can occur and lead to a fire:
Formation of a series arc: When a cable is damaged or an electrical connection is loose, a localised hot spot can develop which starts the carbonisation process of insulating materials in the proximity of the conductor (see Fig. 3). Since carbon is a conductive material it allows electrical current to pass through it.
As carbon is deposited, the electrical currents flowing through it generate electric arcs to facilitate their path. Since each arc amplifies the carbonisation of the insulating material, a chain reaction occurs to the point where enough carbon is built up for an arc to spark a spontaneous fire. A series arc fault results from an arc between two parts of the same conductor.
- Formation of a parallel arc: Once the insulation between two live conductors (copper being the most common of conductors in electrical cables) has become damaged, a significant current is able to flow between the two conductors (see Fig. 4). However, it is too weak to be considered a short-circuit by the circuit-breaker. RCDs cannot detect the anomaly unless the current goes to earth.
Fig. 2: Defective cabling can lead to fire.
While flowing through the insulation materials, these leakage currents optimise their paths by generating arcs that gradually transform the insulating material into carbon. The carbonised insulation then amplifies the leakage current between the two conductors and a chain reaction is produced that amplifies the quantity of arc current and carbon until the first flame appears (i.e., the carbon is lit by one of the arcs as in Fig. 5). A parallel arc fault is produced as a result of an arc between two different conductors.
Much analysis has been performed regarding the nature of these faults and their detection. This analysis focuses on the deformation of electrical current signals (wave forms) to ensure that detection is accurate and that false alarms are avoided.
How arc fault detection devices work
AFDDs constantly monitor and analyse patterns and the high frequency component in electrical current and voltage waveforms. They are on the look-out for the random, non-predictable yet persistent patterns of waveform that denote potentially dangerous arcs.
When the AFDD senses a potentially dangerous wave pattern, it trips, so isolating the faulty circuit. An AFDD can work in conjunction with a circuit-breaker or residual-current circuit-breaker with overcurrent protection (RCBO). It may also incorporate its own switching function.
Fig. 3: Example of a series arc.
AFDDs react very fast to the slightest change in wave patterns. Speed is of the essence as an electrical arc can degrade in a flash (literally), igniting any nearby inflammable material and causing a fire.
Arc fault detection devices are extremely sensitive and designed to sense and respond only to potentially dangerous arcs. They use a specific algorithm to distinguish between dangerous and working arcs – i.e. the harmless sparks you see when you flick a switch or pull a plug (see Fig. 6).
AFDDs cross reference information involving several different electrical parameters to ensure that arc fault detection devices are only activated by the appearance of dangerous arcs.
The various parameters that are analysed include:
- The current of the arc (a series arc is dangerous as soon as its value equals or exceeds 2,5 A).
- The duration of the appearance of the arc (very short durations, for example, are characteristic of the normal operation of a switch).
- The irregularity of the arc (the arcs of motors, for example, are fairly regular and, as such, should not be considered dangerous).
- The presence of disturbances at varying levels of high frequencies is characteristic of the passage of a current through heterogeneous materials (such as cable insulation).
Examples of electric arcs that correspond to normal functioning include arcs created by switches, contactors, impulse switches and other control devices when contacts are opened. Arcs created by motors of the different electrical loads connected to a circuit (like portable electrical tools, or vacuum cleaner motors) are also normal.
Why the new demand for AFDD?
Until recently, it was not technologically feasible to build arc fault protection devices at a reasonable cost and in an acceptable size format. But now, the ability to miniaturise electronic components and the means to digitise such components makes the affordability and reliability of such devices a reality.
Today’s AFDD designs are comparable in size to miniature circuit-breakers (although each has a separate, different function). Innovations in digitisation now permit AFDDs to perform extensive data analysis in real time. That speed is critical to providing an accurate assessment of the environmental conditions.
Fig. 4: Example of a parallel arc fault.
Another driver of AFDDs involves the recognition by government and safety authorities that faulty electrical systems pose a serious threat of fire. Worldwide, electrical system mishaps are the cause of 25% of all fires – with 80% of building fires occurring in dwellings.
Maturation of standards
In 2008, the IEC decided to initiate standardisation work for arc fault detection devices, leading to publication of AFDD product standard IEC 62606 General requirements for arc fault detection devices. To ensure safe behavior, all AFDDs must comply to the IEC 62606 standard since 2013.
A recent amendment of installation standard IEC 60364-4-42 was published in 2014 for countries applying IEC standards (Europe and other parts of the world).
IEC 60364 makes the following recommendations surrounding the installation and application environments of AFDDs in residential and commercial buildings:
- In locations with sleeping accommodations (e.g., hotels, nursing homes and domestic bedrooms).
- In locations with risks of fire due to high quantities of flammable materials (e.g. barns, wood-working shops, stores of combustible materials).
- In locations with combustible constructional materials (e.g. wooden buildings).
- In fire propagating structures (e.g. high-rise buildings).
- In locations where irreplaceable goods are housed (e.g. museums).
It is recommended that AFDDs be installed at the place of origin of the low-voltage final circuit to be protected (i.e. switchboard of an electrical installation).
At this early stage, national standards committees are being given the latitude of deciding if the use of AFDDs is to be made a requirement or a recommendation. Commercial buildings are becoming more common as places where AFDDs are being installed.
Fig. 5: Damaged cables provide fertile ground for arc fault-induced carbon build-up.
Although standards play the vital role of enhancing safety, they are often focused on new installations only, which represent less than 1% of the total number of buildings. Studies show that safety levels in older buildings are far below current standards. AFDDs represent a way to raise safety levels in existing buildings.
Installation hints and tips
During renovation work, safety considerations come into play that will influence the nature of the electrical installation. In addition to upgrading the switchboard with modern circuit-breakers and RCDs (for overcurrent and electric shock protection), the installation of an AFDD (where a substantial arc fault risk is known to exist) is highly recommended.
More specifically, installers should be alerted to the following:
- Protruding cables (risk of knocks).
- Outside cables (greater risk of deterioration).
- Unprotected cables in secluded areas (like storage rooms).
- Shoddy or primitive workmanship.
- Circuits in buildings with wooden structures.
- Ageing, deteriorating wiring or wiring for which the connection boxes are inaccessible.
Manufacturers of AFDDs can assist installers, electrical contractors and electricians with the design and installation of AFDD solutions.
Conclusion
Building modernisation initiatives should consider the importance of low-voltage electrical systems’ safety. Conditions such as overcurrent, electric shock, over-voltage and arc faults are avoidable if proper attention is given to installing up-to-date power protection technologies. Benefits include lower risk of injury to humans, lower risk of damage to building assets and peace of mind for stakeholders who own and manage buildings.
Fig. 6: Anomalies in electric currents that could indicate the presence of potentially dangerous arc faults.
The challenges of electrical safety are real, but the strategies and techniques to address these challenges are tested and proven. New AFDD devices act as a supplement to the overall safety equation by addressing a safety gap covered neither by circuit-breakers nor by residual current devices (RCDs). However, to date, very few buildings have launched initiatives to better protect themselves from arc faults and the fires they can trigger.
Prudent steps for initiating the process of addressing the arc fault issue include:
- Identify a trusted partner and advisor who is familiar with the design and function of these devices.
- If designing a new construction site, consult with experts who can analyse both electrical system safety requirements and costs, and who can validate the design.
- For renovation projects, audit existing sites to determine the level of deterioration in the core wiring systems. Based on audit findings, propose a budget and assemble a project team for implementation of the modernisation and safety plan.
Contact Prisca Mashanda, Schneider Electric, Tel 082 554 6829, prisca.mashanda@se.com
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