What happened? According to direct witnesses of the fire and subsequent investigation, the sequence of events was as follows with high probability. The timber store is a 20-years old building where rodents are not rarity. Wiring became easily accessible for them in one part of the building. Systematic damaging the cables led to cable making thinner with resulting rise in current density in the place of damage. This increased the temperature in the damaged part of the cable, insulation around the damage began to carbonize, and electric arcs began to appear. In fact, ideal conditions were created for fire – sufficient temperature and carbonized plastic that was able to feed the fire. And so the store was set on fire. Temperature detectors and fire-extinguishing system could not prevent the fire.
The primary circuit breaker and the fuse could not switch off, because they are not able to react to such fault. The current in the circuit, which would have activated the circuit breaker or the fuse remained on the standard operating level because of absence of short circuit or overload. Neither the circuit breaker nor the fuse can react on increase in current density (with subsequent fault arcs) in only part of the supply cable. They are not designed for it. In fact, there was not protection. Until now! No the solution exists.
This new solution is the arc fault detection device - AFDD. By contrast to circuit breakers, fuses or residual current circuit breakers, AFDD can detect and switch off such currents and thus prevent fire! The AFDD fills in the safety gap existing in the wiring till now. Should AFDD be installed, the above described event would not have occurred or its probability would have been eliminated substantially.
The shown case of the company TRXe a.s. is fictitious to demonstrate the cause and consequences of such fault simply. However, the mentioned fault or similar mechanisms of cable damage are absolutely realistic.
How the AFDD works?
Generally, the AFDD (arc fault detection device) is a device designed to mitigate the effect of fault arc by circuit disconnection in case of detection of the fault arc. Fault arc is dangerous accidental arc between conductors – light electrical discharge across the insulation environment, usually accompanied with partial evaporation of conductor (electrode) material. Such effect in the conductor may cause inflammation of insulation and subsequent setting of a building on fire. And thus the AFDD eliminates such effects. Examples of causes of fault arcs (and thus possible fire in wiring) are shown in Table 1.
The cases of conductor damage may on principle result in 3 types of fault arcs:
- 1) Series fault arcs (L) are mostly caused by conductor break or loss of contact in series with a load. In these cases, current is smaller than operating current, and the circuit breakers and residual current circuit breakers are not able to detect and switch off the fault. And thus the arc fault detection devices are designed to detect the fault arcs, and to interrupt the circuit before the energy in the point of fault reaches values leading to fire. The event described at the beginning of this article corresponds to this type of fault.
- 2) Parallel fault arcs (L-N) are caused by electric arc due to damaged insulation, which enabled connection of two conductors. Current value is determined by circuit impedance. The circuit is switched off depending on the nominal current of its protection device (e.g. a circuit breaker). If circuit impedance is too high and the breaking current of the protective device is not reached, the breaking may not occur. The AFDD will switch off the fault arc current higher than 2.5 A thus providing reliable protection.
- 3) Parallel fault arcs (L-PE). Phase to earth (PE) fault arcs detected and switched off reliably by residual current circuit breakers. Residual current circuit breakers with IΔn 300 mA ensure protection against fire for many years. The AFDD also detect these types of fault arcs thus providing protection in places where the residual current circuit breakers are not installed. In some cases, overcurrent protection devices (circuit breaker, fuse) do not provide any protection, because the impedance of the faulty circuit can be too high
For overview, see Table 2. Note that in case of parallel fault AFDD does not replace protective features of circuit breakers or residual current circuit breakers, but completes them!
Standards
Standardization bodies paid attention to the advantages of AFDD, in particularly in the environments with higher risk of fire, where fire can propagate easily and where there is a higher hazard to persons or valuable goods.The international standard IEC 60364-4-42:2010+A1:2014 and European standard HD 60364-4-42:2011+A1:2015 highly recommend using arc fault detection devices and consider them as the state-of-the-art devices in this area.
Germany in the national standard DIN VDE 0100-420:2016-02+Amendment A1 has introduced mandatory use of AFDD beginning from 18. 12. 2017. Slovakia in its standard STN 33 2000-4-42/A1 (identical with IEC 60364-42:2010/A1:2014) issued in December 2015 in clause 421.7 in national note says that … let us quote: „The use of the arc fault detection devices (AFDD) in the Slovak Republic within the meaning of this standard will be mandatory after expiration of transient period of three years from the date of issue of this standard...“.
With regard to positive benefits, the installation of AFDD will become mandatory in a rising number of countries and in a rising number of standards where this protection will be mentioned.
OEZ ARC FAULT DETECTION DEVICES
Previous paragraphs concerned general characteristics of arc fault protection device and its key advantage i.e. protection against fire, which filled the functional gap of the existing protection devices - circuit breakers, residual current circuit breakers and fuses - and which moved the installation safety to a higher level. But what a specific device of the OEZ range can be used? And what are its advantages… i.e. beside the key one?
Modular design
OEZ offers AFDD designed as kits. In case of OEZ solution the user assembles the arc fault protection device. It is easy. You can assemble AFDD from
- - arc fault detection unit (AFD unit) and
- - LTS/LTK circuit breaker (MCB) or residual current circuit breaker with overcurrent protection OLE/OLI (RCBO)
The assembly will ensure mechanical and electrical connection of the both parts. By connection of the arc fault detection unit with the circuit breaker or residual current circuit breaker with overcurrent protection, you will create a functional unit – AFDD. Figure 1 explains it better than several sentences.
Figure 1: AFDD assembly. ARC 16 A arc fault detection unit and circuit breaker LTS, 16 A, char. B. The ARC arc fault detection unit is offered with In 16 A and 40 A. On sides, the ARC is equipped with a coding pins protruding from the base to disable, for example, installation of a circuit breaker 32 A on ARC with In 16 A in assembly.
It makes it possible to create hundreds of various versions of AFDD (according to rated or residual current, characteristics, …), with minimum inventory stocks. In case of assembly with the residual current circuit breaker with overcurrent protection OLI/OLE, the customer gets full protection - against fire, overcurrent, and residual currents.
High resistance to unintentional switching-off
AFDD must not only provide reliable protection against fire caused by electricity, but also react only in case of real fault. For OEZ AFDD it means that it must reliably select between fault arcs, for which switching-off is required within the stated limits, and operating arcs (or current behaviour) of electric loads, during which switching-off should not occur. AFDD should not trip in case of any of these signals arising in service or in case of fault arc of the adjacent circuit:
- - arcing on the contacts of a switch, in switching of lights; arc of brush motors; contact of older relays
- - high-frequency (HF) noise - this is about the noise from dimmers, computer LAN cards to low-voltage sockets (power line) etc .
Test for reliable operation
The OEZ AFDD is equipped with a self test, which is started automatically every 15 hours to test the circuits and the detection algorithm. The arc fault detection unit can be also tested manually, by pressing the test button on the front panel of the device (Figure 2).
Clear information about switch-off reasons
LED status indicator on the front panel of the device (which also performs the test button function) provides simple and clear information about switch-off reasons.
Overvoltage protection of loads
The OEZ AFDD is equipped with the overvoltage release which in case of a longer overvoltage, which is higher than 275 V, disconnects the circuit. Thus it protects the appliances (TV, PC or similar) in the installation from destruction.
Easy maintenance
The OEZ AFDD meets EN 62606. So AFDD can be controlled by laymen and does not require maintenance.
Accessories
The accessories are mounted on the LTS/LTK circuit breakers and OLI/OLE residual current circuit breakers with overcurrent protection
- - PS-LT/SS-LT auxiliary and signal switches
- - SV-LT/SS-LT shunt trips and undervoltage releases
- - OD-LT locking inserts
WHERE AND HOW TO USE THE AFDD?
We should not forget one important thing – how and where to use the AFDD. AFDD must be installed at the beginning of the circuit to be protected. If it is possible, we should use one AFDD on one outlet, so that the user can use the benefits resulting thereof:
- - the number of unwillingly disconnected loads and conductors is minimized
- - it is easier to locate the fault
- - unwanted switching-off is reduced due to smaller interference overlap
We recommend protecting above all socket and lighting installations by AFDD. In the area of housing develop- ment (family houses, nursing homes) these are above all the installations of bedrooms and sitting rooms and installations with high current consumption - circuits of dish-washers, dryers, washing machines. For other areas and buildings see Table 3.
