Introduction.
Timely response to the emergence and development of fire hazardous conditions in the monograph [1] was proposed to be carried out taking into account four scenarios, including the consequences of contact failure, short circuit, contact of electric heating devices with flammable materials, as well as equipment overheating.
The purpose of this work was to develop this method to substantiate the principles of monitoring electrical and non-electrical indicators. The chosen approach is quite understandable, but in practice, it encounters a certain crisis in electrical engineering due to the lack of an unambiguous assessment of the fire hazard of arc breakdown in comparison with resistive heating. The presence of disagreements is evident from a comparison of normative documents.
On the other hand, the National Electrical Code NFPA 70 requires that branch circuits supplying outlets or devices in kitchens, family rooms, dining rooms, living rooms, libraries, pantries, bedrooms, sunrooms, recreation rooms, bathrooms, hallways, laundry rooms, and similar areas or areas were protected from arc flash.
- AFCI – arc-fault circuit interrupter,
- AFDD – arc fault detection device (arc fault detection device),
- AFDP – arc flash protection device,
- UZDZ – arc fault protection device,
- UZIS – spark protection device.
The listed arc switches can be composite (combined with circuit breakers, differential current devices or voltage relays) and contain the following basic components:
- AFD – arc fault detection unit (arc fault detection unit),
- DADP – arc breakdown detection unit,
- ADF – arc fault detection device.
The effectiveness of ultrasonic sensors has still not been proven
It should be noted that serial models of the listed devices have been produced for more than 20 years, but there is still no objective data confirming the effectiveness of the method of monitoring and the protective action of arcswitches. To prove the need for these products, suppliers limit themselves to comics, cartoons and distortion of statistics.
Meanwhile, it must be emphasized that official data [2] do not contain any information on the number of fires with an ignition source in the form of an arc breakdown. The closest information concerns only the types of products (devices, materials) on which (from which) the fire occurred. For example, in 2021, 4,701 fires [2] were attributed to electric heating devices (stoves, irons, etc.), for which the examination often shows that the electrical device itself was fully operational, and the fire occurred due to contact with flammable substances and materials, i.e. due to violation of operating rules.
Even for cable products (40,232 fires in 2021), there is no division into cases in which there were short circuits, the current turned out to be greater than the calculated one, or some other reason preceded it. Moreover, such statistics do not reveal in any way how the ignition process occurs.
Note that there is more useful information on the issue of effectiveness in social networks than in the scientific literature. This is because testing arc switches is not technically difficult, and at least not quantitative, but qualitative assessment is quite possible without means of objective control.
The origins of knowledge about arc breakdown suggest that the fire hazard of the process is influenced by self-regulation conditions, including the material and relative position of the conductors, and variations are possible from the axial diagram of V.V. Petrov to parallel P.N. Yablochkova. If the arc breakdown does not die out but is self-regulated, a powerful and long-lasting ignition source arises. On the contrary, the lack of self-regulation excludes arc breakdown from the list of dangerous fire factors.
How to detect sparking using simple methods
However, everyone knows that poor contact is noticeable even without measuring the flicker of lighting fixtures, and in the “International Patent Classification” you can find subclass H02H 3/28 of protection circuits that carry out automatic shutdown and respond to the difference between voltages or currents, for example, on opposite ends of the line, at the input and output of the device. Here it turns out that the identification of short circuits or breaks, which are always accompanied by an arc breakdown, is significantly simplified.
Thus, Sergei Ivanovich Kostruba in 1986 proposed a control principle different from previously known ones. It turns out that to respond to a break or increase in transient resistance, it is enough to ensure a balance of the differential sensor currents only in the absence of defects in the conductors and connections in the circuit.
The use of the named principle of constructing control circuits has led to the emergence of a number of solutions, including differential dipoles and blocks that respond to damage:
– a differential dipole, by definition, is a differential current device in which two poles are connected in opposite directions [11];
– the differential unit consists of at least one differential current device and auxiliary contacts or devices[12].
It is important that differential blocks and dipoles are obtained by combining serial devices and parts, and their sensitivity and performance are orders of magnitude higher than those of arc switches.
2. Research methods and principles
Physical modeling of fire hazard conditions.
Video recording of the arc breakdown and recording of electrical parameters were carried out as shown in Fig. 1. The applied model made it possible to connect conductors and disconnect connections, to use copper and aluminium conductors of different sections and configurations, including together with different terminal parts.
In total, more than a thousand tests were performed. The first tests were performed to verify the functionality of arc circuit breakers from five manufacturers (Eaton, EKF, IEK, Ecolight, and EKM). In this case, instead of metal conductors, graphite welding electrodes with a diameter of 8 mm were installed, and breakdown and self-regulation of the arc were obtained using a Yablochkov candle.
For example in Fig. 4 combines a typical still frame of a video recording, a measurement diagram and an oscillogram of an arc breakdown. The resulting scans of voltage drops U1, and U2 in the current range from 2.5 to 32 A coincided in shape with the oscillogram given in the standard (see Fig. 37 GOST IEC 62606-2016). For the selected models, the time-current characteristics met the requirements for the devices (see Table 1 of GOST IEC 62606-2016). Thus the arc switches were tested and could be used in the following tests.
Short circuit and arcing in real conditions
With a limited load (current up to 32 A) for stationary metal conductors or connection parts used in electrical installations, it was not possible to obtain a long arc, and there were no shutdowns of the arc switches
In earlier publications, the observed breakdown was defined as a single spark, but since the transient process usually spanned several periods on the oscillogram, it was decided to use the term single spark.
It can be noted that the difference from the arc of graphite electrodes lies in the clarity of switching (voltages and currents arise and disappear almost instantly), and there is almost no “freezing” of electrical indicators in the average (non-zero) position. The important thing is that repeating the test many times led to heating, but did not change the shape of the electrical signal. Standard contacts of the poles of switches, relays and other devices give the same oscillogram. Therefore, to avoid false alarms, arc switches are forced to ignore ordinary sparking.
Such an arc, which burns on a graphite electrode, never appears between copper or aluminium conductors in real electrical installations. Therefore, testing devices using a graphite electrode generator is essentially misleading.
Resistance heating is the main cause of fire
The current in a closed circuit operates for a long time and therefore its thermal effect is several orders of magnitude higher than that from ordinary sparking. All tests showed this.
There is no doubt that the destructive ability and fire hazard of resistive heating dominates. To simulate this, the experiments deliberately used circuit breakers that did not provide shutdown when the permissible current load was exceeded. Therefore, resistive heating could result in melting of the conductor. True, in some cases the conductor itself played the role of a fuse, i.e. the chain was broken and the fire-hazardous process stopped.
In other options, heating led to damage to the insulation of the connecting cable along its length, the formation of pyrolysis products and ignition when the conductor melted. Frame-by-frame viewing of video recordings showed that relatively long heating created conditions for the simultaneous ignition of the polymer and gases.
4. Conclusions
1. The fire hazardous operating mode of electrical installations should be considered as a sequence of interconnected stages, the common sign of danger of which is the thermal effect, with resistive heating having a dominant effect on the temperature increase.
2. Timely shutdown of electrical installations ensures the combination of monitoring electrical and non-electrical indicators, and the latter is best handled by fire automatics that control differential current devices [1]. Devices that respond to interference from an arc fault in accordance with GOST IEC 62606-2016 will turn off the power supply after ignition and will not prevent a fire.
3. If it is necessary to monitor the integrity of circuits, methods and devices for monitoring electrical indicators should be used not only at the input, but also at the output of electrical circuits , for example, differential dipoles, blocks [11], [12] and denominators [13], including those located directly in electrical receivers.
