F-line Automatic fire suppression system for the electric cabinets

 

Every year more than 40000 fires occur due to violations of the rules for the installation and operation of electrical equipment, and direct material damages exceed 4 billion rubles. By statistic number of fires this is the second reason of fires followed closely to the careless handling of fire. Significant damage is caused by fires of the electric boards and electrical cabinets.

 

Электрический шкаф после пожара
Pic. 1.
Electric cabinet after fire

 Burning of the electrical equipment and cable in the cabinet (Figure 1) result to disconnection of the electrical consumers for a long time. This in its turn results to significant shutdown material losses of the enterprise. Effective fire-fighting protection of the electrical cabinets is provided using the self-contained gas fire extinguishing systems. As explained by VNIIPO item 8 in Table A.4 of Annex A of the Code of Regulations SP5.13130.2009 should be interpreted as follows: "Electrical boards and electrical cabinets (including switchgears) located in the rooms of the functional fire hazard Class F1.11), a volume of which is less than 0.1 cu.m are to be protected with the automatic fire alarm system (AFAS), and a volume of which is 0.1 cu.m or more are to be protected with the automatic fire extinguishing system (AFES)".

1) The listed equipment is to be protected by the self-contained fire extinguishing installations.

According to the explanation in the Code of Regulations SP5.13130.2009, the self-contained fire extinguishing system is the "Fire extinguishing unit that automatically performs functions of fire detection and fire extinguishing independently of the external power supplies and control systems operation".

 

Selecting the type of the gas fire extinguishing agent (GFEA) by safety

(GFEA) by safety

 

Carbon dioxide CO2, Inergen, Halones 23, 125, 227ea and Fluoroketone FC-5-1-12 (Novec1230) are possible to be used as the gas extinguishing agents (GFEA) in the fire extinguishing systems according to requirements of SP5.13130. Since fire extinguishing of the electrical boards and electrical cabinets by means of the self-contained gas fire extinguishing systems is carried out in the presence of personnel, so when choosing the type of GFEA it is necessary to take into account the fire extinguishing capability, dielectric characteristics and human safety.

 

Carbon dioxide CO2 is still used in the fire extinguishing systems because of its low costs despite the danger of this gas for people. When extinguishing with CO2, a fire extinguishing concentration is of 35%, thereby eliminating oxygen and preventing burning. The concentration of CO2 hazardous to human life is 10%. When carbon dioxide CO2 is released in the room, the temperature in the protected volume decreases sharply, which at certain humidity can result to condensation, which is not permissible in case of extinguishing the live electrical equipment. Inergen contains 52% (vol.) of N2, 40% (vol.) of argon Ar and 8% (vol.) of carbon dioxide CO2. The normative volumetric fire extinguishing concentration for Inergen is 36.5%, and the maximum permissible concentration of GFEA, at which no harmful effects on human beings are observed, NOAEL, is 43% in working concentration. For Inergen it could be noted small difference between the working concentration and NOAEL, in addition, Inergen is stored in high pressure cylinders (250-300 bar), that increases potential hazards to both equipment and personnel. The explosion of the balloon with Inergen at the Museum of Natural Sciences in Turin resulted not only to destruction of the museum objects, but also to destruction of the floors. Only by good fortune there is no harm down because the explosion was occurred at 5 o'clock in the morning.

 

Halons, despite the common name and similar chemical formulas of substances, have different effect on a person. The most common Halon 125 has a NOAEL value of 7.5% with a fire extinguishing concentration of 9.8%, that is this GFEA is dangerous when fire extinguishing in a occupied room and it can also be dangerous when extinguishing fires in the large size cabinets. In 2008 the branch of FGU VNIIPO EMERCOM of Russia in St. Petersburg made tests with Halons 125, 227ea and 318 and investigated safety of GFEA for the laboratory mice and rats. When Halon 125 has been released in "fire extinguishing mode", no one mouse from the test group survived, the 60% of rats survived. Halon 227ea in the tests of the Federal State Institution VNIIPO EMERCOM of Russia showed the best results in mice: 100% of mice survived, but 50% of rats died. NOAEL value for Halon 227aa is 9%, which is not much higher than the extinguishing concentration of 7.2%. Additionally, Halon-125 has a global warming potential of 2.800, and Halon 227aa has a global warming potential of 2.900, they have been stored in the atmosphere for more than 30 years and have been prohibited for use in accordance with the Kyoto Protocol since 2017.

 

From the whole group of halons Halon 23 has the highest NOAEL ratio to the extinguishing concentration, the NOAEL value is 30%, and the fire extinguishing concentration is 14.6%. But Halon 23 cannot be used for extinguishing the electrical equipment, as in the conclusion of the Center for the Safety of Cultural Property there is the phrase: "... its direct contact with open stored items... made of copper and copper alloys… should be limited". In addition, Halon 23 has a large global warming potential of 11.700 and can be maintained in the atmosphere for about 270 years! In accordance with the Kyoto Protocol it is prohibited for use from January 1, 2015.

 

Fluoroketone FK-5-1-12 has a NOAEL of 10% and a fire extinguishing concentration of 4.2%. Thus, even with the use of an increasing factor K4 = 2.25, the concentration of GFEA in accordance with SP5.13130 the safety factor - the ratio of NOAEL to the calculated fire extinguishing concentration (FEC) remains greater than 1, in contrast to all other types of GFEA (Table 1). The safety of fluoroketone FK-5-1-12 was also confirmed experimentally on laboratory animals at the VNII of RR Hygiene, all animals survived the tests. In addition, the fluoroketone FK-5-1-12 has a minimal global warming potential equal to 1, it is resolved in the atmosphere under the UV influence for 3 to 5 days, depending on solar activity. Of course, unlike Freon, FK-5-1-12 is not subject to restrictions in accordance with the Kyoto Protocol.