Frictional sparks

S (NxOy) indicates the quantity of NxOy-molecules produced in the arcs. 

The influence of oxidation processes on particle temperature is demonstrated by the fact that hot particles work preferably as an ignition source in the poor mixture region of gas-air or vapour-air mixtures, i.e. in a concentration range below the stoichiometric mixture. In the rich mixture region above the stoichiometric mixture, particles are not very efficient as ignition sources, due to the lack of oxygen. 

To minimize the risk of ignition by particles generated by frictional effects or impacts, all modem standards for explosion protected electrical equipment restrict the content of aluminium, magnesium and titanium as enclosure materials to a level considered as totally excluding the risk of ignition (e.g. clause 8 in IEG 60079-0 and EN 50014). 

A similar situation exists for the particles generated by fault current arcing in flameproof enclosures after passing the flameproof joints (see Section 6.8). 

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Tramp materials introduced into manway, leading to impact or frictional spark, igniting vapors. 

The general reactivity of the sulfide depends markedly on the physical form, and differences of a factor of 10 may be involved. It is ignitable by friction, sparks or flames, and ignites in dry air if heated close to the m.p., 275 -280°C. The dust (200 mesh) forms explosive mixtures in air above a concentration of 0.5% w/v [1], and maximum explosion pressures of 4.35 bar, with maximum rate of rise exceeding 680 bar/s have been determined [2], The dust can acquire sufficient static electricity from movement for ignition to occur [3],... 

Hard objects coming into shearing contact with one another can cause friction sparks, as in the cases of metal striking metal, metal striking stone, or stone striking stone. Friction sparks are particles of burning material, initially heated by the mechanical energy of friction and impact that have been sheared off as a result of contact. Sparks from hand tools... 

Explosives are generally detonated with the assistance of initiating devices (initiators) that are activated by external stimulation sources (friction, spark, or flame), and may include (1) primers, (2) detonators, (3) electric detonators, (4) safety fuses, and (5) detonating cord. 

Sensitivity to heat, impact, friction, spark and shock. 

Sensitivity of Explosives to Heat, Impact, Friction, Spark and Shock... 

It seems reasonable to expect that expls that are sensitive (eg, sensitive to impact, friction, sparks, etc) will also be sensitive to shock. Generally this expectation is borne out by experience, at least in a qualitative way. Under comparable conditions PETN is certainly more shock-sensitive than TNT. and so on. An example of the parallelism in impact and shock sensitivity of four common expls is shown in the tabulation below. The comparisons are obviously qualitative in that the expls are ranked in descending order of sensitivity. It must be emphasized that reversals in sensitivity ranking can occur if comparisons are made of sensitivity data obtained under non-comparable test conditions... 

Constructional requirements for enclosures cover mechanical aspects to achieve a certain robustness against mechanical damage as well as the avoidance of frictional sparks, and an electrostatic safety referring to enclosures of plastic material only. 

In order to avoid frictional sparks (see Section 1.2.4), Group I enclosures shall not contain, by mass... 

Steel-friction sparks Steel-grinding sparks I I Aluminum/rust-impact sparks... 

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