STATIC PRESSURE
The static pressure within an electrical cabinet is basically a function of the internal volume, this can be expressed by the Ideal Gas Law.
For the purpose of calculations, it is convenient to place the ideal gas law in the form:
where the subscripts i and f refer to the initial and final states of some process. If the temperature is constrained to be constant, this becomes:
which is referred to as Boyle's Law.
If the pressure is constant, then the ideal gas law takes the form
which has been historically called Charles' Law. It is appropriate for experiments performed in the presence of a constant atmospheric pressure.
All the possible states of an ideal gas can be represented by a PvT surface as illustrated below. The behavior when any one of the three state variables is held constant is also shown.
Fig. 1.10
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As can be seen from the above example, static pressure testing only represents one of the effects of an arc, by combining all three test variables represented in this section we can get an accurate representation of the optic strength under a pVT effect which includes a thermoacoutic wavefront.
The simple relationship between pressure and volume can be derived as;
Static pressure increases when volume decreases.
It is important to note that Volume is NOT equivalent to Area and as such the general Equation P=F/A does not apply.
During an electrical arc-flash event the static pressure within an enclosure increases until either; (a) A pre-designed pressure relief system, such as a vent, operates or (b) Physical disintegration of the system causes pressure relief. Fig 1.12 shows an example of a switchgear pressure relief system various iterations of this design type are now incorporated into what is know as “Arc-Resistant” or “Arc-Safe” switchgear.
Fig. 1.12 |
The pressure relief system operates as a result of the Thermoacoustic wave – discussed in the next section – NOT due to the static pressure build-up - as a result, the internal static pressure reduces.
Fig. 1.13 denotes the pressure build-up and release as a function of time inside an Arc-Resistant piece of switchgear. The maximum pressure measured was approximately 19psi.
Fig. 1.13 – Dynamic Representation of Arc-Flash event |
Fig 1.13 clearly shows the increase in pressure associated with an Arc-Flash, and the resultant drop in pressure due to the operation of the relief system.
In order to correctly test an IR window optic for its ability to withstand the static pressures associated with an Arc-Flash event, a Hawk HydroTest must be performed.
As the maximum pressure seen in an arc-flash even is 19psi a 150% increase equates to a minimum static pressure requirement for an IR Sightglass optic of 48psi.
The Hawk IR Sightglass, C-Range have completed a series of successful HydroTests proving their resistance to static pressure far in excess of that seen during an arc-fault event.
DYNAMIC PRESSURE (Thermoacoustic Wave)
Arguably the most destructive force seen during an Arc-flash event is the dynamic pressure increase, known as the Thermoacoustic Wave. The Thermoacoustic Wave is a function of the short-circuit current and is present regardless of the equipment volume.
This shock wave can create impulse sound levels well beyond OSHA's allowable limits. Forces from the pressure wave can rupture ear drums, collapse lungs or cause fatal injuries.
The Thermoacoustic wave is used to operate pressure relief systems built into Arc-Resistant switchgear, which ultimately reduces the internal pressure in the equipment as discussed in the static pressure section.
Fig. 1.14 Pressure vents after an Arc-Flash event. |
Clearly, a static pressure of 19psi will not result in such a ferocious operation. The buckling shown in Fig. 1.15 is the direct result of the shock wave caused under a 20kA for 0.1s Arc-Flash.
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Fig. 1.15 Buckling caused by 20kA Arc-Flash
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The thermoacoustic wave energy is a function of the short circuit current. Each impact of the wave within the cubicle causes an increase in velocity, this effect is known as “pressure piling”. An IR Sightglass must be tested to withstand pressure piling effects of an arc-fault event by way of thermoacoustic modelling.
If an IR window optic has not been arc-tested, it cannot be used as part of an arc-fault energy reduction programme in accordance with such standards as NFPA70E.
