| Rigorous standards for hazardous location equipment have been set. Nationally
recognized testing laboratories conduct actual explosion tests under laboratory
conditions. For each Class I enclosure, they
experiment with different mixtures of gas and air . . . from very lean mixtures (a small
percentage of gas) to very rich mixtures (a high percentage of gas) until they find the
one that creates the greatest explosion pressure.
To pass inspection, the equipment must not only prevent the ignition of the surrounding
atmosphere, but also be able to withstand a hydrostatic test where oil is pumped into the
enclosure at high pressure to test the limits of its strength. The device will not pass
unless it can resist rupture at four times the maximum pressure found in the explosion
tests.
For example, if explosion testing shows a maximum pressure of 250 pounds per square inch
(psi) for a junction box, the box must be able to withstand 1,000 psi of hydrostatic
pressure - FOUR TIMES the maximum anticipated pressure of 250 psi. |
