"Nonincendive" simply means that the equipment does not cause an ignition under normal operating conditions. By this definition, a fair amount of "standard" electrical equipment will qualify as nonincendive, without needing any special design precautions.
This naturally leads to an increased risk of ignition hazard as compared to the other protection methods. In recognition of this, non-incendive equipment is only allowed in the lower-risk Division 2 and Zone 2 areas. The operating premise is that it is very unlikely for an explosive atmosphere and an equipment malfunction to occur at the same time.
Although the nonincendive method operates according to the same general principles as intrinsically safe equipment, there are vast differences between the two methods. Non-incendive equipment has a relatively high risk of ignition, and is only allowed in areas where the risk of an explove atmosphere is very low. Conversely, intrinsically safe equipment is allowed to operate in areas where an explosive atmosphere is continuously present - which means the risk of the equipment causing an ignition must be extraordinarily low.
Recent changes to the harmonized standards have placed "intrinsically safe" equipment (types "ia" and "ib") in the same standard as "non-incendive" equipment (type "ic"). Despite this, the requirements for these types of protection are quite different. Consequently, to help distinguish between the relative difficulty, some still refer to type "ic" intrinsically safe equipment as "non-incendive", in recognition of the fact that type "ic" is quite different from types "ia" and "ib".
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