Many of us got our first glimpse of an intrinsically safe circuit with the addition of Article 504 to the 1990 National Electrical Code. We thought this might be something new. This is the first myth. No, intrinsically safe wiring is not new. It has been around since the 1930's. It all started because of a mine accident in England in the early l900's. Work began on intrinsically safe systems in England in 1911, and by the 1940's certified intrinsically safe systems were available for the industrial market.

This is a common sense statement that is often made by people that do not know the difference between a Class 2 and an intrinsically safe circuit. After all, if an intrinsically safe circuit will not release enough energy to ignite a flammable mixture of gasoline, surely it will not cause a shock or be an ignition source in a nonhazardous location. But, the fact is, AN INTRINSICALLY SAFE CIRCUIT IS NOT NECESSARILY A CLASS 2 CIRCUIT. The two circuits are tested to different standards. Basically, an intrinsically safe circuit is suppose to not cause an ignition of a flammable mixture under any two fault conditions occurring simultaneously while a class 2 circuit is not supposed to shock anyone or be a source of ignition in a nonhazardous location when any number of faults occur. An intrinsically safe circuit is not a Class 2 circuit unless the power supply is durably marked Class 2 as required by the NEC. This does not mean that someone can just mark the power supply Class 2; it means that the power supply must be tested and Listed as a class 2 power supply.

If the intrinsically safe circuit is also a class 2 circuit then the class 2 wiring methods can be used. But if the intriinsically safe circuit is not a Class 2 circuit - Watch Out! This means that the intrinsically safe circuit is more than likely a Class 1 circuit, and Class 1 circuits must be wired like a normal power circuit using the wiring methods of Chapter 3 of the NEC. This means that splices must be in boxes. Open terminals are not allowed, and 22 gauge wires are not allowed. That's right, if an intrinsically safe circuit is not a class2 or class 3 and is not a motor control circuit as defined in section 430-72, we are left with only one option for a remote-control, signaling, or power ]imited circuit: it must be a Class 1 circuit by default (see section 725-3). This means the minimum wire size is No. 18 or other sizes if in a listed class 1 cable. It also means all the relief given for Class 2 circuits in Article 725 do not apply. The intrinsically safe circuit may not ignite the most easily ignitable mixture of gases or vapors for which it is designed for, but it still may be a shock hazard to humans, and therefore, must be wired accordingly.