code article 504

[jack357]

I have a question and hope someone can help me with.

article 504 deals with intrinsically safe systems.How dose this type of system differ from
other systems that are made safe for use in hazardous classified locations??

thanks for any info you can supply.

 

[augie47]

An intrinsically safe circuit is not capable of a spark or thermal effect that will cause ignition and can be installed without using explosion-proof apparatus/wiring methods for the most part.
Intrinsically safe circuits are often interfaced with standard voltage equipment outside of the hazardous area.
Doe that answer your question ?

 

[rbalex]

The basic definition and concept is in Article 504.2:

IntrinsicallySafe Circuit. A circuit in which any spark or thermal effect is incapable of causing ignition of a mixture of flammable orcombustible material in air under prescribed test conditions.

There are several critical auxiliary concepts:

? Intrinsically safe systems must be installed in accordance with a “Control Drawing” [Section504.10(A)] This can’t be over emphasized.
? The conductors must be separated from “NonintrinsicallySafe Circuit Conductors” [504.30]
? Bonding, as opposed to grounding, is virtually the same as that required in the classified location [504.60]

Many of the other descriptors actually do reduce some requirements, but the permissive to use “Any of the wiring methods suitable for unclassified locations…”, isn’t as all encompassing as many would think.

 

[jack357]

Thanks for the quick response, but I still can't see how any electricial circuit

can be made not to have spark or heat potential?

sorry about the spelling , thanks again for the imput.

 

[rbalex]

The key is "... under prescribed test conditions." If you refer to NFPA 497 you will find terms like Minimum Ignition Energy (MIE) and Minimum Igniting Current ratio (MIC ratio) which indicate a spark or heat alone may not be sufficent to "caus[e] ignition of a mixture of flammable or combustible material under prescribed test conditions." This is why it is critical to use the control drawing. It should describe the conditions where the installation is "intrinsically safe." It may be in one application, but not another.