Hazardous area classification confusion


Thread Starter

Robert Zuiderveld

I need clarification concerning the intergration of the international electrical codes and the use of electrical equipment in hazardous area classification according to the world standards and the NFPA/NEC american standards. I have been reading to many articles & codes, and I have spoken to too many organizations like UL, FM, OSHA, ANSI, Dept. of Commerce etc. concerning hazardous area classification and the use of electrical equipment in these areas. So far everybody has been providing me with BS without giving any clear direction. In 1996 the US accepted the world standard for classifying hazardous areas according to Zones, this in order to promote international trade. In stead of the codes and international trade getting easier everybody seems to make an effort in making things more complicated. For example, Class I, Zone 1 equipment has to comply with the same codes and standards as Class I, Division 1 equipment. Yet, according to UL you are not allowed to use Zone 1 equipment in Class I, Div 1 areas since Zone 0 is included in this hazardous area, referring to the NEC. However the 1999 NEC in article 500 only reads that you are allowed to use Zone 0, 1 and 2 equipment in Class I, Div 2 hazardous areas. It makes no mentioning that you are not allowed to use Zone, 0 or Zone 1 certified equipment in Class 1, Div 1 hazardous areas. Are the codes just there to prevent foreign companies to enter the US market? Are the just there to confuse everybody? Can anybody point me to the REAL codes and if possible clarify them for me?
I certainly agree, the Zone classification issues can be confusing. The Zone concept of area classification is an accepted European method its standards body is the International European Community (IEC ). To call it a “world” standard is a hotly contested issue. I mentioned this because equipment built to IEC standards and approved in the USA would bear a ExA verses a Ex label. The use of Zone classification is not mandated in the NEC only offered as an alternate method. The NEC does state that you cannot intermix the two methods, in other words you would never find a Zone 0 boundary in a Div 1 area. It is true you cannot use Zone 1 equipment in a Division 1, but you could use Zone 0 equipment (if you had the money). The reason for this is that the two methods use different approaches to prevent an explosion. In general equipment built for a Division 1 Class 1 location is designed to contain an explosion to an extent that escaping gases or flames will not ignite a larger explosion. It is assumed that during normal operation you will have an explosive concentration of gases for a Zone 1 class 1 location equipment uses “increased safety” to prevent an explosion. For a Zone 0 locations, during normal operation an explosive concentration is present all the time, Zone 1 some of time. To do this the entire wiring method and power supply must be looked at as an engineered system. This includes everything from purchasing the equipment to installing it. The idea is to limit the amount of energy during a fault or prevent the fault entirely. The end result is that you can use less expensive enclosures (e.g. plastic) with a more restrictive wiring method. Some see this as a cost savings. In order to remain competitive in a global market it will be necessary to adopt a universal standard. I’m sure this will be more inline with the IEC standards.

Bruce Durdle

Mike, Under IEC rules there are number of recognised methods of protection. Of these methods, only Intrinsically Safe (equipment that is safe with 2 internal faults (Ex ia) can be used in Zone 0: IS that is safe with 1 internal fault (Ex ib) is confined to Zone 1. Flameproof equipment (Ex d) equipment is also confined to Zone 1, but is identical in principle if not in fine detail to the US explosion-proof equipment which is acceptable in Div 1. Increased Safety (Ex e) equipment uses good wiring practice to avoid hot spots or sparking, and is acceptable in Zone 1 under IEC rules, while a slightly more relaxed version Ex n Non-sparking) is restricted to Zone 2. Only non-sparl=king is recognised under the NFPA system. Ther is no generally-accepted method that can be used in Zone 0 for high-powered circuits. This is an area where there a LOT of inconsistencies and there is no point in trying to rationalise North American practice with that used in the rest of the world.
Answering Robert's questions, we can say that:

1)Are the codes just there to prevent foreign companies to enter the US market?
R: No, although they can be used with this objective too. Every code reflects the country´s culture and knowledge. So, in USA is necessary to follow the NEC requirements.

2)Are they just there to confuse everybody?
R: It is necessary to note that we are referring two cultures (American-NEC, and European-IEC). We can do the same thing in different ways, right? It can be difficult for one to understand other´s culture.

3)Can anybody point me to the REAL codes and if possible clarify them for me?
R: The "real" code is your expertise. It is interesting to remember that API-RP-500 has some notes saying that: "The figures expresses the extentions of classified areas for a typical refinery". But, what is a "typical" refinery? So, you will define the distances, not the code. The codes give a "reference" for your particular job, not the "truth" for all possible cases.
Regarding Zone 0, we can say that this is in practice, only found inside vessels and pipes, in the space above the liquid surface. So, as only sensors will be installed in it, there is no use of high-powered circuits there.
The concept is: if an explosive mixture is always present, it is not allowed any spark with enough energy to ignite the mixture. As this level of energy is very low - mJ - only low-powered circuits will be installed there.