Here are the details you need to know to install a Code-compliant grounding system to serve hazardous (classified) locations.
Hazardous (classified) locations require special attention to grounding. It’s easy to lose sight of the grounding requirements for these locations when you’re managing all the other special rules. Although the Zone classification system (Sec. 505-25 simply requires compliance with Sec. 501-16 and Art. 250) hasn’t added any specific grounding requirements, the complexity makes it easy to lose track of which grounding requirements apply. The following three guidelines will help:
• Decide if your electrical system must be grounded. Do you have to connect one of the circuit conductors to earth? Hazardous (classified) locations don’t add any rules affecting this decision.
• Be sure conductive enclosures, including conduits and other wiring methods, are safe to touch (even under fault conditions). In the case of grounded systems, be sure if a fault to a grounded surface occurs, you have an adequate return path to promptly trip overcurrent protective devices. This is equipment grounding.
Hazardous (classified) locations, however, refine the general concept. We commonly see installations with very high workmanship within these areas, coupled with a lesser attention to detail in home runs feeding the local panelboards. That’s a serious error, since the total grounding return path impedance is the sum of both the portion within the hazardous (classified) area, and the portion in general-purpose areas supplying those locations. You must also be sure while any fault progresses within the wiring system, the return current doesn’t create arcing. This could ignite a local fuel-air mixture.
• Bonding fits closely with equipment grounding in this context. The better you connect conductive parts of enclosures and wiring methods together electrically, the less problems you’re likely to have with external sparks and uncleared internal arcing. That’s why most of the enhanced hazardous (classified) location grounding rules involve additional bonding requirements.
Bonding in the hazardous (classified) location. As covered in Sec. 250-100, you must assure reliable connections in these areas regardless of voltage. For example, if you have a Class 2 signaling circuit, its wiring method needs the same attention to bonding as a power circuit it may control. You’ll also find similar requirements in Sec. 501-14.
Be careful when you apply Sec. 501-14 Ex. on nonincendive circuits. The limited energy circuits in some Class I Div. 2 locations run in ordinary wiring methods. This exception is a wiring method exception only. It doesn’t affect the bonding rules. Even for intrinsically safe circuits, Sec. 504-60 applies similar bonding rules. Intrinsically safe circuit designs often depend on low-impedance grounding paths to carry fault currents shunted by zener diode barrier systems. Intrinsically safe systems also consider the total energy storage available, in terms of inductance and capacitance. An intrinsic-safety barrier manufacturer’s calculations may not apply as intended if you fail to install the appropriate grounding path with the circuit conductors.
How do you comply? Follow the same enhanced rules as those for services in Sec. 250-94. You’ll find an itemized list of acceptable methods, as covered in this section, in the 1999 NEC:
• Bonding to a grounded service conductor. This normally won’t apply to hazardous (classified) location wiring, unless you have the actual service location involved. You should avoid this wherever possible—given the high power levels and available fault currents inherent to service installations. Follow the advice in Sec. 500-3(a) (FPN) and arrange your installation so the service falls outside of hazardous (classified) areas.
• Wrenchtight connections into threaded bosses and couplings. You’ll probably use this technique the most. Even if you’re using the new allowances in Sec. 501-4(a) for special Type MC cabling in Class I Div. 1 areas, you still need to make up your mechanical connections wrenchtight. Be certain you engage at least five full pipe threads in Class I Div. 1 areas. Sec. 500-3(d) won’t allow you to leave a loosely threaded connection, even if you use a bonding jumper. Why? Loose threads won’t complete an explosionproof enclosure (Fig. 1, on page 36 of original article).
• Threadless connectors and connectors made up tight for metallic wiring methods. You’ll use this in Class I Div. 2 areas away from enclosures containing arcing components. Class I Div. 1 locations assume periodic explosions within the wiring method. This means joints need to contain the pressures of an explosion. These systems allow the hot gases to escape slowly and through a path with adequate heat-sinking ability. They end up cooled below the ignition temperature of the gases or vapors outside the enclosure.
There are two ways to do this. You can use flat, carefully milled surfaces bolted together, or you can use five fully engaged threads. The first method, widely used for factory-built enclosures, isn’t practical for field wiring. The concept of a threadless connector clearly doesn’t involve threads. Class I Div. 2 wiring, where not exposed to internal explosions, does use this principle. You could use compression couplings on a conduit run on the supply side only of an explosionproof seal you installed to complete the enclosure around a switch with arcing contacts.
• Other approved means, such as bonding locknuts, wedges, and bushings. Here, you’ll be using this method in Class I Div. 2 locations. Sec. 501-4(b) doesn’t require explosionproof enclosures in these areas except to enclose devices having normally arcing, sliding, or separating contacts. These devices could generate sparks during routine operation. When you do use conventional enclosures, you cannot rely on conventional locknuts.
Don’t confuse this rule with allowances for over 250V to ground in Sec. 250-97 Ex. for double locknuts or for listed boxes with concentric knockouts rated for use without additional bonding. Sec. 250-100 points to Sec. 250-94 and nowhere else. You can use a bonding locknut on a clean knockout, and bonding wedges or bushings around a concentric or eccentric knockout only. Sec. 501-16(a) disallows the double-locknut procedure in Sec. 250-97 Ex.
Some manufacturers claim their listing on concentric knockouts brings them into compliance with the wording in the last paragraph of Sec. 250-94. That paragraph only imposes the bonding rule on concentric knockouts that “are punched or otherwise formed so as to impair the electrical connection to ground.” The UL guide card restrictions only point to use over 250V to ground. That clearly points to Sec. 250-97 Ex. Don’t use these products to satisfy either service bonding in Sec. 250-94 or hazardous (classified) location bonding in Sec. 250-100.
Bonding of flexible connections. Sec. 501-4(b) allows liquidtight flexible metal conduit and flexible metal conduit to be used in Class I Div. 2 locations where you need limited flexibility, such as at motor terminals. The Code requires bonding jumpers any time you use these wiring methods as the equipment grounding path from or within a hazardous (classified) location.
The only exception to this occurs on control equipment with overcurrent protection not over 10A. You can use listed liquidtight flexible metal conduit for these connections without an additional bonding conductor, provided you use fittings listed for grounding.
The equipment grounding return path. Sec. 250-100 applies within the hazardous (classified) location. Sec. 501-16 (and Sec. 502-16 for combustible dusts, and Sec. 503-16 for combustible flyings) augments those rules in two ways. First, it specifically disallows using locknuts for bonding purposes, and calls for bonding jumpers. Second, and most importantly, it requires the same enhanced bonding all the way back to where the local equipment grounding system originates.
There are three points where your equipment grounding system might originate.
• First, very service originates an equipment grounding system. If you’re using a grounded electrical system, you connect that system to earth and to the equipment grounding system using a main bonding jumper per Sec. 250-28. If the system in your hazardous (classified) location isn’t in another building fed from a remote service, and it isn’t fed from a separately derived system, then it originates at the service and enhance bonding rules apply.
• Second, a separately derived system, by definition, has no conductors in common with the system that provides its source of power to the site. Therefore, any fault current returning over an equipment-grounding path must return to that separately derived source. The enhanced bonding rules apply only up to the separately derived source—not beyond it.
• Third, the source could be at a second building fed from another, but only if you reground your electrical system at that building. The 1999 NEC severely limits this practice . Now, you can only do this in cases where there aren’t any common conductive paths between buildings, either equipment grounding conductors or items like bonded metal piping systems.
Don’t confuse this concept with providing a grounding electrode at second buildings, per Sec. 250-32(a). You always have to do that to establish a local grounding reference for the equipment grounding system.
If you don’t reground at the second building, then the enhanced bonding goes all the way back to the service. If you do reground at a second building, then the requirements apply as far back as the building disconnect, where Sec. 250-32(b)(2) requires the interconnection. From that point back, the grounded circuit conductor carries any fault currents.
This all looks much more complicated than it really is. Simply trace back the ground return path to the point where it no longer follows a raceway or a green wire, and instead follows a white wire. That’s how far back the enhanced grounding and bonding rules apply. In the case of an ungrounded (or high-impedance grounded) system (no white wire used as a circuit conductor), apply the rules back to the grounding point for the system source. In accordance with Sec. 250-24(d), Sec. 250-30(b)(1), Sec. 250-32(c), or Sec. 250-36(f) as applicable for ungrounded (or impedance-grounded) systems, that point occurs at or on the line side of the principal disconnecting means.