With all the emphasis on explosion proof lighting usually revolving around the fixtures themselves, it is very easy to overlook the fact that electrical safety and compliance with regulations relevant to the operation of electrical equipment in hazardous locations requires much more than simply choosing and installing the properly classed and approved light fixtures. Lighting is only a part of a bigger system wherein electrical current is supplied, routed, and fed to the equipment it is intended to power. In almost all cases involving hazardous classed locations, not only is equipment required to carry the proper approvals and certification for operation in those areas, but the wiring, conduit, connectors, junctions, switches, plugs and outlets too must also be properly approved and installed as well. While it is important to ensure that equipment is capable of preventing the occurrence of accidental ignitions, it is just as important to ensure that the equipment and hardware supplying power to devices is equally capable of providing protection.

First and foremost, it is critical that the proper grounding techniques are applied within hazardous locations.  For instance, have you determined if your explosion proof lighting system must be grounded? Does it require an earth ground? Is equipment grounding comprehensive, meaning, are all connected parts of an enclosure safe to touch, even if a fault is present?  In the case of equipment and attached accessories, there must be a return path capable of activating protective devices such as over-current breakers or relays.  A good ground is an extremely important part of ensuring effective protection because it helps to provide a clear path for current to follow and safely exit along should a fault occur. When the various parts of fixtures are poorly grounded, including connecting cables, hoods, reflectors, and conduits, return current can cause arcing between these various components as well as internally, resulting in the ignition of flammable atmospheres.

For hazardous locations, equipment bonding is a part of NEC requirements and is similar to grounding in its context as compared to grounding, and requires similar methods for the connecting of enclosure components as well as power supply, wiring and conduit. Section 250-100 of the NEC requires there to be effective connections in these locations regardless of how much voltage is present, so even though a switch may only operate with 110 VAC, if the equipment it controls deals with 220 VAC, connections must be similarly bonded. Suffice to say, there should be similar load carrying capacities and capabilities among all the connections in a system in order for there to be effective bonding in place which can help to prohibit overloads and arcing in the instance of a fault.

For connections among devices and fixtures in hazardous locations, wrench-tight connections are the most common and usually found at the threaded bosses and couplings where wiring or conduit is intended to enter into an enclosure or fixture. All mechanical connections must be wrench-tight, and in Class 1 Division 1 locations you must ensure that at least 5 threads are engaged in the coupling for secure connection. Even when an added jumper is used, you cannot have a loosely threaded connection as it won’t complete an explosion proof enclosure or fixture and can compromise the compliance of the system. Thread-less connectors, by their name, obviously do not utilize threads in their composition. Threaded connections are intended to continue the main principal of explosion proof protection through the creation of a flame path which is designed to contain an internal ignition and slow the expansion and eventual escape of the heated gases from the fixture so that when they exit they are too cool to cause ignition of the outside atmosphere.

Non-threaded or tight connections rely on interference and compression type fitting and are suitable for use in Class 1 Division 2 locations where exposure to internal explosions is not expected. These thread-less connections are suitable for use on the supply side of a conduit or explosion proof seal in order to finish the enclosure surrounding a sparking or arc prone set of contacts.

Class 1 Division 2 locations can also use flexible connections provided they are properly bonded. Flexible metal conduit is suitable for Class 1 Division 2 areas where this flexibility is needed to support connection to devices such as motors or other equipment where movement and vibration may be an issue. Top use flexible conduit you must also include jumpers in order to form the grounding path to or from the hazardous location. Control equipment that is fitted with over-current protection 10A or below is not required to use jumpers, however, the fittings utilized must be listed as suitable for grounding.  

Grounding paths can include three main points including the main grounded electrical system, a separate system with no conductors in common with the source that powers the site, and a source that derives power and supplies it to the site from a third separate main source. In the first case of a main grounded electrical system, that system must be grounded to earth as well as the equipment ground via a main bonding jumper. In the second case of a separate system with no common conductors, it is necessary to provide a ground path whereby faults can return to that system, not the originating source of the fault ie the equipment system. In the third case, it is possible to re-ground the electrical system to the third source beyond the secondary source, however, the NEC limits this practice and allows it only if there are no common conductive connections in any form between the sources.

As can be seen, just installing properly approved equipment in a hazardous location is not enough to meet compliance. There is a complex relationship between enclosure and fixture approval, the proper installation of that equipment, and how it is connected to the local power source. Different classes of locations have different requirements, and it is necessary when installing lighting equipment to pay close attention not only to the proper approvals for equipment, but the methods for installing that equipment as well if compliance and safety is to be maintained.