Intrinsically safe remote controls

by Emily Wilson
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it’s been more than 30 years since portable radio remote control systems (PRRCs) first made their appearance in the grain industry. In those three decades, PRRCs have proved themselves a necessary element for improving productivity and efficiency in the transfer of grain from the elevator to the railcar. However, by their very nature, PRRCs had to meet another challenge before they could be accepted for use in grain handling.

The grain loading/unloading environment contains extremely combustible particles that fill the air during normal grain transfer operations. It is not an understatement to say that the slightest spark or electric emission could have tragic consequences for anyone or anything in close proximity.

Since PRRCs operate with battery-powered current in a potentially hazardous atmosphere, why is there not even the possibility of a spark or a short that could cause an explosion? The answer is that the systems are designated "intrinsically safe" and in this situation, they have to be.

Defining intrinsically safe

‘Intrinsically safe’ is far more than a descriptive designation: it is a term applied to a low-voltage PRRC that can release neither heat nor sparks that could cause an explosion of any flammable particles in a hazardous area. The same designation applies to usage where there are volatile gasses.

In order to qualify as an intrinsically safe system in the United States, a PRRC must be stringently tested by a third party, which bases its tests on the National Electric Code (NEC). Worldwide, the International Electro-technical Commission sets the international standards. The intrinsically safe designation is determined when tests conclusively prove that the PRRC cannot cause an explosion, regardless of how hazardous the environment — even under fault conditions. That’s the major difference between "intrinsically safe" and "explosion proof." The latter term refers only to an apparatus in a case that can withstand an explosion of gases or vapors.

When is intrinsically safe required?

More than common sense dictates that you need an intrinsically safe PRRC during grain transfers. Consider the elements that create hazardous environment: flammable substances, oxygen, and an ignition source. Article 500 of the NEC guidelines is very specific about those locations it has determined to be hazardous, and only intrinsically safe electrical equipment can be operated in those areas; there are no exceptions.

The locations are divided into three classes and divisions. While some do not necessarily apply to grain handling, several others do.

Within these divisions, Class II, Division 1, Group G is most applicable for PRRCs in grain transfer processes. Underwriters Laboratories provides these elements, which define Class II, Division 1 hazardous locations:

• "ignitable concentrations of combustible dust can exist in the air under normal operating conditions;

• ignitable concentrations of combustible dust may exist because of equipment breakdown that simultaneously causes the equipment to become a source of ignition; or

• electrically conductive combustible dusts may be present in hazardous quantities."

Other than intrinsically safe equipment, only dust ignition-proof or pressurized equipment is permitted in grain transfer operations.

Testing experience

Manufacturer of PRRCs, Cattron-Theimeg Inc., Sharpsville, Pennsylvania, U.S., describes below how its controllers were thoroughly tested by Underwriters Laboratory, using United States and Canadian standard testing requirements, based on Article 500 of the NEC.

"To the uninitiated, some of the tests may have seemed like old-fashioned low-tech methods for figuring out just how safe a solid-state, low-energy PRRC is," Cattron-Theimeg said. In fact UL examiners dropped its controllers 10 times from a height of one meter (approximately 39.5 inches). The battery packs met the same fate. They were dropped an equal number of times from the identical height. These were not tests to see if the PRRCs would function after the drops. Tests for shockproofing are usually performed when the systems are manufactured. Instead, these tests showed that the PRRC’s internal heat would not begin to rise following the series of drops.

Tester examinations weren’t limited to the controllers. PRRCs contain several possible energy sources such as capacitors, inductors and the battery pack. These, too, were extensively tested and evaluated. Examiners used a series of charts showing maximum allowable energy based on the specific classification. Chart statistics were compared with potential energy sources to make certain that these sources could not produce a spark that could cause a catastrophic explosion."

The PRRC in operation

At Cargill Fertilizer in Florida, one operator, standing near the train, uses a PRRC to control the locomotive pulling hopper cars beneath discharge chutes.

This operation clearly falls under the NEC’s description of hazardous areas with particles and vapors in abundance. Yet, because there is no spark emission of any type, the threat of an explosion as a result of their use is non-existent. The Cattron-Theimeg MP 96 used in this application contains up to 96 outputs and 48 inputs as part of a microprocessor system. This type of system conceivably could, were it not intrinsically safe, generate heat that goes beyond the limits set in the Underwriters Laboratory tests.

The company cites a similar situation at ADM Growmark in Louisiana, where a solo operator also controls locomotive speed as it pulls the hopper cars to discharge chutes, which fill the cars with grain. As at Cargill, the operator stands alongside the train observing the entire procedure. His PRRC is obviously exposed to the elements, however it functions without danger to workers, equipment and the elevator.

Another hazardous location where the remote controller must be intrinsically safe is at ports where grain ships are routinely loaded or unloaded. Here, fuel is an additional consideration to the hazardous elements found in a confined grain operation.

PRRCs have long been lauded for their efficiency, control and intuitiveness, all of which have led to the increase of portable radio remote controls in the business of agriculture. So has the evolution of technology, which has meant even greater capabilities, such as those displayed by the multiprocessor systems. However, the emphasis must always be on the safety of the system and its operational use; which is why portable radio remote controls used in the grain industry must have intrinsically safe designation.


Article 500 of the National Electric Code: locations determined to be hazardous

Class I - Division 1:

• Group A — Atmospheres containing acetylene.

• Group B — Atmospheres containing hydrogen, fuel and combustible process gases that contain in excess of 30 percent hydrogen by volume or propylene oxide and acrolein.

• Group C — Atmospheres such as ethyl ether, ethylene or gases or vapors of equivalent hazard.

• Group D — Atmospheres such as acetone, ammonia, benzene, butane cyclopropane, ethanol, gasoline, hexane, methanol, methane, natural gas, naphtha, propane or gases or vapors of equivalent hazard.

Class II – Division I:

• Group E — Atmospheres containing combustible metal dusts including aluminum; magnesium and its commercial alloys or other combustible dusts whose particle size, abrasiveness and conductivity present similar hazards in the use of electrical equipment.

• Group F — Atmospheres containing combustible carbon-based dusts including carbon black, charcoal, coal or dusts that have been sensitized by other materials so they present an explosion hazard.

• Group G — Atmospheres containing combustible dusts not included in the preceding two groups such as flour, grain, wood, plastic and chemicals.

Class III – Division I:

Atmospheres containing easily ignitable fibers or materials producing combustible flyings.