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A Wisconsin wastewater treatment plant is saving time with a combination plug/receptacle and disconnect switch that makes motor and generator connections safe, fast and easy, whether in the plant or at lift stations around town. The Meltric Decontactor Series switch-rated motor plug allows workers to safely make and break electrical connections, even under full load, and also provides the NEC-required "line of sight" disconnect.
The $23.6-million plant serves Watertown, Wis., a city of about 23,000 residents. Submersible mixers in the aeration basins originally were hard-wired. When one of the mixers had to be replaced, it put the tank out of service for about a day while the mixer was disconnected and a new one re-wired.
As a result, the facility installed DSN30 (30A, 480V, 10-hp rated) Decontactors on all its aeration tank mixers. These devices allow the mixers to be connected and disconnected safely with plug-and-play simplicity. Now, mechanics can easily replace or service the mixers without an electrician and without the cumbersome electrical personal protective equipment, as required by NFPA 70E.
"When the first mixer failed, we had to shut everything off and disconnect all the wiring before we could pull it out and drop in a replacement," said Kevin L. Freber, assistant water systems manager of wastewater. "Now, we just pull the plug, crank the mixer up and plug in a new one. We're ready to go in minutes, and there's never any exposure to live power."
Breaking the circuit
Disconnecting a motor is a simple operation that is initiated by pressing a pawl on the unit, which causes it to break the circuit and eject the plug to its rest position. Then, a simple quarter-turn of the plug allows it to be completely withdrawn from the receptacle in complete safety because the circuit is already dead. When the plug and receptacle are separated, a safety shutter prevents access to live parts.
The units incorporate spring-loaded, silver-nickel butt-style contacts that provide consistently superior electrical performance over thousands of operations and are resistant to wear, corrosion, oxidation and other factors that contribute to premature failure of pin and sleeve-type devices.
Success with the aeration basins led the utility to equip its portable emergency generators and remote lift stations with similar connectors. Typically, they are located below ground level with a control panel above ground.
A power failure may make it necessary to bring the portable generators to power the pumps at some lift stations. Previously, these stations were equipped with conventional pin and sleeve connectors; however, they could not be locked easily to prevent tampering or injury to children or vandals who might try to remove the plug.
"The generators deliver 100-amp service, and with the plugs we had before, there was no way of locking the two parts together," Freber said. "Any child could walk up and pull it apart."
The units are easy to lock to prevent tampering, and they are safe when separated.
"You have to twist it to open it, and even if someone could get it apart, they never could get at the live contacts," Freber said. This is due to their dead-front construction and enclosed arc chambers.