Water and live power are two things you don’t want to mix. Just ask the safety-conscious employees at the Watertown Wastewater plant in Watertown, Wisc.
The $23.6 million facility serves Watertown, a city of about 23,000 residents located midway between Milwaukee and Madison on the Rock River. Completed in 2004, the 5.2 mgd facility is expected to accommodate future growth well beyond the next 20 years and is located on a 40-acre site that will permit future expansion.
During construction of the plant, which began in 2002, submersible mixers in the aeration basins were hard-wired. Shortly after operations began, one of the mixers had to be replaced, which put the tank out of service for a day while the mixer was disconnected and a new one re-wired.
At about the same time, Jim Arndt, a department maintenance technician, attended a trade show and saw a switch-rated motor plug that allowed workers to safely make and break electrical connections, even under full load, and also provided the NEC-required “line of sight” disconnect. Arndt suggested installing them on the mixers to prevent delays on future mixer replacements.
As a result, the facility installed DSN30 (30A, 480V, 10 HP rated) Meltric Decontactors on all its aeration tank mixers. The combination plug/receptacle and disconnect switch makes motor and generator connections safe, fast and easy, whether in the plant or at lift stations around town.
“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,” explains Assistant Water Systems Manager-Wastewater Kevin Freber. “If one failed on a weekend, the weekend staff couldn’t handle it, so we either had to wait until Monday or call in an electrician. 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.”
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 needing an electrician and without the need for cumbersome electrical personal protective equipment, as required by NFPA 70E.
Disconnecting a motor is a simple operation that is initiated by pressing a pawl on the Decontactor, 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 Then, a simple quarter-turn of the plug allows it to be totally withdrawn from the receptacle in complete safety, since the circuit already is dead. When the plug and receptacle are separated, a safety shutter prevents access to live parts.
“They have been online for more than a year without any problems,” says Freber.
Safer and Faster Connections
Success with the aeration basins led the utility to equip its portable emergency generators and remote lift stations with similar connectors. The city is underlain by 105 miles of sewers that collect wastewater from a 12-square-mile area. The collection system also includes 18 remote lift stations.
The lift stations, located around the city, use submersible pumps to elevate the wastewater in the sewer lines and facilitate gravity flow to the treatment plant. Typically, they are located below ground level with a control panel above ground. Freber says the total capacity of the pumps is 27 million gallons per day, although flow generally averages between three and three-and-a-half mgd.
While some lift stations are equipped with stationary generators to provide emergency power, a power failure can make it necessary to bring the department’s portable generators to the other lift stations and connect them to power the pumps until service is restored. 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. Any child could walk up and pull it apart,” says Freber.
Freber points out that the new plugs are easy to lock to prevent tampering and also are safe when they are 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,” he says.
Easily accessible contacts on the previous connectors had the potential to expose workers or others to live power, so switching to the new plugs also helped the utility to simplify compliance with NFPA 70E arc flash requirements.
Arc flash can be a concern when it becomes necessary to switch power connections, but the city’s lift stations that use mobile generators for emergency power are constructed to minimize or eliminate this risk. Freber says that wiring typically comes up from the pump into the bottom section of the control panel, which is constructed so that the related starters and other electronics are segregated in a sealed area.
“Because of the new arc flash laws, we have them separated so our technicians can open the outside panel without danger from arc flash. There is also a walking beam inside, so when we switch from city power to emergency power, the power can’t back feed,” Freber explains. “Using the Decontactor to connect to the generator with this arrangement we can switch safely from city to emergency power.”
Now, it takes only minutes to connect a generator and begin pumping. The ability to connect or disconnect quickly and safely makes it easier to move generators around to various lift stations for monthly test runs or if necessary during a prolonged or widespread power outage.
Bill Fortman, PE, MBA, is the marketing manager for Meltric Corp. Since 1982 Meltric has manufactured a complete line of industrial duty plugs and receptacles in Franklin, Wisc. Meltric’s UL/CSA switch rated plugs and receptacles are safe to make and break under load up to 60 hp or 200 amps.