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Sewage ejector system with only one wire3/28/2024 Over time, the sewage-ejector pump or switch seal can deteriorate and fail, allowing moisture and corrosive wastewater to penetrate the float-switch or pump cavity. Ground Leakage and Differential Current from Seal Failure ![]() Any differential current indicates a leakage current, which is typically a shunt to ground-either along the ground conductor itself-or through another grounded object in contact with the energized surface. In addition, the hot and neutral currents should always be identical in magnitude. The ground wire should never carry any return-load current, except during an emergency ground-fault or short-circuit. In the case of an insulating-plastic (ungrounded) sump basin as shown above in Figure 1, the only safety-ground path is from the pump ground wire (via the plug-in receptacle) to the circuit-breaker panel where the neutral and ground conductors bond together at a single point. A neoprene, nitrile rubber (NBR), or similar watertight gasket prevents moisture from entering the motor cavity or coming in contact with electrical wires. ![]() ![]() Pump Seal Keeps Moisture Away From Electrical Componentsĭuring normal pump operation, the hot and neutral conductors are electrically isolated from the sewage-pump housing, which is typically constructed from a combination of heavy-duty cast iron or cast bronze and/or engineered plastics. Selection of the backup-battery chemistry and construction-such as the newer sealed Absorbent Glass Mat (“AGM”), gel-cell, and Lithium-Ion (“Li-ion”) technologies-are beyond the scope of this article, with each offering tradeoffs of cost, ease-of-maintenance, discharge characteristics, and longevity. Optionally, the backup pump may also be a battery-operated DC or AC/DC pump powered, for example, by a 12-volt wet-cell battery (marine battery) and trickle charger (Figure 3). For best reliability-and to avoid tripping the subpanel circuit breaker when both pumps are active-the backup pump should also connect to a dedicated GFCI-protected branch circuit (typically 120 VAC, 15 A or 20 A). In the event of a primary-pump mechanical or electrical failure, Figure 2 illustrates a recommended back-up pump to prevent the basin from overflowing into the basement or crawlspace. Proper hysteresis allows sufficient time to empty the effluent basin without short-cycling the sump pump. Hysteresis is the difference between the initial switch-on wastewater level and the subsequent switch-off level-similar in principle to an HVAC thermostat regulating temperature when cycling a furnace every 10 to 20 minutes. The former switches only the hot conductor, whereas the later switches both the hot and neutral conductors to the pump-providing an extra measure of protection.ĭischarge Hysteresis and Recommended Backup Pump Depending on the pump model, the float switch may be single-pole or double-pole. When sewage or other wastewater fills an empty sump tank, after a few minutes a level-sensing float-switch activates the pump, which remains active until the fluid level discharges below a preset threshold. Sewage-ejector and dewatering sump pumps are common in basements and below-grade drainage basins (Figure 1). The full case study, measurement methodology, and results follow below. ![]() If left unresolved, abnormal leakage current presented a serious safety hazard that could result in electrical shock or electrocution. As the sewage pump worked reliably for years without tripping the original GFCI-and recognizing that replacing the GFCI did not remedy the new tripping problem-BUILDERA measured suspected leakage current to determine whether a possible pump electrical short was the GFCI-tripping root cause. Upon replacing the GFCI with a new one, the power-tripping problem persisted. After 10 years of reliable operation, a California homeowner experienced nuisance (ghost) tripping of a basement Ground-Fault Circuit-Interrupter (“GFCI”) whenever activating the sewage pump.
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