ELECTRICAL SYSTEMS

Primary electrical systems that should be considered in the design of a pump station include systems that:

• provide surge protection for pump station equipment during electrical service irregularities or failures from the utility company,

• provides for phase failure,

• provides protection against lightening strikes,

• provide automatic power transfer, and

• allow the standby engines to start automatically in response to sensing a power outage and a change in water level in the wet well, since most pumping stations are unattended.

11.6.1 Protection Systems

A switching system should minimize damage to the pumps and limit the extent and duration of interruptions in the electrical service supplied to the station. A system must isolate any affected portion of a station system while maintaining normal service for the rest of the system. It must minimize the danger of short-circuit currents and may provide alternate circuits to minimize the duration and extent of outages.

System protection is provided by the following devices:

• a fuse, which is the simplest of all protective devices, performing both sensing and interrupting functions.

• circuit breakers, which are interrupting devices and must be used in conjunction with sens ing devices to fulfill the detection function. Most low- voltage applications use either

molded-case circuit breakers or other low- voltage circuit breakers having series sensing devices built into the equipment.

Circuit breakers and fuses are employed to disconnect the affected parts of the power system.

The use of computer-controlled circuitry to perform the sensing and timing functions can help optimize control conditions and allow for easier testing, monitoring and adjustments.

When two utility sources from separate supply sources and over separate lines are connected, an automatic transfer switch is usually located in the main distribution equipment. Special consideration should be given to the ability of the switch to:

• close against high inrush currents,

• carry full rated current continuously from normal and secondary source,

• withstand fault currents without contact separation, and

• have adequate interrupting capacity.

11.6.2 Engine and Motor Switches

Since most pumping stations are unattended, engines must start automatically. The engines are designed to start at a predetermined low speed or load. After a suitable time delay the throttle is opened to its operating speed. A selector switch is included on the control panel.

During a power outage, a trans fer switch should be provided with an accessory control with switches that disconnect the main pump motors during the energized condition, prior to transfer, and reconnect them after transfer when the residual voltage has been substantially reduced. A time delay of 2 to 10 minutes should be provided to prevent transfer back to normal source when normal service is recovered after an outage.

11.6.3 Water Level Sensors

The various types of pump control sensors, which regulate pump activity, include:

• Float switch

• Electrode probes

• Ultrasonic devices

• Tilting bulb with mercury switch

• Bubble-tube

• Entrapped air pressure switch

11.6.3.1 Float Switch

A float switch consists of a floating device attached to a moving tape or wire and pulley

system. The float rises and falls with the water level in the sump. When the water level reaches a given height, the pumps are activated by a control switch and the pumps continue pumping water from the intake area until the water level drops. One particular specification of which the

hydraulic designer should be aware is that float switches should not be used if the number of pump cycles is expected to exceed 4 per hour because they are typically relatively slow to respond. Also, the float will respond to fluctuations caused by turbulence in the sump. Thus designers will usually ensure that a minimum height of about 150 mm (6 inches) is used between successive start elevations of pumps and successive stop elevations of pumps.

11.6.3.2 Electrodes

Electrode probes are activated when the rise in water level causes an electric current to pass between the electrodes of the sensor thereby activating the pumps. The electrodes are suspended vertically from the top of the pit with their lower ends positioned at the level elevations at which the control device is to be actuated. These sensors can tend to corrode and can be fouled by floating debris and pollutants.

11.6.3.3 Ultrasonic Transducers

Ultrasonic devices emit a mechanical pulse that registers a change in the water level in the intake box. The sensor registers a change in frequency due to the rise in water level and the pump controls detect this change and activate the pumps.

11.6.3.4 Mercury Switch

A tilting bulb with mercury switch (Figure 11-6) registers a change in the orientation of the liquid mercury within the bulb caused by a change in the water level in the intake box. This type requires a significant change in level to open or close. Thus, the designer will usually apply a water level change of at least 150 mm (6 in.) between successive pump start elevations and between successive pump stop elevations.

Figure 11-6. Tilting bulb switch

11.6.3.5 Bubble Tube

A bubble tube regulates pump activity by registering a change in the orientation of the bubble within the bubble-tube caused by a change in the water level. A bubble-tube is placed inside a conduit installed between the wet well and dry well of dry-pit stations. The tube enters the wet well from the top and terminates in the dry well at an elevation above the highest possible water elevation. When the wet well and the dry well do not have a common wall then a bubble tube presents the best alternative.

11.6.3.6 Air Pressure Switch

The switch registers an increase in pressure due to the compression of the air in the inflow pipe resulting from an increase in the height of water in the wet well. When the water level reaches a given height and corresponding air pressure the switch is activated and the pump starts. When the water level drops, the switch closes and the pump stops.

11.6.4 Sensing and Alarm Systems

Sensors are provided for critical operating systems within the pumping station in order to detect changes in standard operating procedure. One such system is the gas detection system.

Wet-pit stations with a large, closed, rectangular pump pit, require fuel presence alarms. Stations with a ventilated below-pavement storage unit, such as the dry-pit station, do not require a gas detection system.

Detection and alarm systems are typically used for:

• gas detection,

• heat detection,

• pump failure,

• pump usage monitoring,

• indicating high water, and

• electrical failure.

Information from the sensor (detector) can be made available at a remote point by telemetry.

11.7 MECHANICAL CONSIDERATIONS