Engineering of Water Systems

Published On: June 16, 2023By Categories: Pumps and Water Systems, The Water Works

Electrical System and Control Methods: Panel Design

By Ed Butts, PE, CPI

In past installments of The Water Works, we have outlined various methods of control of pumping systems including PLCs, SCADA, hardwire and relay control, and instrumentation.

We will now examine development of a control panel in this column. However, before committing to implementing a control panel, it is important to determine if a single panel with components is preferred to a group of individual devices for circuit protection, motor starters, and controls. This is a decision based on available space, wiring, and labor, number of devices, control and interfacing requirements, and desire for modular construction.

Assuming a singular panel has been selected, we will review the basic requirements of control panel design and preventing electrical signal interference.

Fundamentals of Control Panel Design

Figure 1. Basic PLC controlling two motors.

Whether it’s municipal, agricultural, industrial, or domestic pumping equipment—all pump motors and their controls require some type of control panel or controller to manage their starting and operation. But what exactly defines what a control panel is and how it is designed and assembled?

A control panel is basically a combination of devices which use electrical power to control and monitor the various functions of specific electrically driven equipment. It is usually modular and contained within a single panel structure, generally consisting of a sheet metal, stainless steel, aluminum, or plastic enclosure with equipment mounting provisions to the enclosure itself or an integral back panel that contains and supports vital electrical components that may start, control, and monitor many electrical and mechanical processes of specific pumping equipment.

The enclosure is the outer body and housing of the control panel assembly, which is generally equipped with a UL 508A, IEC/EN 60529 (IP designation is for ingress protection), or an applicable NEMA classification rating. These listings classify their permitted environmental exposure and approved uses such as indoor or outdoor use for water, dust, hazardous, or potentially explosive exposures printed or labeled on the enclosure.

Figure 2. PLC controlling a VFD with closed loop feedback.

These various ratings are usually assigned a proof tagline behind the applicable exposure class, such as dustproof, waterproof, and explosion-proof. NEMA-rated enclosures are available for non-hazardous exposure ratings in NEMA rated 1, 2, 5, 12, 12K, and 13 ratings for indoor use and NEMA 3, 3R, 3S, 3X, 3SX, 4, 4X, 6, and 6P ratings for outdoor use.

Table 1 illustrates the permitted exposures and uses for NEMA-rated enclosures with IP equivalents. The back panel is a fabricated sheet metal plate mounted inside the enclosure that provides structural support for DIN rail or hard (direct) mounting and wiring ducts. The back panel permits mounting, wiring, and testing of the components as an assembly in a separate environment for later installation in the enclosure.

A DIN rail is a standardized metal rail widely used for mounting circuit breakers, relays and timers, PLCs and I/O modules, and other industrial rated control equipment inside equipment racks.

The other components of a basic electrical control panel are incoming protection and switching devices, power distribution systems, and circuit and load protection. All primary electrical currents fed into the control panel should pass through a main circuit breaker or fused disconnect switch before distribution to other devices. These are disconnecting devices that protect and disconnect the incoming source of electrical power.

Examples of Effective Control Panel Designs

Figure 3. Control panel with hardwired components.

There are few established rules when laying out a control panel. Obviously, the first criterion is to provide adequate space for mounting of equipment, wiring, and cooling of the components. The next consideration is providing a panel with neat and orderly wireways, straight wire runs, and secure equipment mounting. This facilitates future troubleshooting.

Industrial control panels come in all dimensions and with various types of equipment. An example of a good control panel design is illustrated in Figure 1. It shows a panel for two-motor control with a PLC. Note how the incoming disconnect switch is mounted at the top and the schematic is permanently affixed to the door.

Figure 2 illustrates a cabinet enclosed VFD with a PLC control system. In this case, the controls and VFD are mounted to separate back panels, plus there is a pocket in the door for storage of larger schematics and instructions.

Figure 3 is a control panel with Din rail mounted hardwired components. Note the effective use of wireways, aligned components, neat and straight wire runs, and identification tags at the terminals.

Finally, Figure 4 demonstrates a large control cabinet with a VFD and adjacent controls. Note the cooling fan over the drive to provide cooling of the cabinet.

Design Step No. 1.
Evaluate and Define the Operational, Regulatory, and Jurisdictional Requirements

Figure 4. Control cabinet with VFD.

The design process should always begin with an evaluation or creation of the technical or performance specifications or narrative (if any) to ascertain the operational and functional requirements of the control system along with the jurisdictional requirements and regulatory standards.

The narrative should consider all operational requirements based on a process diagram or narrative along with safety and ancillary functions. A major goal of an effective control panel design is to limit the switches and other control and sensing devices to those needed. This will help to prevent internal and unnecessary heat buildup from extraneous devices.

Verification of the jurisdictional and regulatory requirements should be performed as certain control panels may require use of a certified panel shop. Although many jurisdictions do not require specific labeling on a control panel, most do. In some cases, this may be limited to UL (Underwriters Laboratories), CSA (Canadian Standards Association), or ETL (Electrical Testing Laboratories). This typically requires a shop with labeling authority from the applicable testing lab.

Design Step No. 2.
Develop a Schematic, Panel Layout, and Process and Instrumentation Drawing (PI&D)

The following initial design decisions represent the primary considerations in developing an industrial control panel, although additional considerations specific to individual applications often apply.

Schematics and drawings are developed to outline the specific configuration of wiring, circuits, controls, and every other aspect of the final control panel.

Schematics generally follow a standardized format using ladder logic and rungs to each circuit or device with designations or descriptions adjacent to the circuit. Refer to Figure 5, which shows a typical schematic for a PLC with the circuit description next to the applicable rung.

When dealing with single- or three-phase power circuits, one-line power diagrams (Figure 6) are often used. This avoids the repetition of showing multiple conductors when a single line will suffice.

Next, control panel designs are often performed with proprietary (specific brand) equipment planned. In this case, the schematic should include wiring and circuit configuration to match the specific device. See Figure 7 for an example of an Allen-Bradley model 525 variable frequency drive.

When designing a control panel, it is critical that the designer consider all possible outcomes from unplanned events such as equipment failures or process shutdowns. This includes both operational and safety functions. If personnel safety may be possibly compromised or endangered from this type of event, most codes permit a controlled shutdown as a protective measure.

Drawings should include a Functional PI&D Drawing, Physical Space Requirements and Layout, and Control Schematic as good design addresses both the electrical wiring and physical space requirements. At a minimum, these drawings should include:

  • Programmable logic controller (PLC) with power supply and DIN rail or panel hard mounting
  • (+) I/O (input/output) modules for PLCs and space allocation.

Or:

  • Alternative to PLC system: hardwired relays, timers, switches, custom controls, etc.
  • Incoming power supply and distribution with space allocation, including circuit breaker, fuses, AC step-down transformer or DC power supply when needed, vertical and horizontal wireways, and terminal blocks and boards with connection assignments
  • Control cabinet three-dimensional (height, width, depth) space requirements and back panel layout
  • Ancillary remote/local and safety control or motor control equipment as described below.

An accurate schematic and PI&D are the principal foundations for the subsequent development of an industrial control panel. In addition to the basic equipment that comprises a control panel, ancillary equipment may be needed for specific functions, including thermal management systems for enclosure heating and cooling (particularly if a VFD is to be used), safety systems with manual or automatic overrides, bypasses, or lockouts, programmers, timers, relays, contactors, and motor starters and controllers.

In addition to the motor starting and control method, each motor must be equipped with branch circuit protection against short circuits. This is generally accommodated using a circuit breaker or fuses.

Motors will generally be supplied at the primary voltage, typically 230-volt, single- or three-phase or 460-volt, three-phase 60 Hz. Motors will typically utilize full voltage, across-the-line or reduced voltage starting methods. Methods of reduced voltage motor starting, such as variable frequency drives (VFDs), autotransformers, wye-delta, or part wind starting, will require separate consideration and space requirements. For three-phase motor circuits, it is unlikely that the neutral of the power supply would be routed to the motors, although an equipment grounding conductor is always required to each motor.

Figure 5. Example of PLC wiring schematic.

Other additional components may include:

  • Surge suppression or lightning arresters: Protects the electrical components inside the panel from damage caused by overvoltage due to lightning strikes, faults, or utility power surges
  • Power conditioning: Corrects power factor with capacitors (PFCs), reactors, and passive or active filters for VFDs
  • Additional transformers or power supplies: Manages the voltage by changing it to the required type and level
  • Terminal blocks: Organizes and distributes the wires from various field devices to different electrical devices
  • Interposing relays and contactors: These are on/off interfacing devices that manage and direct the functionality based on commands from the PLC. Smaller relays control minor functions like lights and fans while larger relays are called contactors that control more advanced functions like motors.
  • Communication devices and network switches: Acting as the communication hub in a control panel, they facilitate communication between the PLC and the other network-compatible remote devices in the system.
  • Human machine interface (HMI): Components such as mice, switches, joystick, buttons, and keyboards that help the human operate or manage the various functions of the system or process
  • Computer or programmer: A separate device used for interfacing and programming the system or components is often provided, particularly for PLCs and VFDs. This is usually in the form of a door-mounted keypad programmer or adjacent computer and monitor.
  • Field device wiring: In most cases, a control panel for water-related functions must provide an interconnection method between the panel components and the field devices. This is generally accomplished at terminal boards and can consist of an input or output signal in discrete or analog values. Common types of field devices include microswitches, solenoids, pressure switches or transmitters, flowmeters, and pump motor start/run commands and verification.
  • Specialized or proprietary equipment: Occasionally, specialized equipment is needed to interface the control devices with the process variable. For control over water processes, this is generally performed for flow, pressure, or water level by using analog or discrete signals. Specialty devices such as submersible and gauge pressure transmitters, induction level controls, float switches, or capacitive level sensors are often used for control over pressure or water levels.

Schematics should be prepared to maintain voltage consistency on the drawing. Because there are so many elements to a schematic, a table of contents, bill of materials, and identification key are also recommended.

Figure 6. Typical one-line power diagram.

Design Step No. 4.
Equipment Layout and Wiring

Placement and mounting of the various components in a control panel play a crucial role. The initial use of a temporary template with space allocation for each device will often help to facilitate the panel layout.

To maintain consistency and help in troubleshooting, AC and DC power segregation and distribution across the components in the panel should be consciously designed. An arrangement using terminal boards is used to evenly distribute the incoming power and control circuits across the various components of the control panel.

Power and control wiring should not be daisy-chained from device to device as this wiring arrangement will potentially inhibit troubleshooting and could potentially result in failure of the entire control or power circuit.

For example, a central terminal board is often used to receive power from the main circuit breaker and distribute the power to the various components. High voltage-rated components should be placed near the main incoming power disconnect switch which is usually placed at the top. AC and DC circuits and wiring should be identified and segregated to avoid induced voltage or crossed circuits and troubleshooting errors. DC and AC circuits should be assigned and tagged with unique identification labels to prevent a cross connection and ensure the proper routing of field wiring.

As a better understanding of the control panel functions are met, it is important to understand the quality of each component. Always procure a control panel or its components from a trusted source. Hierarchy in schematics should be followed. Accordingly, the breaker for the specified power type and level, distribution breakers, fuses, and terminals should be arranged in an orderly manner.

It is suggested to place them in the same order as here listed. Considering that some components might expand with a temperature rise, adequate spacing should be allowed between each of the components to provide adequate heat dissipation and cooling.

Itʼs important to understand the heat sensitivity of each component and those with higher sensitivity should be placed away from high voltage or current rated components. I/O terminals and PLC racks are suggested to be placed below the power distribution components to allow the passage of heat and help maintain a stable temperature within the control panel.

In addition to the placement, there are a few more points to consider.

  • Labeling: Make sure the enclosure and all wires, power supplies, breakers, and other panel components are properly identified and labeled. Usually abbreviations, prefixes, and numbers are used corresponding with the line number, PLC address, and terminal group, which helps in identification.
  • Intra-panel spacing: As discussed above, enough space should be provided between components to avoid excessive heat concentration. While designing a panel, ensure adequate space both horizontally and vertically for effective placement of all equipment exists using proper arrangement of wires.
  • Organizing wireways: The right amount and type of wireway ensures a good control panel design. This will guarantee convenience in terminating internal wiring to panel components and terminal boards for remote device interface, leaving adequate room for NEC wire bending radius requirements, plus any additional wires that may be added at the time of or post installation. This will also ensure that enough room exists for field I/O wiring and panel wiring to be routed to the I/O terminals.

Figure 7: Typical VFD schematic with remote terminal board connections.

Sources of Electrical Interference and Control Panel Layout

When assembling a control panel, the layout and separation of control devices of differing voltage (EMF) types and values and signal strengths is a critical factor. Since power voltage is generally 240- or 480-volt, single- or three-phase and control voltage is typically 120-volt, single-phase AC or 12- or 24-volt DC, the possible propagation of strong magnetic fields and high-frequency waves is not easily controllable.

They often create local disturbances in the region in which the monitoring and control equipment must be able to reliably operate. This is particularly true for sensitive DC electronic sensors and components that operate at analog values below 100 mA or 50VDC and high-speed digital signals.

All analog signals are potentially susceptible to electrical interference and a voltage signal is certainly no exception. Devices such as motors, relays, high current feeders, and noisy power supplies can induce voltages onto signal lines that can degrade a voltage sensor signal.

All electrical equipment located in this loop will be passed through by a current identical to the original current. Its energy may be significant if the winding is formed by a power cable and may be superimposed onto an adjacent low voltage device. In these situations, use of a resistor-capacitor (RC) network to shunt the inductive reactance may be necessary.

There are numerous ways of ensuring process electromagnetic compatibility. The dedication of panels by power class is the most efficient measure to obtain an excellent result. Moreover, separate routing of disturbing and sensitive cables ensures minimum coupling potential. A metal raceway ensures equipotential bonding of the panels and efficient conduction of LF and HF interference.

Analog sensor signals and data flows are also sensitive to interference, thus shielded cables should always be used to convey them. These cables are also used to execute variable speed drive/motor links to generate less interference. If high-power and low-power devices are not segregated without taking precautions, and if cables of different kinds are routed in the same raceway, serious malfunctions are likely.

Partitioning of the panel into two zones, power and low level, is another alternative. A metal partition will be able to further improve the isolation by confining each zone of power. However, even on installations in perfect condition, a flow of 60 Hz current can be observed on certain earth conductors due to the presence of stray current. This current can be as much as several amperes at a few millivolts if the conductor is sufficiently long. This current can interfere with low power analog circuits (4-20 mA or 0-10VDC sensor lines, etc.) if they are wired without taking adequate precautions.

Control Panel Testing and Verification

Upon completion of the control panel, testing for determining and verifying functionality and safety parameters should be initially conducted at the assembly location. This is a critical step, particularly to ensure there are no safety issues that could potentially cause physical harm as well as verify functionality under controlled conditions.

The initial verification testing conducted before energization should consist of a megohmmeter test between the incoming line terminals and enclosure to verify a dead-short does not exist, particularly to high voltage power circuits.

Depending on the specific functions and equipment used in the panel, extending this test to downstream electrical equipment—such as branch circuit protection devices, transformers, motor controls, isolated or sensitive power supplies and electronics, and environmental regulation equipment (i.e., enclosure heaters and cooling fans)—may be warranted. This is for protection against potentially damaging faults and surges that could destroy sensitive equipment as much as personnel safety.

The tests for power to ground isolation should be repeated once the panel has been installed and wired in the field. These tests should always be conducted before the application of power. Although simulation testing and calibration of field parameters is often desirable, operational testing and system proving should be conducted using the actual field instrumentation and conditions.

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This wraps up this edition of The Water Works. We will introduce several methods of control over a water system with multiple sources in the October 2023 issue of Water Well Journal.

Until then, keep them pumping!

Learn How to Engineer Success for Your Business
 Engineering Your Business: A series of articles serving as a guide to the groundwater business is a compilation of works from long-time Water Well Journal columnist Ed Butts, PE, CPI. Click here for more information.

Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.

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