In Part 1 of this two-part series, we explored PLC basics and voltage sag susceptibilities. In this segment, we present PLC voltage sag test results along with guidelines for making PLC systems more robust to voltage sags. The tests, conducted by EPRI and funded by the California Energy Commission (CEC), helped establish PLC baseline performance guidelines for improving system compatibility. These results are complementary to similar tests conducted by EPRI in 1995.

Voltage Sag Test Results

EPRI conducted voltage sag tests on five common PLCs. The PLCs (referred to as A, B, C, D, and E) were subjected to voltage sags in a test setup using a portable sag generator with and without power conditioning. The setup included additional relays, power supplies, and motor starters to make the load of the system reach a target of 2A.

To characterize the operation of the PLCs during a power quality disturbance, a PQ test algorithm was programmed into each PLC. The program monitored the status of various AC input module channels and controlled the status of various AC output channels. A general-purpose control relay was wired to an AC output module channel, and an AC input module channel monitored relay status. The written program provided the latching logic to hold the output relay “on” after receiving the appropriate input.

If the relay momentarily opened as a result of the voltage sag, and the input channel detected the opening of the relay, the program latch would drop the output module signal. The test device recorded this condition as a system failure since such an event could easily upset an automated process. But if the PLC power caused a shutdown as a result of the sag, the device noted the condition as a CPU failure.

Without Power Conditioning

Excluding PLC D, the responses of the remaining PLC power supplies are more robust than the overall system response (denoted as system failure). These units appear to ride through the voltage sag based on the output voltage of the DC power supply rather than the AC input voltage. This result means the PLC CPU may continue to operate even if the voltage sag has disturbed the I/O and field devices. Such an event in a process control system can lead to a possible malfunction or process shutdown.

Unlike the remaining units, PLC D forced a shutdown when a 2-cycle, 78%-of-nominal (or less) voltage sag occurred. This PLC shuts down based on the AC input voltage. Denoted as CPU failure, this response ensures the PLC will shut down before the control system malfunctions.

In comparison to earlier EPRI tests, the I/O racks contained fewer I/O modules (six I/O modules in earlier tests versus two modules in this series of tests). Therefore, the PLC DC power supply modules in these tests were more lightly loaded.

With Power Conditioning

EPRI repeated voltage sag tests on the five PLCs when the power source underwent conditioning. Test conductors used a constant voltage transformer (CVT), two offline UPS units, and one online UPS to mitigate voltage sags.

CVT test results. All PLCs exhibited superior voltage sag ride-through with the 500VA CVT power conditioner installed. On average, induced shutdown levels on the five PLCs' power supplies improved from an average of 62.6% of nominal without power conditioning down to 23.2% of nominal voltage with the CVT. Furthermore, the system failure shutdown level dropped from an average of 73.6% without power conditioning to an average of 33.4% of nominal with the CVT in place.

UPS test results. The results of the offline and online UPS tests were very different. During the voltage sag, the two offline UPS units used in the project produced a “simulated” sine-wave output. Furthermore, the offline units required about 4 ms (¼ cycle) to switch over to the battery source. Typically, this duration is not critical since most control equipment can withstand voltage sags of such short duration.

Although the two square-wave output UPS units kept the PLCs powered and averted a CPU failure, some of the PLC 120VAC input cards could not resolve the square wave. This led to the PLC detecting logic level “0” instead of logic level “1.” As a result, the output relay was dropped when the UPS kicked on and the inputs could not be resolved. In all, three out of the five PLCs tested could not resolve the square wave on the AC input card and experienced system failures as soon as the UPS transferred. On the other hand, the tests found the line-interactive UPS to be compatible with all PLCs tested, allowing continued operation even in a complete power outage.


Voltage sags as short as one cycle can affect PLCs, leading to the shutdown of automated control systems and the loss of production time and money. However, proper hardware and software integration leads to vast improvements in the response of these systems to power disturbances.

When attacking voltage sag-related problems, it is important to ensure that the PLC and the I/O control power are robust. Techniques such as using power conditioners for the AC source voltage or employing a robust DC control power source provide effective mitigation. In addition, you must carefully select power conditioning devices to ensure the entire system will not be made less compatible as a result. Power conditioning devices that produce square-wave outputs are not compatible with all PLC systems. Square-wave outputs are available on many battery-based UPS systems as well as on newer technology capacitor-based power conditioners.

Top Ten Guidelines For Improving PLC Voltage Sag Performance

  1. Avoid mismatched control power voltages. If the actual PLC system nominal voltage is lower than the expected nominal input voltage, the control system will be more susceptible to sags. Such mismatches can occur when control power transformers are tapped low or a 230VAC input PLC power supply is connected to a 208VAC source. For relays and contactors, a mismatch of 10% of voltage equates to an increase in susceptibility by 10%. But in DC power supplies, the energy stored in the internal capacitors can be as much as 18% lower when the input voltage is mismatched by as little as 10%—directly equating to a reduction in ride-through time.

  2. Provide a robust power source for PLC power supply and I/O control power. Ensuring the PLC power supply response to voltage sags will be robust without considering the I/O control power is only a partial fix. Although the PLC CPU may survive voltage sags, the system is still likely to suffer process upsets. Therefore, you must consider the control power and I/O power.

  3. Consider using DC to power the PLC and I/O. EPRI tests have confirmed that using a DC power scheme for the PLC power supply and I/O control power is an ideal embedded solution for solving voltage sag-related shutdowns. The best way to design the system with this approach is by the integrator, since the PLC power supply must be specified as a DC input type. However, you must specify the I/O modules, sensors, relays, solenoids, and motor starters for DC control voltages. In systems were the I/O control voltage is already DC, the solution is as easy as replacing the AC input power supply module with the comparable DC input power supply module. It is important to ensure the DC power source, typically 24VDC, is robust as well.

  4. Use universal input switching power supplies, wired phase-to-phase. Typically, the universal input-type power supply has a voltage range of 85VAC to 264VAC. When connected phase-to-phase in a 208VAC system, the power supply can continue to operate down to 41% of nominal. Specify this type of supply for DC-powered instrumentation, I/O control voltage, and external DC PLC power supplies.

  5. Do not overload DC power supplies. Since the amount of voltage sag ride-through time available from a typical linear or switch-mode DC power supply is directly related to the loading, DC power supplies should not be running at maximum capacity. Oversizing by at least two times the expected load will help the power supply ride through voltage sags. This is only critical for systems that do not use a universal input power supply wired in a phase-to-phase configuration.

  6. Use a robust control relay for the master control relay (MCR). The importance of selecting robust control components for the MCR circuit cannot be understated.

    When used in the safety circuits or as subsystem power contactors, the selection of the MCR can make a large difference in the control system's ability to survive voltage sags. Before installing a relay or contactor into the design scheme, the integrator should bench test the units for voltage sag immunity. One way to improve the MCR circuit is to avoid general-purpose “ice cube” relays as the MCR because they are too sensitive to voltage sags—use a small 4-pole contactor instead. In general, a small contactor can survive voltage sags as low as 40% to 55% of nominal.

  7. Properly maintain PLC battery. Many PLCs utilize lithium-ion batteries to maintain their control programs and nonvolatile memory data in the event of a power loss or voltage sag-induced shutdown. Such a loss of the PLC program may result in extended downtime because you'll need to locate the latest backup, then reload and restart the process. Since the active process data will be lost in this situation, scrapping of product may be inevitable.

  8. Use a state-machine programming method and/or nonvolatile PLC memory. When properly coded, this type of programming ensures the system will not lose its place in the event of a voltage sag/outage and will result in quicker restart times.

  9. Consider the power source for analog input signals. For analog signals, ensure the source is robust. If you use 2-wire transmitters, the DC power supply should be lightly loaded or naturally robust, as noted in guidelines 4 and 5. If you use 4-wire transmitters, consider providing power conditioning for the AC voltage source.

  10. Only use compatible power conditioners. A properly sized CVT or line-interactive UPS greatly enhances a PLC system's ability to ride through voltage sags. Avoid power conditioners with square-wave outputs since the AC input module channels on the PLC may not be able to resolve the square-wave signal. Only use square-wave output power conditioners with the PLC manufacturer's assurance that the PLC power supply and I/O cards are compatible.