Electronic Design
Protect PLCs from Transient Surges

Protect PLCs from Transient Surges

  Using careful layout and the proper protection circuitry as well as properly designed isolation can ensure years of uninterrupted service life for industrial control systems.

 

Industrial automation relies on programmable logic controllers (PLCs) that have been hardened against the harsh environments where they are installed. Much of the protection is mechanical to guard against the elements such as extreme temperatures and humidity. However, unseen elements such as electromagnetic interference (EMI), electrical fast transient (EFT) bursts, and electrostatic discharge (ESD) can generate large voltage transients that can destroy unprotected equipment.

Background

PLCs have been around since the Jacquard Loom was invented in 1801, using punch cards to control hooks that wove patterns into fabric. Much later, microprocessors entered the workforce and electronics replaced mechanical controls. Small industrial computers used programming languages designed to simplify process and machine controls, revolutionizing manufacturing.

It was also quickly recognized that the physics that produced the semiconductors could lead to their demise in industrial applications. These dangers arise from transient surges of current caused by various electromagnetic events found in industrial environments. Surges can induce voltages on other cables, causing large voltages to appear on circuits that were never designed to handle them. Damage to semiconductor devices often is inevitable if the absolute maximum levels are exceeded.

Surge Types

Several types of transient surges can damage electronic equipment. The most common, ESD, is caused by the buildup of charge on people or equipment. When discharged, extremely large voltages can be produced, damaging or destroying semiconductor components. Damage can be caused by natural phenomena such as lightning or by simply wearing insulating shoes while walking around and then touching equipment—especially connectors.

EFT bursts occur when an inductive load such as a motor or solenoid is disconnected by relays or switches where the contacts are “opened” to stop the flow of current. Inductors store energy, and when the circuit is opened, the current will continue to flow as the inductor sheds energy:

V = L(di/dt)

where V is in volts, L is in Henrys, i is in amperes, and t is in seconds.

At the moment the circuit is opened, di/dt can become extremely large (theoretically infinite). This results in extremely high voltages across the air gap of the relay contacts (or semiconductor switch). In the case of relay or switch contacts, the voltage can grow large enough to ionize the air and form an arc.

The arc has negative resistance, so as current flows, it will reduce the resistance, once again closing the circuit momentarily until the arc extinguishes. This may go on for many cycles, each causing extremely large voltage spikes (Fig. 1). The drive circuits are protected against this reverse EMF pulse. However, cables often are bundled together, and these spikes will appear across all the wires in common mode.

1. An electrical fast transient (EFT) burst (top) repeats until the energy in an inductive load dissipates. This can take many cycles and continue for many milliseconds.

So even though the aggressor signal (the originator of the surge) is protected and on a completely different circuit, the EFT burst can induce voltages onto other (victim) wires capable of destroying the electronics where they are connected. It is very common to run both load-carrying cables along with signal wiring, which can be exposed to these large spikes.

Unlike ESD exposure, these transients occur continuously during normal operation. Whenever a switch, relay, or other control opens the circuit of an inductive load, these types of spikes will occur.

Other Sources of Surges and Damaging Currents

Lightning, user induced ESD, EFT bursts, and other EMI sources pose threats to I/O modules. Yet other sources such as ground differentials caused by transients and other static sources can damage interface and sensitive measurement electronics.

What is considered “ground” is completely relative to the measurement. “Ground” to a power supply is local to the return connection. But often, the grounds between equipment can be many volts apart due to resistance in the ground path—in severe cases hundreds of volts or higher. Resistance in the earth ground causes this ground differential, which can be static or dynamic.

In Figure 2, current (IG) flowing to ground will encounter a resistance such as soil or poor earth ground connections. As current flows through this resistance, a voltage appears across it, moving the ideal ground (0 V) to something different (higher or lower depending on polarity).

2. Current flowing from V1 to ground through RG can move the ground potential between two points (VG1 and VG2), creating a differential voltage between grounds. This can reach hundreds of volts or higher in transient conditions.

Lightning-induced currents are transient and can shift ground differentials with extreme voltages. Even though these surges are transient, they can do significant damage due to their high currents. This is especially true in cabling where the shield or neutral wire is in use (not floating).

Protection Strategies

 

Standards such as IEC61000-4-4 stipulate the levels that equipment must withstand on cabling entering a chassis. These standards can test connections with transients of 4000 V (Fig. 1, again). Since both ESD and EFT bursts have overall low energy, these pulses are best clamped to ground using transient voltage suppressor (TVS) diodes.

These diodes have very low capacitance and should be placed as close to the point of entry as possible, near connectors or headers where wires are terminated. For differential signals, a TVS diode should be placed in common mode (across the differential pair). There should be adequate connections to the ground plane to prevent surges from manifesting additional loop currents within the printed-circuit board (PCB), resulting in additional large voltage spikes.

To protect against ground offsets, isolation is the only answer. Isolation can be accomplished in several ways. Transformer coupling found in isolated power supplies works well to allow the system ground to “float” relative to the earth’s ground. Transformers are only good for low-frequency isolation and require an ac signal to cross the isolation barrier. For dc control or high-frequency applications such as communications, optical isolators are useful.

But in many applications, the LEDs will degrade with age due to elevated operating temperatures, which is something to avoid in high-reliability industrial control systems. The photodiode receiver section of the isolator often is limited in frequency response, limiting the maximum operational data rate through the device.

Alternatively, semiconductor isolators such as the ISO7131CC provide galvanic isolation through capacitive coupling within the device. They can provide the more than 2500 VRMS of isolation required by many standards and data rates in excess of 40 to 50 Mbits/s.

The advantage of solid-state devices is the absence of the LEDs used for optical isolation that can degrade and lose performance over time. These devices also allow both sides to be completely isolated with independent grounds and VCC connections.

Conclusion

Designers of industrial-grade PLC systems need to worry about the mechanical environment where a system will be installed, in addition to the electromagnetic environment. There are potential damaging surges created by a multitude of events such as lightning, ground loops, ESD, and EFT bursts.

Designers must consider both the standards to which they are building and the real-world events that can damage systems exposed to these environments. Using careful layout and the proper protection circuitry as well as properly designed isolation can ensure years of uninterrupted service life for industrial control systems.

References

• Download the ISO7131CC datasheet

• Programmable logic controllers (PLCs)

Electrostatic discharge (ESD)

• Electrical fast transient (EFT)

Richard Zarr is a technologist at Texas Instruments focused on high-speed signal and data path technology. He has more than 30 years of practical engineering experience and has published numerous papers and articles worldwide. He is a member of the IEEE and holds a BSEE from the University of South Florida as well as several patents in LED lighting and cryptography.

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