Protecting AC Power Lines in Industrial Applications

What may not be apparent in a seemingly optimal industrial environment are the many potential electrical issues that can threaten the reliability of industrial-based power systems and communication lines. 

Behind the scenes, industrial applications frequently connect to large motors, high-power heaters, a complex lighting infrastructure and a host of other heavy electrical loads that, when switched, can cause the equivalent of mini-lightning surges on power and communication lines.  On top of this, nearby lightning events can occur potentially causing damaging electrical surges onto power and communication lines. And, industrial settings in remote locations or at higher elevations only increase the probable frequency and intensity of these surges.

These issues are not restricted to large factories -- even a simple industrial application can experience these problems.  Many of today’s industrial sensors and communication / control ports may not be designed to protect sensitive core electronic circuits.  While some equipment is equipped with adequately “hardened” ports, other equipment will require enhanced port protection in the harshest of industrial environments.

Circuit protection is a little like buying insurance.  Better protection will cost more, so design engineers must balance requirements for enhanced up-time and reliability against budgetary constraints with the understanding that unplanned downtime can be quite costly.  Achieving the best outcomes – and hardening the equipment before a failure – comes down to selecting the optimal protection solution from the range of options available today.

AC Power Input Protection

The power grid in an industrial environment often acts like a huge antenna. It has a tendency to collect all manner of noise and energy spikes from the switching of large electrical loads, or from the induced voltages of neighboring lines and equipment or nearby lightning events.  Inverter drives and variable frequency drives can be especially problematic.  The frequency and intensity of these surges can greatly exceed those found in commercial and residential settings.

Commercial and consumer equipment are most often protected from overvoltage events by small Metal Oxide Varistor (MOV) devices.  These devices are usually effective in the relatively benign consumer and commercial settings, but can age very quickly in an industrial environment.  MOVs are under stress whenever they are connected to the AC line.  In industrial applications, this can mean 24/7 duty.  High temperatures and humidity can significantly increase the aging.  (Most MOVs are only rated to 85ºC, which is not adequate for most factory use!)  The noise and spikes on the line – even if they pose no real threat to the equipment –accelerate the aging of the MOV.  Aging manifests itself as increased leakage, which causes the MOV to warm, further increasing the leakage current.  Eventually the MOV will fail-short in a runaway thermal event.

Premium MOV devices are available with temperature ratings up to 105ºC or even 125ºC.  Designers typically specify voltage ratings well in excess of the expected line voltage to extend the life of the MOV.  Often, they will also specify higher surge ratings to further guard against premature aging.  Such overspecification can result in higher clamping (protection) voltages that make the voltage protection less effective. Plus, downstream equipment must be “hardened” to higher voltages resulting in increased costs for higher voltage-rated components.

There are hybrid protectors now available that integrate a GDT in series with the MOV.  The GDT effectively keeps the MOV disconnected from the AC line until a significant voltage surge occurs so it is not aging while disconnected.  This type of solution is ideal for applications in high temperature and high humidity areas.  When a surge that could threaten the equipment occurs, the GDT triggers and reconnects the MOV for the duration of the surge.  Properly designed, this arrangement also allows designers to reduce the voltage margin as temporary overvoltage events will not trigger the GDT.  This, in turn, allows lower voltage downstream components to be selected.

For superior voltage clamping on an AC power line, designers typically specify solid-state Power TVS (PTVS) devices.  These rugged bidirectional silicon diodes offer precise clamping without aging.  Certain suppliers, such as Bourns, offer PTVS devices with ratings up to 15 kA and breakdown voltages of 470 V providing excellent protection on industrial 277 VAC lines.

This article has been focused thus far on overvoltage protection as overcurrent protection is often managed at the branch circuit breaker level.  Should local fusing be required, the selected fuse must be surge rated at the same level or higher than the overvoltage protection scheme to avoid nuisance tripping during surge events.

Control / Sense / Communication Line Protection

Control, sense or communication lines can be grouped into a general “signal line” category.   Some older systems use 120 VAC signaling – those would be protected more along the lines of the AC protection outlined above.  Modern industrial signaling uses DC voltages, 4-20 mA current loops or RS-485 protocols. 

Protection needs to be customized to the protocol used based on three key considerations:

1.       Voltage range of the signal

Signal transceivers are designed for a certain operating signal voltage range.  Those same transceivers are capable of tolerating voltages beyond the operating range before damage occurs.  Circuit protection must not conduct in the operating signal range and must keep voltages below the damage threshold level of the protected equipment.

2.       Bandwidth of the signal

Signal transceivers also have designated bandwidth.  Circuit protection must not present a significant impedance across the protected line that could reduce the range of the signal, especially when they must operate over long distances.

3.       Power capability of the signal

Some signals are impressed on power lines – either AC or DC.  PLC (Power Line Communication) systems are an example of AC while 20 mA current loops and USB lines are examples of DC.  When power is present, care must be taken to choose a protection scheme that won’t interfere by latching up or otherwise interrupting the power.

MOV devices are generally not the first choice to protect signal lines.  They do not have the precise clamping voltages demanded by signal lines where the operating voltage range and damage threshold are not far apart.  Instead, silicon solutions are generally preferred. 

The common choices are TVS diodes and TISP thyristor devices.  TVS diodes can be unidirectional or bidirectional.  Protection is achieved from clamping or limiting voltages by lowering their impedance once their breakdown voltage is exceeded.  Thyristors are voltage-operated switches.  When their trip voltage is exceeded, they switch to become a near short circuit, shunting incoming surge energy to ground.  They turn off and return to their high impedance state when the current through them is reduced below their holding current threshold.

The advantage of the TVS diode clamping function is that it cannot latch up – so TVS diodes are a common choice on lines with DC power present.  TVS diodes are available in tightly-spaced voltage ranges so that protection can be tailored with precision. 

Thyristors are often a better choice when latch-up is not a consideration either because no power is present or the current available is less than the holding current.   For a given protection voltage and package size, thyristors will have far less capacitance and better surge capability.  Table 1 shows a comparison of devices suitable for use on nominal 12 Volt signal lines using Bourns parts as examples.

For challenging RS-485 installations, transient blocking unit (TBU) devices are robust protection solutions.  Bourns TBU High-Speed Protectors are FET-based devices that can react quickly as faster-than-lightning resettable fuses.  For example, the Bourns Model TBU-RS integrates a TBU HSP with a TVS device allowing designers to customize protection for RS-485 protocol voltages.

Click image to enlarge

Figure 2: The recommended Bourns TBU HSP protection circuit.  The block labeled “PHY” represents the RS-485 transceiver.  Also shown is a line termination resistor.  The Bourns Model TBU-RS uses a set of TVS diodes that protect the 12 V/ 7 V maximum voltage signals allowed by the RS-485 protocol.  These TVS diodes assure protection of the PHY under surge conditions.  When excessive appears across one or both TVS diodes (to ground), the diodes will begin to conduct.  This action will trigger the TBU device elements into their blocking state, protecting the TVS diodes from damage
 

To complete the protection scheme, the primary protector is then chosen to protect the TBU HSP elements from overvoltage during a surge event.  There are a number of primary protection options to balance device cost with surge withstand capability, including MOVs and the hybrid GDT+MOV devices covered earlier. 

As shown in this article, there are a wide variety of advanced circuit protection options that need to be considered for AC power and signal lines. With these helpful design tips, there are ample available solutions developed to keep industrial system designs operating reliably even in certain harsh environments. 

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