TTI Inc. has submitted this post. Written by Lee Bourns, Director of Circuit Protection Technical Marketing at Bourns.

Today’s industrial applications use advanced sensors, high bandwidth communications and intelligent control systems, but they still must contend with potential electrical events that can cause reliability and uptime issues in industrial power systems and on communication lines. 

In factories, it is a common practice to connect power and communication applications to large motors, high-power heaters, a supportive lighting infrastructure and other types of heavy electrical loads that, when turned on, can cause the equivalent of mini-lightning surges. These internally-induced threats are in addition to real nearby lightning events that can cause damaging electrical surges. And if the industrial power and communication equipment is housed in a remote location or at a high elevation, then there is a higher probability these surge threats occur more frequently and with increased intensity. 

These issues affect all types of industrial applications, not just systems in large factories. Therefore, it is a critical requirement for industrial sensors and communication control ports to protect sensitive core electronic circuits. While some equipment is designed with “hardened” ports, other equipment does need enhanced port protection for high reliability operation in extreme industrial environments.

Superior protection will cost more, so it is important to balance an application’s uptime and reliability requirements against budgetary constraints taking into consideration the costs associated with maintenance calls and warranty issues. The right approach to hardening an industrial application before a failure falls on designers to select the optimal protection solution from the range of options available today.

AC Power Input Protection

The power grid in an industrial environment 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 equipment or nearby lightning events. The frequency and intensity of these surges can greatly exceed those found in commercial and residential settings.

Metal Oxide Varistor (MOV) devices are popular overvoltage protection solutions. They are typically most 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. And most MOVs are only rated to 85 °C, which is not adequate for many industrial applications. 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 and can eventually lead to a thermal runaway event.

Higher-end MOV devices are available with temperature ratings up to 105 °C or even 125 °C. To extend MOV life, designers typically specify voltage ratings well in excess of the expected line voltage. They will also select MOVs with higher surge ratings to guard against premature aging. The problem with this overspecification is that it can result in higher clamping (protection) voltages that make the voltage protection less effective. Plus, downstream equipment must be “hardened” to higher voltages that further increase design costs.

A solution for this issue is with a new generation of hybrid protectors that integrate a gas discharge tube (GDT) in series with the MOV, as in the Bourns IsoMOV. In this protector, the GDT effectively keeps the MOV disconnected from the AC line until a significant voltage surge occurs to prevent premature aging.  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.

Another option that provides superior voltage clamping on AC power lines are Power Transient Voltage Suppressor (PTVS) devices. These rugged bidirectional silicon-based diodes offer precise clamping without aging. Bourns’ PTVS portfolio offers ratings up to 15 kA and breakdown voltages of 470 V as an optimal protection solution for industrial 277 VAC lines.

Note that overcurrent protection is often managed at the branch circuit breaker level, but if local fusing is required, then the fuse must be surge rated at the same level or higher than the overvoltage protection scheme. This is to avoid nuisance tripping during surge events.

Protection for Control, Sense or Communication Lines

In industrial applications, control, sense or communication lines are grouped into a general “signal line” category. Today’s industrial signaling uses DC voltages, 4 – 20 mA current loops or RS-485 protocols where protection often needs to be customized to the protocol used. There are three key considerations when selecting the proper protection:

1. Voltage range of the signal: Signal transceivers are designed for a certain operating signal voltage range and are capable of tolerating voltages beyond the operating range before damage occurs. The circuit protection solution must keep voltages below the damage threshold level of the protected equipment and must not conduct in the operating signal range.
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 for operation over long distances.
3. Power capability of the signal: Some signals are impressed on power lines, either AC or DC.  Power line communication (PLC) 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 not the best choice to protect signal lines as they do not have the precise clamping voltages demanded by signal lines. Instead, silicon solutions such as TVS diodes or Bourns’ TISP thyristor devices are preferred. TVS diodes are clamping or voltage limiting devices that can be unidirectional or bidirectional. They offer protection by lowering their impedance once their breakdown voltage is exceeded. Thyristors, on the other hand, are voltage-operated switches. When their trip voltage is exceeded, they switch to become a near short circuit, shunting incoming surge energy to ground.  Thyristors 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, making TVS diodes a common choice on lines with DC power present. Bourns offers TVS diodes in tightly-spaced voltage ranges so that protection can be tailored with precision. 

Thyristors are a good 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.

For challenging RS-485 installations, Bourns’ transient blocking unit (TBU) High-Speed Protectors (HSPs) are robust protection solutions. These FET-based devices are able to react extremely quickly. 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.

To complete this 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 Bourns IsoMOV devices. 

For industrial AC power and signal line protection, designers have a broad range of advanced circuit protection options available to them. The design tips provided in this article are meant as a helpful resource for the development of highly reliable and efficient industrial systems. A knowledgeable circuit protection design strategy will help ensure industrial power systems and communication lines achieve their maximum uptime goals even in certain harsh environments.