5 Steps to choosing the right signal line Surge Protection Device (SPD) for your application

The right Surge Protection Devices (SPDs) help ensure continuous operation of critical systems, even in the event of a direct lightning strike.

While power SPDs provide protection against surge voltages on the electrical installation, damage via connected metallic signal and telecommunication lines is often overlooked.

Electronic systems are in nearly every aspect of our lives at work and at home. Because we rely on the continuous and efficient running of computers, electronic process controls and telecommunications networks, protection of these systems from damage is essential. One of the most common causes of system damage is transient overvoltages, which are short duration, high magnitude voltage peaks with fast rising edges often referred to as surges. Although lightning strikes are the most common source of extreme transient overvoltages, electrical and electronic equipment are also continually stressed by hundreds of transients that occur every day on the power supply network through electrical switching.

Transient overvoltages, whether caused by lightning or by electrical switching, have the potential to cause similar negative effects such as system downtime, data loss, damage to critical equipment and, in extreme cases, loss of life. Industry standards such as IEC/BS EN 62305 detail protection measures against lightning and switching transients on metallic service lines using SPDs. Additionally, IEC 60664 electrical installation standards and local country installation regulations such as the UK’s IET Wiring Regulations, require all new electrical system designs and installations, as well as alterations and additions to existing installations, to be assessed against transient overvoltage risk. Where surge protection is required, SPDs are selected and installed in accordance to these regulations.

To help with this selection, here are five key things to consider:

1. How many signal line SPDs are needed?

A typical signal and telecommunications line SPD is designed to protect two wire systems – one pair of live cables and a screen/cable shield, if present. For the system, two SPDs are needed – one SPD connected at each end of the signal line to protect the connected equipment. If the system has three wires present such as a Resistance Temperature Detector (RTD) measurement sensor, two 2-wire SPDs are needed at each end, leaving an extra, unused (or redundant) pair. To avoid this redundancy, save cost and space, Furse® offers a range of SPDs to accommodate many wire systems. If the system has three wires, the Furse SL (Slim Line) 3-wire series or RTD range (see more information here) is available. If four wires are present, the traditional solution would provide two, 2-wire SPDs on each end, for a total of four. Furse offers the Furse Q (Quad) which has 4-in-1 at both ends, saving cost, space and installation time given its fast fit, screwless or Push Terminal (/PT) option (more information here).

If an odd number of lines need protection, in a typical installation, one channel at each end will be unused. If one of those lines is 0 Volts or earth/ground, it may be possible to route it via the ‘S’ (screen) terminal of the SPD. The ‘S’ or screen terminal is also used to terminate the screen. Pairs of wires can be grouped together (i.e., TX+ and TX-).

2. Choose the correct voltage rating of the SPD to best protect the system

A role of the SPD is to ensure its voltage protection level or let-through voltage is low enough to safeguard equipment against damage. Equally important is that the SPD’s working or nominal rated voltage must be high enough, so the SPD doesn’t treat the nominal signal operating voltage itself as a transient and interfere with system operation. By factoring in the system’s damage threshold, if known, the SPD can be chosen based on its voltage protection level.

When choosing SPD protection, it’s also important to consider the system’s signal peak working or operating voltage requirements. For example, a Furse ESP 30D or ESP 30E are options (more information here) to protect a 2-wire system with a 52 Volts damage threshold that has a working or operating voltage of 5 Volts. This is because the ESP 30D and ESP 30E protection level voltages of 43.4 Volts & 44.3 Volts respectively, and are below 52 Volts, and their working voltages of 30 Volts are above the 5 Volts system operating voltage; therefore, neither will impede the signal operating voltage.

But damage thresholds are rarely known, so SPDs are usually chosen to equal or be slightly higher than the system’s normal peak working voltage. For example, a 6-Volt system can be protected with a Furse ESP 06D or ESP 06E and both a 12-Volt system and a 15-Volt system can be protected with a Furse ESP 15D or ESP 15E. This SPD allows for system tolerances and prevents disruption of the signal transmission if actual voltage is higher than expected. Refer to the system manufacturer or manual and relevant industry standards to determine the appropriate operating voltage.

3. Is the SPD line resistance a problem?

SPDs insert resistance into the signal line of the protected equipment. If resistance is too high, the signal level is reduced and function can be disrupted. The Furse ESP E Series SPD prevents this by inserting only 1 Ω into each line (more information here). If the SPD has two channels, it will introduce double this resistance into a circuit. When surge protection is required at both ends of a circuit, SPDs are necessary at both ends and resistance is added at both ends. With a Furse ESP E series SPD at each end of a circuit, the overall circuit loop resistance will only be 4 Ω.

  • Consult the system manufacturer’s documentation for an acceptable loop resistance value.
  • Perform an ‘ohms law’ voltage drop calculation (V=IR) based on the system current ‘I’ present, and the resistance ‘R’ introduced into the circuit, as a result of introducing the SPD. The resistance is acceptable if the voltage drop ‘V’ is negligible relative to the signal level voltage.
  • Compare the resistance to cable resistance in the system. If the resistance of the SPD is negligible relative to the cable resistance that is present, introducing the SPD is unlikely to make any difference to the system.

4. Is the maximum operating current and bandwidth of the SPD being exceeded?

It’s a fact that heating occurs when current flows through the resistance of the SPD. The maximum operating current of the SPD should never be exceeded and is based on a conservatively acceptable amount of heat to ensure safe, continuous operation.

Additionally, SPDs insert series resistance and shunt capacitance into a circuit. These form a low-pass filter with a range of frequencies, known as bandwidth, within which a signal can be transmitted. Outside of this bandwidth, an SPD will interfere or stop signal transmission. No problems occur if the bandwidth of the SPD exceeds the system’s maximum signal frequency by at least a factor of ten. If the factor is less than ten, a higher bandwith SPD may be required.

Keep in mind, if the SPD is inserted into a non-traditional 50 Ω system, its actual bandwidth will change. Non-sinusoidal signals may contain frequencies higher than expected. Conservatively, we specify the -3 dB bandwidth for our Furse SPDs in a 50 Ω system for a sinusoidal signal.

5. Choose the correct termination, enclosure and approvals

The final consideration is to evaluate the SPD environment to determine the termination, enclosure and approvals required.

Telephone signal lines typically terminate on a Private Branch eXchange (PBX) type LSA-plus disconnection frame. The Furse ESP KT Series is a good solution since it provides for easy plug-in and cost effective protection (more information here).

Screw terminals are the most common for general signal line terminations, and increasingly fast-fit screwless push terminals are used in many applications. Furse ESP PCB SPDs provide protection directly on the circuit board to have discrete protection, such as military applications (more information on our PCB protectors here).

SPDs sometimes have exposed terminals that should be in an enclosure to protect them from dirt, water and other elements for electrical safety purposes. This protection can be provided within the instrumentation panel or kept within a Furse ESP WBX weatherproof box enclosure, made of polycarbonate and which carries a minimum IP56 rating (more information here).

Finally, ensure that your SPD has been extensively tested and meets the correct local standards, including UL certification in the United States and ATEX certification in Europe.


Safeguard critical electrical systems from transient overvoltages

The above key considerations can help ensure every signal line SPD you choose offers simple system integration, is cost-effective and achieves the highest levels of surge protection to protect your data and telecommunications equipment against transients.

For more information on the Furse range of SPDs, visit here.

Note: This guidance does not ensure compliance with relevant global safety standards or guarantee equipment is protected against transient overvoltages. The electrical specifier should use their own judgment, consulting recognized safety standards (IEC 60364 on electrical installation of buildings, IEC/BS EN 61643 on low voltage surge protection devices (SPDs) and IEC/BS EN 62305 on protection against lightning) to determine the correct selection and installation of SPDs.
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About the author

Samad Khan

Samad Khan is a Chartered Engineer graduated from Loughborough University in 1996 with a Masters degree in Electrical and Electronic Engineering. Since then, Samad has worked for the ABB Installation Products Division supporting the Furse lightning protection product solutions and building upon Furse’s over 125 years legacy. His roles have included R&D, global field sales engineering, project management, marketing and specialist technical support. Samad is the BSi principle UK expert for Surge Protective Devices (SPDs) since 2001, representing the national committee in European and international meetings for the development of lightning protection standards including the application of SPDs.
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