Surge protection for measurement and control systems

Measurement and control systems are vulnerable to transient surges and therefore require robust lightning and surge protection to avoid plant outages.

In addition to providing high-performance protection circuits and equipment status monitoring, it is especially important to ensure that surge protection systems are easily installable and expandable.

Ensuring production plant availability is becoming increasingly important. Plant outages in any industrial sector generally entail substantial financial loss. Many demands are placed on surge protection devices (SPDs) in measurement and control systems.

Lightning and surge protection requirements

In order for lightning and surge protection equipment to do its part in ensuring production facility availability, the SPDs must have high-performance protection circuits. Moreover, the protection systems also have to be monitored. It is not sufficient just to report an outage after it has already occurred; preventive information about the status of the installed SPDs is also required.

The facilities to be protected are typically widely distributed. It is thus advantageous to be able to report and monitor the status of SPDs remotely to avoid having to perform constant on-site inspections of the protection systems. The additional effort required to install remote monitoring systems should incur as little time and cost as possible.

There are numerous types of SPD status monitoring systems on the market. These systems must continuously monitor the status of the SPDs. Most systems only report surge protection failures. The new Plugtrab PT-IQ family of surge protection systems provides a status report when protection devices have reached their performance limit. A yellow LED indicates this status.

Installing protection systems quickly and error-free

In large production plants or facilities that are difficult to access, such as offshore wind power installations, performing on-site visual inspection of the surge protection equipment is time-consuming and costly. In such situations, it makes sense to use remote indication contacts to transmit the SPD status.

The floating contact design allows users to select the transmission signal of their choice. Other components make it possible to integrate the remote indication contacts into intelligent reporting systems using cellular telephony or other wireless technologies.

In addition to providing high-performance protection circuits and equipment status monitoring, it is especially important to ensure that surge protection systems are easily installable and expandable.The latest technology integrates the remote indication contacts into the housing design of the surge protection device.

Doing this requires using connection terminals that could have been used for protecting other signalling lines. The obvious solution is to put the remote indication and auxiliary power equipment into a separate external controller.

Doing this is highly beneficial for the protection design, since controllers can be used to supply auxiliary power to multiple SPDs, making possible a group indication for the connected SPDs. This frees up all SPD connections to protect the signal lines.

Up to five signal lines can be connected to the SPD, thus providing space and cost savings. The Plugtrab PT-IQ product family uses a T-Bus as a carrier rail connector to minimize the power supply and status indication installation costs.

Protection system easy to expand

If maintaining plant expandability is an important consideration, the surge protection system should also be uncomplicated and expandable without requiring a great deal of installation effort.

Such protection systems are also advantageous for handling last-minute planning modifications, which occur frequently nowadays.

The T-Bus design also allows the protection system to remain flexibly expandable in these situations. To integrate additional SPDs into the protection design, the T-Bus can simply be placed onto the carrier rail and connected to the other connectors. Additional SPDs can then be installed on it.

The challenges in detail

When selecting SPDs, it is important to consider not only the protection level but also the pulse discharge capacity. These properties, as well as many others, are established through standardised tests in accordance with the product standard.

Plant outages in any industrial sector generally entail substantial financial loss. Many demands are placed on surge protection devices (SPDs) in measurement and control systems.Users must differentiate between the types of standardized pulses that were used to determine the protection level. For example, due to the internal design, the protection level for a C2 pulse of 10 kA is significantly higher than the protection level for a C3 pulse of 50 A.

Since the expected pulse strength often depends on the installation location, well-documented SPDs ideally carry specifications for a number of standard pulses.

Measurement and control installations have small cable cross-sections, meaning that the resistance per unit of length (Ω / m) is relatively high. Comparatively moderate current pulse peaks of a few kA may therefore be assumed.

However, the devices to be protected against surges are more sensitive than line-powered devices. The protection level of the SPDs must therefore be as low as possible. A good protection level can be achieved using voltage-limiting diodes (TVS – transient voltage suppressors).

The current-carrying capacity of these devices is limited to several amperes. Gas discharge tubes (GDTs) provide good current-carrying capacity, but some require a relatively high trigger voltage of over 100 V.

Two-stage surge protection devices combine the advantages of both components in one device: the low protection level of the TVS diode and the high current-carrying capacity of the GDT.

Surge protection for measurement and control systems: How to designing systems for easy integration and expandability

The schematic diagram of a two-stage surge protection device illustrates this: the limited current-carrying capacity of the protective diode has large resistors (R) that create a large voltage drop. These resistors are in the signal path and influence the signal.

A resistance value that is too high also causes elevated losses in the applied signal. The losses and the low current-carrying capacity of TVS diodes meant that earlier two-stage devices were limited to low rated currents. By precisely matching the individual components, modern SPDs can achieve signal currents up to 1 A.

This makes it possible to protect not only devices having low rated currents such as sensors, but also typically low-resistance actuators such as valves, contactors, and motors. Smaller signal path losses make thermal management at the installation site easier and more economical.

Since each current pulse causes wear to the gas discharge tube as a function of the pulse strength, the increased current-carrying capacity of the protective diode and the protective resistors help extend the service life of the SPD. This is possible because the diode covers a larger current pulse amplitude range.


The requirements for the highest level of production plant availability call for high-quality, high-performance surge protection products that are easy to install. These products must provide system-dependent advantages over standard products.

Phoenix Contact’s new Plugtrab PT-IQ product family now offers a surge protection system for all current measurement and control application areas, which can be integrated into the plant’s protection equipment design and expanded as required.

[This feature was supplied by Phoenix Contact.]