Surge Protection for Measurement and Control Systems
Designing Systems for Easy Integration and Expandability
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 highperformance 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
Surge protection for measurement and control systems – easy to integrate
outages in any industrial sector
and expandable as required.
generally entail substantial
financial loss. Many demands are placed on surge protection devices (SPDs) in
measurement and control systems [1].
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, by contrast, 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.
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 signaling lines.
The obvious solution is to put the remote
indication and auxiliary power equipment into a
separate external controller. Doing this is highly
Fig 1: Using the carrier rail connector, it is possible to
beneficial for the protection design, since
expand the surge protection for the measurement and
controllers can be used to supply auxiliary
control system into a protection system that can be
installed quickly and error-free.
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 new Plugtrab PT-IQ
product family uses a T-Bus as a carrier rail connector to minimise the power supply and
status indication installation costs (Figure 1).
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 (Figure 2).
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 [1].
Users must differentiate between the types of
standardised pulses that were used to
Fig 2: The pluggable surge protection devices have
continuous status monitoring of the protective elements
and provide protection for up to five signaling lines.
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, welldocumented SPDs ideally carry specifications for a number of standard pulses. The pulse
strength for various lightning protection zones is provided in [2]. 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.
The schematic diagram of a two-stage surge protection device (see sidebar figure) 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.
Sidebar Figure: Two-stage surge protection devices how do they work?
The limiting diode (TVS) has a fast response
characteristic in the range of a few ns, thus
ensuring a low protection level. However, the
performance of available diodes is limited to a
pulse performance of a few kW. The maximum
diode pulse current for 24-volt devices is
approximately 100 A. To discharge currents of
several kA, a gas discharge tube (GDT) is
Two-stage circuit for measurement and control
arresters – operational principle.
required. Depending on the slope of the current
pulse, the GDT triggers at voltages of more than 100 V. Commutation resistors (R) are
used to build this voltage. If the current flows through the terminal In 1, it flows through the
upper resistor, then through the diode, and then through the lower resistor to the terminal
In 2. The sum of the voltages at the diode and the resistors grows until the gas discharge
tube triggers and creates a low-resistance path through which the major portion of the
pulse energy is drained off.
Summary
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 easily integrated into the plant’s
protection equipment design and expanded as required.
Sources:
[1]
IEC 61643-21: “Low-voltage surge protective devices – Part 21: Surge protective devices connected to
telecommunication and signaling networks – Performance requirements and testing methods”
[2]
IEC 61643-22: “Low-voltage surge protective devices – Part 22: Surge protective devices connected to
telecommunication and signaling networks – Selection and application principles”
If you are interested in publishing this article, please contact Becky Smith:
bsmith@phoenixcontact.com or telephone 0845 881 2222.