Surge Protection among electronic circuits and Fieldbus Systems

This application note discusses suitable surge protection techniques to
protect electronic circuits and equipment within the fieldbus system and
the associated fieldbus trunk from high voltages and surge currents
induced by lightning and other forms of transients.
Most process control or telemetry installations are interconnected by
power and signal cables and busses which run on trays, in ducting,
underground or via overhead poles. Lightning strikes, static discharges and
induction from power cabling are typical sources of transient voltages that
can be coupled into signal cables and hence transmitted to electronic
equipment. Field transmitters and computerised control equipment, etc.
containing low-power semiconductor devices can be damaged by
overvoltages of only tens of volts.

The longer the cables, the higher the magnitude of high voltage transients
through shifts in earth potential, therefore devices controlling or
monitoring events in remote locations are more likely to suffer from
overvoltages and consequent component failures. Significant damage can
also be found in equipment connected by relatively short cables if the
circuits or components are particularly sensitive.

Electronic systems can be damaged or disrupted by what are referred to
as “surges”. These are voltages which are much greater than the normal
working voltage and which appear in a system such as fieldbus for a short
period of time, and hence are also sometimes referred to as “transient
over-voltages”. These surges can arise from switching of nearby heavy
electrical equipment or from the clearance of an electrical short circuit
fault (e.g. by a fuse blowing), but the most potent source is lightning.
It is important to appreciate that although catastrophic damage can
indeed result from a direct lightning strike to a building, this is relatively
rare. More usually is the substantial damage to electronic components
inflicted by a strike to ground within a distance in the order of a kilometre
or so. This can produce a surge on cables feeding vulnerable electronic
equipment, resulting in damage. Typical damage to a circuit board consists
of such items as scorched and vapourised copper track, burned and opencircuit
resistors, integrated circuits with part of their package blown away,
and semiconductor junctions failed short-circuit. At a lower level, there can
be latent damage to semiconductors, which subsequently fail perhaps
months later, as can happen with electrostatic discharges.
Surge protection consists of the use of hardware devices, increasingly
termed surge protection devices (SPDs, see glossary for other terms),
which, correctly positioned and installed, limit surge voltages reaching
protected equipment to a safe level.

FIELDBUS BACKGROUND
Users of industrial process control systems are now able to specify proven
and standardised fieldbus technology for their field instruments. Buses
conveying both communication signal and power on the same pair of wires
are among the most popular. An international standard physical layer used
by Foundation Fieldbus and Profibus PA is specified by IEC 61158-2,.

A major influence on the economy of fieldbus installations is the number of
field devices which can be connected on a single bus segment and hence
to a single controller input/output (I/O) channel. In particular applications,
users may choose to use only a small number of devices on a segment, in
order to achieve faster scan cycles or for reasons of system integrity. In
most cases, economic considerations lead to the desire to connect as
many devices as possible on to a single cable and fieldbus port.

The IEC 61158-2 specification allows for up to 32 devices to be connected
to the single pair of wires forming a bus segment (see Appendix 1, Table 1),
most practical installations to date have between 6 and 12 bus-powered
devices. There may be limitations depending on the design of the host
system, but in practice, in IS (Intrinsically Safe) circuits the need to supply
power to all of the devices limits the possible number of bus-powered
devices on one segment.

DC voltage drop in the fieldbus cable reduces the supply voltage to the
most remote devices (see Appendix 2, Table 2), and the situation requires
greater consideration in Intrinsically Safe (IS) systems. Intrinsically Safe
power supplies typically provide lower voltage and some versions include
current-limiting resistors which contribute significantly to the voltage drop.
Section 5 shows where careful selection of surge protection devices
increases the availability of the fieldbus system and reduces / eliminates
the adverse effects of surges.

WHAT IS A SURGE PROTECTOR?
Electronic equipment can be protected from the potentially destructive
effects of high-voltage transients. Protective devices, known by a variety of
names (including ‘lightning barriers’, ‘surge arrestors ‘, ‘lightning protection
units’, etc.) are available. The ‘correct’ name (accepted internationally) is
‘surge protection devices’ or ‘SPDs’ – and this nomenclature is used
throughout this publication.

Surge protection devices should ideally operate instantaneously to divert a
surge current to earth, and control voltage to a level, which will not damage
the connected equipment. Once the surge current has subsided, the SPD
should automatically restore normal operation and reset to a state ready
to receive the next surge.

Atlantic Scientific specialise in the design and manufacture of SPDs.
The range of products available includes models for virtually all applications.

They are based on a combination of gas discharge tubes (GDTs), voltageclamping diodes, and metal-oxide varistors (MOVs) which feature rapid
operation, accurate voltage control and automatic resetting once the
overvoltage has ceased.

The working voltage of a surge protection device is the normal working
voltage of the application without affecting the circuit in which it is placed.
It is also the maximum voltage between lines or from line to earth for the
specified leakage current.

Limiting voltage is a measure of how good a surge protection device is at
removing surges. Also known as ‘Let-through Voltage’, this is the peak
output voltage after injection of a test impulse from a 6kV/3kA 8/20µS
combination waveform generator or some other specified voltage and
current.
A good limiting voltage should be not much higher than the working voltage
of the device. In general, however, it is usually around twice the working
voltage.

A correctly selected surge protection device should not change the
characteristics or reliability of an application, whether it is for the
protection of AC power systems, signal data systems such as fieldbus and
4-20mA, aerials (antennae) or telephony and communications systems.
The magnitude of lightning discharges around the world have been
measured from 2kA to more than 200kA, with rise times to peak current
of less than 10µs.
IEC 61024 gives the following data:
1% of strokes exceed 200kA
10% of strokes exceed 80kA
50% of strokes exceed 28kA
90% of strokes exceed 8kA
99% of strokes exceed 3kA