9.4 Lightning and surge protection retrofitting for
sewage plants
Resources of drinking water running short require
a more efficient treatment. Therefore, sewage
plants play a central role in the circle of drinking
water. The necessary high efficiency of sewage
plants (Figure 9.4.1) requires the optimisation of
the operating procedure at a simultaneous reduction of the running operating costs. For this purpose, considerable financial efforts were made for
electronic measuring equipment and decentralised
electronic control and automation systems in
recent years. Compared to conventional technology, however, the new electronic systems provide
only a low resistance against transient surges. The
structural conditions of the spacious open-air
plants with wastewater treatment technology and
the spread measuring devices and controls
increase additionally the risk of interferences due
to lightning discharges or surges. Thus, a failure of
the complete process control system or parts of it,
is highly probable to expect, if no protective measures are taken. The consequences of such a failure
can be far-reaching. They can reach from the costs
for the recovery of the system function to the
undefinable costs for the removal of ground water
contamination. In order to come up to this threat
effectively and increase the availability of the systems, external and internal lightning protection
must be provided.
Lightning protection zones concept
In order to obtain the best technical and economical protection, the sewage plant control is divided
into lightning protection zones (LPZ). Subsequently, a risk analysis is carried out for each LPZ and for
the relevant types of damage. For the risk analysis
acc. to IEC 62305-2 the software tool DEHNsupport
can be used. Lastly, the mutual dependences of the
LPZs are examined and the finally required protection measures are defined in order to reach the
necessary protection aim in all lightning protection zones. The following areas were assigned
lightning protection zone LPZ 1 and lightning protection zone LPZ 2:
⇒ Electronic evaluation system in the control
room (LPZ 2)
pumping draw works
rough / fine rake
rain overflow basin
ventilation / sand /fat catcher
black water basin
sewage plant control
primary sedimentation tank
precipitant tank
sedimentation tank
outlet
activated sludge basin
nitrification – denitrification
Fig. 9.4.1 Schematic structure of a sewage plant
244 LIGHTNING PROTECTION GUIDE
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sewage plant control
230 V
DG MOD 275
DEHNguard
DG MOD 275
N
DEHNguard
L
PE
measuring point
1’
3’
1’
3’
1’
3’
1’
3’
1’
3’
protected
protected protected protected
2’
2’
2’
4’
2’
BXT ML4 BE 24
4’
BXT ML4 BE 24
2’
BLITZDUCTOR
BXT ML4 BE 24
BLITZDUCTOR
4’
BLITZDUCTOR
4’
BXT ML4 BD
EX 24
BLITZDUCTOR
4’
BXT ML4 BE 24
BLITZDUCTOR
protected
2
4
2
4
2
4
2
4
2
4
1
3
1
3
1
3
1
3
1
3
MCS
O2-value
Fig. 9.4.2 Division of a sewage plant control into lightning protection zones
⇒ Oxygen measurement in the aeration tank
(LPZ 1)
⇒ Interior of the control room (LPZ 1)
According to the lightning protection zones concept of IEC 62305-4 (EN 62305-4), all conductors at
the LPZ boundaries must be provided with appropriate protective measures against surges, (Figure
9.4.2).
Risk assessment for the sewage plant control
The following example was calculated by using IEC
62305-2 (EN 62305-2). It should be pointed out
that the procedure is only described as an example. The solution presented is in no way binding
and can be replaced by any other equivalent solutions. The following states only the essential characteristics of the example.
First, a questionnaire with relevant questions on
the structure and its utilisation was discussed with
the operator and fixed in writing. This proceeding
ensures the elaboration of a lightning protection
concept that is comprehensible for all parties
involved. This concept represents then the minimum requirements, which, however, can still be
technically improved anytime.
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Site description
The complete process control of the sewage plant
is situated centrally in the sewage plant control.
Characterised by the extended cable connections
to the measuring stations as well as substations,
considerable partial lightning currents and surges
are imported by these lines into the control rooms
at a lightning strike. In the past, this resulted again
and again in destruction of the installation and
system failures.
The same applies to the power supply line and the
telephone line (Figure 9.4.3).
The sewage plant control itself shall be protected
against damage by fire (direct lightning strike),
and the electric and electronic systems (control
and automation system, telecontrol) against the
effects of lightning electromagnetic pulses (LEMP).
Additional conditions
⇒ Protective measures against effects of lightning actually are already existing (external
lightning protection, surge protective devices
(SPD), (previously class B), type VGA 280/4 at
the service entrance of the 230/400 V power
supply line, SPD, (previously class C) type
LIGHTNING PROTECTION GUIDE 245
fixed telecommunication network
230 / 400 V power supply
sewage plant control
O2 value
1’
L
3’
measuring point
N
protected
4
3
DG MOD 275
DEHNguard
DG MOD 275
BXT ML4 BE 24
2
1
DEHNguard
4’
BLITZDUCTOR
2’
PE
230 V supply
4 - 20 mA
Fig. 9.4.3 Electrical lines going into the sewage plant control
VM 280 in the switchgear cabinets of the measuring and control system).
(this is the same result as stated in VdS publication 2010)
⇒ The following types of damage are relevant:
L2 for loss of services (water supply and water
disposal) and L4 for economic losses (buildings
or structures and their contents). Type of damage L1 (loss of human life) was excluded, since
the installation should run fully automatically
in future operation.
⇒ Installation of SPDs Type 1 according to EN
61643-11 (power supply) and SPDs, category
D1 according to IEC 61643-21 for the data processing lines (data lines of the measuring and
control system and telecommunication lines)
The result after calculating the actual state is that
the calculated risk R for L2 for loss of service is still
well above the tolerable risk RT .
Now, possible protective measures are initiated in
order to obtain R < RT whereas with respect to L4
loss of economic values the most cost effective
solution has to be selected:
⇒ Installation of a lightning protection system
Class III according to IEC 62305-3 (EN 62305-3)
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⇒ SPD Type 2 according to EN 61643-11 (power
supply) and surge protective devices, category
C2 according to IEC 61643-21 for the data processing lines (data lines of the measuring and
control system and telecommunication lines)
Lightning protection system
The existing lightning protection system of the
sewage plant control was upgraded in accordance
with the requirements of lightning protection systems Class III (Figure 9.4.4). The existing, indirect
connection of the structures mounted on the roof
(air conditioning systems) via isolating spark gaps
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αϒ
80
70
60
50
Class of LPS
40
30
I
20
II
III
IV
10
0
0 2
10
20
30
40
50
60 h[m]
room, no more partial lightning current can flow into
the structure and cause damage. Due to the size of the
control room (15 m x 12 m),
the number of down conductors (4) did not have to
be changed. The local earthing system of the sewage
plant control was checked
at all measuring points and
the values were recorded.
Also, no upgrades had to be
made here.
Lightning equipotential bonding for all cables entering
from the outside
In principle, all conductive systems entering the
sewage plant from the outside must be integrated
into the lightning equipotential bonding (Figure
9.4.5) The requirements of lightning equipotential
bonding are fulfilled by direct connection of all
Fig. 9.4.4 Protective angle method according to IEC 62305-3 (EN 62305-3)
was removed. The protection against direct lightning strikes was realised by means of air-termination rods in compliance with requested separation
distances and protective angles. Consequently, in
the case of a direct lightning strike into the control
lightning equipotential bonding
external lightning protection system
EBB
power
supply
water
gas
Z
cathodic protected tank pipe
foundation earth electrode
Fig. 9.4.5 Lightning equipotential bonding according to IEC 62305-3 (EN 62305-3)
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LIGHTNING PROTECTION GUIDE 247
do not have to be expected, even at a failure. For
the application of surge protective devices, the
cross section of the earth conductor for equipotential bonding must be minimum 6 mm2 Cu for SPDs
for power supply systems, and minimum 4 mm2 Cu
for SPDs for information technology. Moreover, in
areas with potentially explosive atmospheres the
connections of the equipotential bonding conductors must be secured at e.g. equipotential bonding
bars against self-loosening (e.g. by means of spring
washers).
Fig. 9.4.6 DEHNventil installed into a switchgear cabinet for protection of the power supply system
metal systems and indirect connection of live systems via surge protective devices. The SPD Type 1
(power supply system) and the SPD Type D1 (information technology) must have a lightning current
discharge capability of test waveform 10/350 μs.
The lightning equipotential bonding shall preferably be installed near the entrance into the building or structure in order to prevent a penetration
of lightning currents into the inside of the building.
Equipotential bonding
In the entire sewage plant control, a consistent
equipotential bonding is carried out according to
IEC 60364-4-41 and IEC 60364-5-54. The already
existing equipotential bonding is tested to avoid
potential differences between different as well as
extraneous conductive parts. Also, supporting
parts of the building and parts of the construction,
pipelines, containers, etc., are included in the
equipotential bonding, so that voltage differences
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Surge protection for the low-voltage power
supply
In the described application, the SPD type VGA
280/4 installed at the service entrance of the building is replaced by an SPD Type 1 DEHNventil M TNS
255 (Figure 9.4.6), since the “old” SPD does no
more comply with the requirements for lightning
protection systems according to IEC 62305-3
(EN 62305-3). The SPDs Type 2, (previously class C),
Type VM 280, were tested with an arrester test
unit, type PM 10. Since the test values were still
within the tolerances, there was no reason to
remove the SPDs. If further SPDs are installed for
protection of the terminal equipment as in the
present case, they must be coordinated among
each other and with the terminal equipment to be
protected. The corresponding instructions given in
the enclosed installation instructions must be
observed.
Otherwise, the use of surge protective devices in
low voltage consumer's installations shows no
peculiarities compared to other applications and
has already been described many times (for more
information, please also see publication DS649 E
“Surge Protection – Easy Choice”).
Surge protection in data processing systems
From the protection point of view, the transfer
interface of all data processing lines to the sewage
plant is the service entrance. At this point SPDs
(category D1) type DRL 10B 180 FSD are used,
which are capable of carrying lightning currents.
From the transfer interface, the cables are led
directly to the switchgear cabinets and are connected there. In accordance with the performed
risk analysis, the incoming cables must be led via
SPDs, types DCO RK ME 24 (20 mA signal) or DCO
RK MD 110 (telecontrol). These are suitable for use
in the lightning protection zones concept (catego-
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Fig. 9.4.7 DCO ME 24 surge protective device installed into a
switchgear cabinet for protection of the complete
measuring and control system
Fig. 9.4.8 DCO ME 24 surge protection device installed into a
switchgear cabinet, incoming lines from double bottom
ry C2), and are system compatible (Figures 9.4.7
and 9.4.8).
This ensures a complete surge protection concept
for the data processing cabling.
Additional applications for protection of sewage
plants can be found in publication DS107 E.
This can be downloaded from our website:
www.dehn.de.
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