CMT MANUAL
1.6 EARTHING AND PROTECTION
TABLE OF CONTENTS
INDEMNITY ............................................................................................................................................ 5
DOCUMENT INFORMATION.................................................................................................................... 5
ACKNOWLEDGEMENTS .......................................................................................................................... 5
1.6.1 INTRODUCTION ............................................................................................................................. 6
1.6.2 EARTHING ..................................................................................................................................... 6
1.6.2.1 THE EARTHING ROUTE ................................................................................................................... 6
1.6.2.2 EARTH IMPEDANCE ........................................................................................................................ 7
1.6.2.3 DIFFERENT VOLTAGES .................................................................................................................... 7
1.6.2.4 WIRE SIZING ................................................................................................................................. 10
1.6.2.5 WIREWAYS ................................................................................................................................... 10
1.6.2.6 EARTH WIRING ............................................................................................................................. 10
1.6.2.6.1 EARTH WIRING - LV – 400 VAC.................................................................................................. 10
1.6.2.6.2 EARTH WIRING - LV – 230 VAC.................................................................................................. 12
1.6.2.6.3 EARTH WIRING - ELV – 24 VAC.................................................................................................. 12
1.6.2.6.4 EARTH WIRING – 1 to 40 VDC ................................................................................................... 13
1.6.2.6.5 EARTH WIRING – COMMUNICATION ........................................................................................ 15
1.6.2.7 CENTRAL EARTHING POINT – EARTHING BUSBAR ....................................................................... 16
1.6.2.8 THE EARTH TERMINAL ................................................................................................................. 17
1.6.2.8.1 FOUNDATION ELECTRODE ........................................................................................................ 17
1.6.2.8.2 RING EARTH ELECTRODE ........................................................................................................... 17
1.6.2.8.3 VERTICAL OR HORIZONTAL EARTH ELECTRODES ...................................................................... 17
1.6.2.9 THE METHOD OF USING HORIZONTAL EARTH ELECTRODES (EARTH RODS) ............................... 18
1.6.3 SURGE PROTECTION .................................................................................................................... 26
1.6.3.1 SURGE PROTECTION – FIRST LAYER ............................................................................................. 26
1.6.3.2 SURGE PROTECTION – SECOND LAYER - DISTRIBUTION BOARD AND IRRIGATION SWITCH
BOARD ...................................................................................................................................................... 26
1.6.3.3 SURGE PROTECTION – THIRD LAYER ............................................................................................ 26
1.6.3.3.1 UPS – GENERAL DESCRIPTION .................................................................................................. 26
1.6.3.3.2 LINE-INTERACTIVE UPS ............................................................................................................. 28
1.6.3.3.3 DOUBLE CONVERSION ON-LINE UPS ........................................................................................ 31
1.6.3.3.4 SURGE PROTECTION - POWERLINE PROTECTOR ...................................................................... 33
1.6.3.3.5 SURGE PROTECTION – COMMUNICATION ............................................................................... 33
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1.6.3.3.6 LIGHTNING PROTECTION – BUILDING....................................................................................... 33
1.6.3.3.7 LIGHTNING PROTECTION – WEATHER STATION ....................................................................... 33
REFERENCES ........................................................................................................................................ 37
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LIST OF TABLES
TABLE 1. MINIMUM EARTH WIRE SIZE .................................................................................................... 11
TABLE 2. DC SENSOR WIRES – DIGITAL INPUT SENSORS ......................................................................... 14
TABLE 3. SENSOR WIRES – ANALOGUE INPUT SENSORS ......................................................................... 14
TABLE 4. FACTOR λ FOR RODS IN A STRAIGHT LINE FOR USE IN EQUATION 2 ........................................ 21
TABLE 5. FACTOR λ FOR RODS ARRANGED IN A SOLID SQUARE FOR USE IN EQUATION 2 ..................... 21
LIST OF EQUATIONS
EQUATION 1. SOIL RESISTIVITY ................................................................................................................ 20
EQUATION 2. EARTH RESISTANCE OF TWO OR MORE RODS .................................................................. 21
EQUATION 3. ‘α FACTOR’ IN EARTH RESISTANCE EQUATION 2............................................................... 21
LIST OF FIGURES
FIGURE 1. THE EARTHING ROUTE .............................................................................................................. 7
FIGURE 2. SCHEMATIC OF DIFFERENT VOLTAGES IN A TYPICAL IRRIGATION AND CLIMATE
CONTROL SYSTEM ...................................................................................................................................... 8
FIGURE 3. SCHEMATIC HIGHLIGHTING THE 400 VAC WIRING POSITIONS .............................................. 10
FIGURE 4. SCHEMATIC HIGHLIGHTING THE 230 VAC WIRING POSITIONS .............................................. 12
FIGURE 5. SCHEMATIC HIGHLIGHTING THE 230 VAC WIRING POSITIONS .............................................. 12
FIGURE 6. SCHEMATIC HIGHLIGHTING THE DC WIRING POSITIONS ....................................................... 13
FIGURE 7. SCHEMATIC HIGHLIGHTING THE COMMUNICATION WIRING POSITIONS .............................. 15
FIGURE 8. SCREENED TWISTED PAIR - STP CABLE .................................................................................... 15
FIGURE 9. SCREENED SHIELDED TWISTED PAIR CABLE CROSS SECTION ................................................. 16
FIGURE 10. SCREENED UNSHIELDED TWISTED PAIR / FOILED TWISTED PAIR CABLE CROSS
SECTION ................................................................................................................................................... 16
FIGURE 11. DRIVING AN EARTH ROD WITH STRIKING CAP INTO THE GROUND...................................... 18
FIGURE 12. AN EARTH ROD COUPLER ...................................................................................................... 19
FIGURE 13. EARTH WIRE CLAMPED TO AN EARTH ROD .......................................................................... 19
FIGURE 14. EXAMPLE OF EARTH RESISTANCE VS. NUMBER OF RODS .................................................... 20
FIGURE 15. SOIL RESISTIVITY AND EARTH RESISTANCE EXAMPLE IN AN MS EXCEL SPREADSHEET. ....... 25
FIGURE 16. THE CURRENT FROM THE PROVIDER TO THE CONTROLLER ................................................. 27
FIGURE 17. THE CURRENT FROM THE PROVIDER TO THE CONTROLLER VIA A UPS ................................ 27
FIGURE 18. BASIC COMPONENTS OF A LINE-INTERACTIVE UPS .............................................................. 28
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FIGURE 19. LINE-INTERACTIVE UPS OUTPUT WITHOUT GOING TO BATTERIES ...................................... 29
FIGURE 20. LINE-INTERACTIVE UPS OUTPUT, WHEN GOING TO BATTERIES ........................................... 30
FIGURE 21. DOUBLE CONVERSION ON-LINE UPS – BASIC COMPONENTS ............................................... 31
FIGURE 22. DOUBLE CONVERSION ON-LINE UPS OUTPUT WITHOUT GOING TO BATTERIES ................. 32
FIGURE 23. A LIGHTNING ROD PROTECTING THE WEATHER STATION ................................................... 35
FIGURE 24. FIXING THE LIGHTNING ROD TO THE WEATHER STATION POLE ........................................... 36
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INDEMNITY
Netafim South Africa (Pty) Ltd has taken all reasonable care in ensuring the integrity and reliability of
the information contained in this document. Despite this Netafim South Africa (Pty) Ltd. takes no
responsibility for any damage or loss that may result from the use of this manual.
This document should be regarded as the property of Netafim South Africa (Pty) Ltd. This document is
not intended for further training and neither may it be reproduced nor copied in its current form or
temporary form. This document may not be revealed and/or carried over to any third party without
the explicit written consent of Netafim South Africa (Pty) Ltd.
This document is presented with the exclusive aim of notifying selected potential clients
regarding Earthing and Protection. Receipt or the possession of this document does not imply
rights and the contents should be viewed as a proposal only. This document is neither issued as
a guarantee, nor does it confirm any legal obligations on Netafim South Africa (Pty) Ltd whatsoever.
Netafim South Africa (Pty) Ltd reserves the right to make changes in its products and in the Earthing
and Protection without prior notice.
DOCUMENT INFORMATION
Version:
1.01
Last updated:
19 Jan 2012
ACKNOWLEDGEMENTS
Netafim South Africa
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1.6.1 INTRODUCTION
There is no such thing as total lightning and surge protection. Although the word ‘protection’ is
commonly used, ‘mitigation’ or ‘risk reduction’ are probably better terms. The word ‘protection’ is
continued to be used here but any protection measures taken reduce the risk; they do not eliminate
it.
Netafim gives suggestions here to reduce the risks where Netafim equipment is involved. It is
necessary though for the broader-based entire electrical system, including non-Netafim equipment,
to have sufficient protection against surges and lightning. For these purposes, it is recommended that
the services of a lightning protection system (LPS) designer are used. An LPS designer or an LPS
installer are able to issue an LPS installation safety report and maintenance certificate according to
SANS 10313. (9)
1.6.2 EARTHING
The cornerstone of surge and lightning protection is earthing. Any surge and lightning protection is of
no value unless there is adequate earthing. For the Netafim equipment in an installation, the earthing
is more involved than the actual surge protection. Once the earthing system has been established,
the surge protection methods are relatively more straightforward.
1.6.2.1 THE EARTHING ROUTE
Voltage surges must be shunted immediately to earth before causing damage to electrical and
electronic equipment. The basis of the earthing is that earth wires should take a route that runs from
each electric component to a central point and thence to earth (7), which itself is usually at one point.
(14)
The earthing route:
•
•
•
•
•
From
Via
To
Thence via
Finally to
- Electric component / equipment,
- Earth wire,
- Central point, (earthing busbar)
- Earth wire,
- Earth electrode.
Except with a lightning rod:
•
•
•
From
Via
To
- Lightning rod
- Earth wire
- Earth electrode
The central point would generally be the earthing busbar in the main electrical switchboard panel and
the earth electrode would generally be at or near the consumer’s earth terminal, which is provided at
the point of the incoming electricity supply. A lightning rod’s earth electrode would be at its base. See
FIGURE 1.
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Electric
component
Electric component e.g. weather
Electric
component station or antenna with lightning rod
Electric
component
Earthing busbar
Earth electrode
Earth electrode for
lightning rod
FIGURE 1. THE EARTHING ROUTE
1.6.2.2 EARTH IMPEDANCE
The route’s earth impedance – the difficulty at which the surge is shunted to earth - must be as low
as possible. This can be enhanced (lowered) by:
•
•
•
•
Large surface areas of conductors. Lightning surges tend to flow over the surface, so the
larger the better.
Lack of sharp bends in conductors – large radii in bends. Minimum radius of 30cm. (14).
Tight clean connections at all connection points.
Large surface area in contact with the earth. In practice this is achieved with deeper, longer
driven earth rods and multiple rods thereof.
1.6.2.3 DIFFERENT VOLTAGES
Alternating current voltages are categorised as follows:
•
•
•
•
0 VAC
50 VAC
1,000 VAC
44,000 VAC
< Extra low voltage (ELV)
< Low voltage (LV)
< Medium voltage (MV)
< High voltage (HV)
(5) (7)
≤ 50 VAC
≤ 1,000 VAC
≤ 44,000 VAC
Direct current voltages are categorised as:
•
•
0 VDC
120 VDC
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< Extra low voltage (ELV)
< Low voltage (LV)
(7)
≤ 120 VDC
≤ 1,500 VDC
Earthing and Protection
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Transformer
Electric supply
board
ELV DC
Analogue inputs
ELV 24 VAC
Analogue inputs
- climate box
Earth
electrode
LV 230 VAC
NMC controller /
dosing unit
ELV DC
Earth wire
LV 400 VAC
LV 230 VAC
Earth wire
Earth wire
LV 400 VAC
LV 230 VAC
ELV 24 VAC
PC Communication
Distribution LV 230 VAC
board
LV 400 VAC
Earth wire
ELV 24 VAC
Analogue inputs
- rain sensor
Digital inputs
Digital outputs
Irrigation
switchboard with Digital outputs
earthing busbar in switchboard
FIGURE 2. SCHEMATIC OF DIFFERENT VOLTAGES IN A TYPICAL IRRIGATION AND CLIMATE CONTROL
SYSTEM
In Netafim’s automation service experience, the lower the voltage, the higher is the risk for lightning
or surge damage. Problems are more likely to arise with communication equipment and controller
commands for instance, than with three-phase motors.
A conductor (wire) with a lower voltage is affected by another conductor that is running parallel to it
and carrying a higher voltage. This is called electromagnetic interference (EMI). (Lightning has a
similar and potentially much greater effect). When taking power to the various electric components /
equipment, differing voltages should be kept separate and not share the same cables or wireways
where feasible. For example, the 230 VAC power to a climate controller should not share the same
cable or bundle of cables as that controller’s analogue inputs for instance, which are 1 to 40 VDC.
Netafim’s equipment may be broadly grouped into 5 voltages.
•
•
•
•
•
Low Voltage Three Phase
Low Voltage Single Phase
Extra Low Voltage
Extra Low Voltage
Communication
– 400 VAC
– 230 VAC
– 24 VAC
– 1 to 40 VDC
400 VAC and 230 VAC are treated much the same, as they are both 230 VAC per phase.
LV – 400 VAC. Motors.
•
•
•
•
•
NMC-Junior FertiKits 400 VAC
NMC Pro NetaJets 3G 400 VAC
NMC Pro NetaJets 3G – HP 400 VAC
NMC Pro NetaJet HF 400 VAC
NMC Pro NetaJet HF - HP 400 VAC
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LV – 230 VAC. Motors. Controllers.
•
•
•
•
•
•
•
•
Transformer 50 VA (400 V/230 V to 24 VAC)
NMC Junior Irrigation Controller 230 VAC
NMC Pro Irrigation Controller (L) 230 VAC
NMC Junior Climate Controller 230 VAC
NMC 64 Climate Controller (L) 230 VAC
NMC-Junior FertiKits 230 VAC
NMC Rain Sensor 230VAC Power Supply
NMC – RS485 PC Communication Unit + Box
ELV – 24 VAC. Commands. Digital outputs. Backflush timers.
•
•
•
•
•
•
AC backflush timers
AquaNet Valves – AC
Aquativ AC solenoid
Baccara 8 Watt NC Solenoid
NMC Inside Temperature & Humidity + Box (fan only)
NMC EC and pH Transmitter – SP
ELV – 1 to 40 VDC. Sensors. Analogue inputs. Digital inputs.
•
•
•
•
•
•
•
•
•
•
•
Digital inputs:
Water Meter Reed Switch White (EV) Only
NMC Rain Collector
NMC Wind Speed and Direction Sensor (Speed only)
Analogue inputs:
NMC Wind Speed and Direction Sensor (Direction only)
NMC Solar Radiation Sensor - Netafim
NMC Solar Radiation Sensor - Davis
NMC CO2 Sensor EE82 0 – 5000 ppm
NMC Inside Temperature & Humidity + Box (except fan)
NMC Outside Temperature Sensor + Shield
NMC Outside Temperature and RH Sensor + Shield
NMC Rain Sensor (excluding 230 VAC Power Supply)
ELV – Communication
•
•
•
•
NMC – RS485 PC Communication Unit + Box
NMC – USR External 56 K Modem
NMC 64/Pro/DC RS485 Card at NMC Controller
NMC Junior RS485 Card for NMC Controller
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1.6.2.4 WIRE SIZING
Wire sizing for live / phase conductors is not dealt with here. However, the earth wire sizes below are
often linked to those of the live / phase wires. The phase wires should be sized according to
SANS10142-1:2008. (7). See the sections 1.6.2.6.1 EARTH WIRING - LV – 400 VAC, 1.6.2.6.2 EARTH
WIRING - LV – 230 VAC and 1.6.2.7 CENTRAL EARTHING POINT – EARTHING BUSBAR.
The maximum voltage drop from the point of supply to the outlet or appliance under design load
shall not exceed 5%:
•
•
Voltage drop for a 400 VAC system ≤ 20.0 V
Voltage drop for a 230 VAC system ≤ 11.5 V
1.6.2.5 WIREWAYS
Buried wireways (sleaves) are preferred over aerial wireways. Where aerial wireways are used inside
or outside a structure, a metal cable tray is preferred. The metal wireway should itself be earthed (7).
1.6.2.6 EARTH WIRING
1.6.2.6.1 EARTH WIRING - LV – 400 VAC
Transformer
Electric supply
board
ELV DC
Analogue inputs
ELV 24 VAC
Analogue inputs
- climate box
Earth
electrode
LV 230 VAC
NMC controller /
dosing unit
ELV DC
Earth wire
LV 400 VAC
LV 230 VAC
Earth wire
Earth wire
LV 400 VAC
LV 230 VAC
ELV 24 VAC
PC Communication
Distribution LV 230 VAC
board
LV 400 VAC
Earth wire
ELV 24 VAC
Analogue inputs
- rain sensor
Digital inputs
Digital outputs
Irrigation
switchboard with Digital outputs
earthing busbar in switchboard
FIGURE 3. SCHEMATIC HIGHLIGHTING THE 400 VAC WIRING POSITIONS
400 VAC only applies to Netafim products where the product contains a three-phase electric motor.
These wires are used from the point of supply to the irrigation switchboard and from there to the
motor’s starter and motor.
South Africa follows the TN-C-S earthing system. Up to the point of supply, there is a combined
protective earth and neutral. This though does not affect the installation of Netafim’s equipment.
Beyond the point of supply, a separate earth and neutral must be run to each appliance. (7)(5).
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From the point of supply to the earthing busbar in the switchboard, for overload protection the
minimum size of the earth wire may be 10 mm2 (7). However for lightning protection the minimum
size of this earth wire is 16 mm2. (10).
From the earthing busbar in the switchboard to the electric motor, the minimum size of this earth
wire is 6 mm2. (12)
However in the case of between the point of supply and the switchboard, it could be larger. If the
earth wire forms part of a flexible cable, its size shall be the same as the same as the other live cores
(7).
If the earth wire does not form part of a cable, then the size indicated in TABLE 1 shall apply. (7)
TABLE 1. MINIMUM EARTH WIRE SIZE
Minimum nominal cross-sectional area of copper continuity conductors
TABLE 1.
10
Rated
current of
protective
device (A)
16
25
35
50
Maximum length of earth
continuity conductor (m)
10
805
16
503
20
2
(mm )
Rated
current of
protective
device (A)
10
16
25
35
50
Maximum length of earth continuity
conductor (m)
63
128
204
319
447
639
805
80
101
161
252
352
503
402
644
100
80
129
201
282
402
25
322
515
125
64
103
161
225
322
32
252
402
629
160
80
126
176
252
40
201
322
503
704
200
64
101
141
201
50
161
258
402
563
250
80
113
161
Notes for TABLE 1
•
•
•
•
NOTE 1. This table is not to be used to determine the maximum length of live conductors
because the voltage drop may be excessive for the current that they carry.
NOTE 2. The values in the table are based on a fault current of 2.5 times the rated current of
the protective device and a touch voltage of 30 V. (Ref. 7. Section 3.81).
NOTE 3. This table applies to over-current protective devices and might not be appropriate to
other types of protective device.
NOTE 4. If the full load current rating of the protective device is non-standard, the maximum
length of the earth continuity conductor shall be taken as that applying to the next higher
standard rating.
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1.6.2.6.2 EARTH WIRING - LV – 230 VAC
Transformer
Electric supply
board
ELV DC
Analogue inputs
ELV 24 VAC
Analogue inputs
- climate box
Earth
electrode
LV 230 VAC
NMC controller /
dosing unit
ELV DC
Earth wire
LV 400 VAC
LV 230 VAC
Earth wire
Earth wire
LV 400 VAC
LV 230 VAC
ELV 24 VAC
PC Communication
Distribution LV 230 VAC
board
LV 400 VAC
Earth wire
ELV 24 VAC
Analogue inputs
- rain sensor
Digital inputs
Digital outputs
Irrigation
switchboard with Digital outputs
earthing busbar in switchboard
FIGURE 4. SCHEMATIC HIGHLIGHTING THE 230 VAC WIRING POSITIONS
230 VAC applies to Netafim controllers and Netafim products where the product contains a singlephase electric motor or electric component such as a rain sensor. These wires are generally used from
the main irrigation switchboard to that electric component. If there is a single-phase supply, they
would also run from the point of supply to the switchboard.
A separate earth and neutral must be run to each appliance. (7)(5).
For lightning protection the minimum size of the earth wire between the earthing busbar and the
point of supply is 16 mm2. From the earthing busbar to the controllers and electric components, the
minimum size is 6 mm2. (12)
1.6.2.6.3 EARTH WIRING - ELV – 24 VAC
Transformer
Electric supply
board
ELV DC
Analogue inputs
ELV 24 VAC
Analogue inputs
- climate box
Earth
electrode
LV 230 VAC
NMC controller /
dosing unit
ELV DC
Earth wire
LV 400 VAC
LV 230 VAC
Earth wire
Earth wire
LV 400 VAC
LV 230 VAC
ELV 24 VAC
PC Communication
Distribution LV 230 VAC
board
LV 400 VAC
Earth wire
ELV 24 VAC
Analogue inputs
- rain sensor
Digital inputs
Digital outputs
Irrigation
switchboard with Digital outputs
earthing busbar in switchboard
FIGURE 5. SCHEMATIC HIGHLIGHTING THE 230 VAC WIRING POSITIONS
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24 VAC wires are always from the controller to a digital output.
•
•
•
•
Relays – Use a 3 core cable earthed to the controller’s earth terminal. Sometimes, the output
is a 24 VAC relay that is mounted inside the same switchboard that provides 230 VAC power
to the controller or 400 VAC to a dosing unit. Where feasible, do not use the same wireway
for the 24 VAC and either of the 230 or 400 VAC. Keep the wireways at least 10 cm apart.
Solenoids – Aquativ AC solenoid / AquaNet Valve – AC. These solenoids do not facilitate an
earth wire. Use a 2-core wire. If possible, lay the wires in a metal cable tray and earth the
cable tray.
Solenoids – Baccara. These solenoids facilitate an earth wire in addition to the live and
neutral wires. Use a 3-core wire earthed to the controller’s earth terminal.
NMC Inside temperature and humidity box fan. The 24 VAC wires to the fan do not facilitate
an earth wire. Use a 2-core cable from the controller but if feasible, keep it separate (at least
10 cm) from the wires to the temperature and humidity sensors, which are DC.
1.6.2.6.4 EARTH WIRING – 1 to 40 VDC
Transformer
Electric supply
board
ELV DC
Analogue inputs
ELV 24 VAC
Analogue inputs
- climate box
Earth
electrode
LV 230 VAC
NMC controller /
dosing unit
ELV DC
Earth wire
LV 400 VAC
LV 230 VAC
Earth wire
Earth wire
LV 400 VAC
LV 230 VAC
ELV 24 VAC
PC Communication
Distribution LV 230 VAC
board
LV 400 VAC
Earth wire
ELV 24 VAC
Analogue inputs
- rain sensor
Digital inputs
Digital outputs
Irrigation
switchboard with Digital outputs
earthing busbar in switchboard
FIGURE 6. SCHEMATIC HIGHLIGHTING THE DC WIRING POSITIONS
Within an AC system, Netafim’s DC equipment mainly comprises of sensors – digital and analogue
inputs and the NMC’s actual CPU and busboard.
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TABLE 2. DC SENSOR WIRES – DIGITAL INPUT SENSORS
DC sensor wires - Digital input
DC leadout cores
With earthing
Water Meter Reed Switch White (EV)
Only
2
No
NMC Rain Sensor + Power Supply +
2
No
230 VAC Power Supply for the NMC
Rain Sensor
NMC Wind Speed and Direction Sensor
– wind speed *
NMC Rain Collector - Including Shelf
No
Includes 1 x earth
wire
2
Additional AC wires to be
kept separate from DC
2 wires x 9 VAC from the
power supply.
2 wires x 230 VAC to the
power supply
No
+ The incoming 230 VAC power to the Power Supply for the NMC Rain Sensor should not share the
same wireway as any DC sensor cable. Keep them at least 10 cm apart.
TABLE 3. SENSOR WIRES – ANALOGUE INPUT SENSORS
DC sensor wires - Analogue input
NMC Outside Temperature Sensor and
Shield
NMC Outside Temp. and RH Sensor and
Shield
NMC Wind Speed and Direction Sensor –
wind direction *
DC leadout cores
2
3
With earthing
Additional AC wires to be
kept separate from DC
Supplied with
shield for earth
Supplied with
shield for earth
Includes 1 x earth
wire
Includes 1 x earth
wire
Supplied with
shield
NMC Solar Radiation Sensor - Netafim
4
NMC Inside Temperature and Humidity
with Box** (except fan***)
4
NMC Solar Radiation Sensor – Davis
3
No
NMC CO2 Sensor EE82 0 – 5000 ppm
4
No
2 wires x 24 VAC power
input for the fan.
24 VAC power supply
*The wind sensor has both digital (speed) and analogue (direction) inputs.
**In the NMC Inside temperature and humidity sensors and box (except fan), each of the sensors has
its own shielded cable terminating inside the box. Join the two sensors’ shields together in one
terminal. To connect to the controller, use one shielded 4 core x 0,5mm2 copper cable for these two
sensors and earth the shield at one end to the two sensors’ shield and at the other end to the earth
terminal at the controller.
***Where possible, the 24 VAC power from the controller to the fan of the temperature and
humidity sensors climate box should not share the same wireway as the above sensors cable. Keep
them at least 10 cm apart.
All shields / earth wires from each sensor should be earthed at the controller. Where the sensor has
neither an earth wire nor a shield, there is little that can be done to protect that sensor other than
where possible, lay the wires in a metal wireway and earth that wireway.
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1.6.2.6.5 EARTH WIRING – COMMUNICATION
Transformer
Electric supply
board
ELV DC
Analogue inputs
ELV 24 VAC
Analogue inputs
- climate box
Earth
electrode
LV 230 VAC
NMC controller /
dosing unit
ELV DC
Earth wire
LV 400 VAC
LV 230 VAC
Earth wire
Earth wire
LV 400 VAC
LV 230 VAC
ELV 24 VAC
PC Communication
Distribution LV 230 VAC
board
LV 400 VAC
Earth wire
ELV 24 VAC
Analogue inputs
- rain sensor
Digital inputs
Digital outputs
Irrigation
switchboard with Digital outputs
earthing busbar in switchboard
FIGURE 7. SCHEMATIC HIGHLIGHTING THE COMMUNICATION WIRING POSITIONS
Communication wiring:
•
•
•
•
•
•
Do not share any communication cable’s wireway with LV or ELV cables. (7). Keep them at
least 10 cm apart.
Screened shielded twisted pair (S/STP) with an earth strand is preferred (see FIGURE 9) or at
least use screened unshielded twisted pair (S/UTP or FTP – foiled twisted pair) (see FIGURE
10).
The twist rates of each pair must be different.
When bending, STP cable has a minimum radius stipulated by its manufacturer.
Earth the STP cable’s earth strand and shield at each end at the communication card or PC
communication unit as the case may be. (See Section 7.1.2.4 NMC Communication Lighting
Protector for details).
The earth route to ground from the communication wires is through their earth terminal in
the NMC RS485 PC communication unit and built-in to its earth terminal for the 230 VAC
incoming power supply. (See Section 7.1.2.4 NMC Communication Lighting Protector for
details).
FIGURE 8. SCREENED TWISTED PAIR - STP CABLE
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FIGURE 9. SCREENED SHIELDED TWISTED PAIR CABLE CROSS SECTION
FIGURE 10. SCREENED UNSHIELDED TWISTED PAIR / FOILED TWISTED PAIR CABLE CROSS SECTION
1.6.2.7 CENTRAL EARTHING POINT – EARTHING BUSBAR
The irrigation and climate system’s main electric board or distribution board must contain an earthing
busbar. It is a collection point for all earth wires from the components of the system, each of which is
linked to this busbar. (7). Unless specified, the manufacturer of the switch board may not necessarily
provide an earthing busbar. (3)
The minimum cross section of copper earth wires from internal metal installations to the earthing
busbar is 6 mm2. (12)
The busbar itself is connected by one copper earth wire to the earth terminal. The minimum cross
sectional area of this wire is 16 mm2. (12). It may not carry any current other than fault currents. See
Earthing – LV – 400 VAC.
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1.6.2.8 THE EARTH TERMINAL
Netafim equipment requires a targeted earth resistance at the earth terminal of 3.0 Ω or less but a
maximum of 5.0 Ω.
There are three types of earth terminal: (8)(5)
•
•
•
Foundation electrode. The reinforced steel network (rebars) of the building’s concrete
foundations and floor.
Ring earth electrode around the outside of the building.
Vertical or horizontal earth electrodes.
o One or more vertical rods.
o One or more horizontal rods or wire “trench earth”.
1.6.2.8.1 FOUNDATION ELECTRODE
The network of rebars in a concrete foundation can be used for lightning discharge currents, but not
for earth fault currents. (8)(11). This type of earth electrode is feasible in a new installation with a
new building, in which case the services of an LPS designer should be used.
If earthed to existing rebars, it is possible that the resistance of 3.0 Ω might not be achieved, in which
case earth rods may need to be added in addition to this electrode.
1.6.2.8.2 RING EARTH ELECTRODE
A ring electrode is a 50 mm2 copper stranded earth wire closed loop around a building, one metre
from the building’s perimeter of which at least 80% is buried, preferably to a minimum depth of 0.5
m. (11). This electrode is more feasible with an existing installation. However as with the foundation
electrode above, it is possible that should the resistance of 3.0 Ω not be achieved, additional earth
rods may needed.
1.6.2.8.3 VERTICAL OR HORIZONTAL EARTH ELECTRODES
Earth rods driven into the ground are the simplest form of earth terminal and the quickest to install.
The standard earth rod supplied by Netafim is 1.2 m long x 14 mm diameter copper-coated steel. (6).
It is necessary that the installer of the rods has an earth resistance meter.
The electricity service provider needs only an earth resistance of 15 Ω. (5). This is insufficient for the
controllers and other electronic equipment.
The target earth resistance for Netafim equipment is 3.0 Ω or less. It is sometimes possible to achieve
this with just one rod but normally, several rods will be required. One keeps adding rods - thus
lowering the earth resistance - until the desired resistance is achieved.
The deeper the rod is buried, the better. Two x 1.2 m rods coupled end-to-end to form one rod driven
2.4 m into the ground will achieve lesser resistance than two x 1.2 m rods placed side-by-side
connected with copper earth wire. However the depth to which the rods can be driven would be a
limiting factor due to soil depth or other physical impediments in the soil. When the extended rod
can be driven no deeper and the desired earth resistance has still not been achieved, it will be
necessary to add further rods adjacent to each other.
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1.6.2.9 THE METHOD OF USING HORIZONTAL EARTH ELECTRODES (EARTH RODS)
Equipment required:
•
•
•
•
•
•
•
•
Copper coated earth rods 1.2 m x 14 mm threaded at each end.
14 mm striking cap.
One 14 mm coupler for each earth rod.
14 mm earth rod clamp.
16 mm² insulated earth wire – for connecting above ground to the electrical system.
16 mm² uninsulated earth wire – for connecting individual earth rods.
Earth resistance meter.
Heavy hammer - 1.5 kg or more.
Fit a 14 mm striking cap onto one rod and drive its entire length into the ground if possible. After
driving the entire length into the ground, measure the rod’s earth resistance. If it is more than 3.0 Ω,
replace the striking cap with a 14 mm coupler and couple a second rod. Fit the striking cap to the new
rod, hammer it 2.4 m into the ground and measure the resistance. Repeat this process until 3.0 Ω is
achieved or one can drive the rod no deeper.
FIGURE 11. DRIVING AN EARTH ROD WITH STRIKING CAP INTO THE GROUND
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FIGURE 12. AN EARTH ROD COUPLER
Any further reduction in earth resistance can only be gained by adding more rods adjacent to each
other and connecting them with buried 16 mm² uninsulated earth wire clamped to each rod. See
Figure 13. The addition of each adjacent rod lowers the earth resistance at a diminishing rate. See the
graph in Figure 14.
FIGURE 13. EARTH WIRE CLAMPED TO AN EARTH ROD
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Space permitting, the distance between adjacent rods must where possible, be at least equal to one
rod’s length. For example, no two earth rods that are 2.4 m long should be closer than 2.4 m apart.
Example of earth resistance vs. number of rods
200
180
160
140
Earth
resistance
in Ω
120
100
80
60
40
20
0
1 rod
2 rods
3 rods
4 rods
5 rods
FIGURE 14. EXAMPLE OF EARTH RESISTANCE VS. NUMBER OF RODS
It may not be feasible to achieve 3.0 Ω as the number of rods required may be uneconomical. If this is
the case, the user would need to judge the cost of the rods versus the resistance achieved.
To this end, it is possible to estimate the number of adjacent rods that would be required, by
calculating the soil resistivity in ohm metres (Ω.m) for the total depth of one vertical rod. (2)
EQUATION 1. SOIL RESISTIVITY
𝜌=
ρ
R
L
d
loge
=
=
=
=
=
𝑅2𝜋𝐿
(log 𝑒 �8𝐿�𝑑 � − 1)
soil resistivity (Ω.m)
the earth resistance of one vertical rod (Ω)
the effective buried length of the vertical rod (m)
the rod diameter (m)
natural log
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EQUATION 2. EARTH RESISTANCE OF TWO OR MORE RODS
𝑅𝑛 = 𝑅 �
Where
1 + 𝜆𝛼
�
𝑛
EQUATION 3. ‘α FACTOR’ IN EARTH RESISTANCE EQUATION 2
Rn
R
n
λ
=
=
=
=
s
=
𝛼=
o
o
𝜌
2𝜋𝑅𝑠
the earth resistance of n rods (Ω)
the earth resistance of one rod (Ω)
the number of rods
factor in:
TABLE 4 for rods in one straight line or
TABLE 5 for rods arranged in a solid square
the distance between the rods (m)
TABLE 4. FACTOR λ FOR RODS IN A STRAIGHT LINE FOR USE IN EQUATION 2
n
λ
2
1.00
3
1.66
4
2.15
5
2.54
6
2.87
7
3.15
8
3.39
9
3.61
10
3.81
TABLE 5. FACTOR λ FOR RODS ARRANGED IN A SOLID SQUARE FOR USE IN EQUATION 2
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Number of rods along each side of the square
n
λ
2 rows x 2 rods per row
4
2.7
3 rows x 3 rods per row
9
5.9
4 rows x 4 rods per row
16
8.5
5 rows x 5 rods per row
25
11.4
6 rows x 6 rods per row
36
14.0
7 rows x 7 rods per row
49
16.9
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Example 1. An example of high earth resistance overcome by several earth rods.
The installer discovers that it is not possible to drive a 14 mm diameter earth rod longer than 2.4 m
into the ground. The earth resistance meter measures the resistance of this rod as 27.7 Ω. This is
higher than 3.0 Ω and further adjacent rods must be added.
Calculate the soil resistivity:
𝜌=
𝜌=
𝑅2𝜋𝐿
(log 𝑒 �8𝐿�𝑑 � − 1)
27.7 𝑥 2𝜋 𝑥 2.4
(log 𝑒 �8 𝑥 2.4�0.014� − 1)
𝜌=
417.71
(log 𝑒 (1,371.429 ) − 1)
𝜌=
417.71
(7.224 − 1)
𝝆 = 𝟔𝟕. 𝟏𝟏 𝜴. 𝒎
Attempt to bring the earth resistance down to less than 3.0 Ω by calculating the required number of
earth rods necessary to meet this resistance.
1 + 𝜆𝛼
�
𝑛
𝑅𝑛 = 𝑅 �
where
𝛼=
𝜌
2𝜋𝑅𝑠
The earth resistance of 4 rows of 4 earth rods 2.4 m apart and arranged in a square, 16 rods in total
would be as follows.
𝑅16 = 27.7 �
where
𝛼=
67.11
2𝜋 𝑥 27.7 𝑥 2.4
𝛼 = 0.16
𝑅16 = 27.7 �
This is higher than the target of 3.0 Ω.
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1 + 8.5𝛼
�
16
1 + (8.5 𝑥 0.16)
�
16
𝑹𝟏𝟔 = 𝟒. 𝟏 𝜴
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The earth resistance of 5 rows of 5 earth rods 2.4 m apart and arranged in a square, 25 rods in total
would be as follows:
1 + 11.4𝛼
�
25
𝑅25 = 27.7 �
Where α remains the same as with 16 rods above
𝛼 = 0.16
1 + (11.4 𝑥 0.16)
�
25
𝑅25 = 27.7 �
𝑹𝟐𝟓 = 𝟑. 𝟏 𝜴
These calculations would put the user in the ballpark of how many rods theoretically would be
required to achieve the desired earth resistance. 16 rods would give a resistance of 4.1 Ω and 25 rods
a resistance of 3.1 Ω. The user may be unwilling to buy 25 rods and may settle for less protection. The
actual measured results would vary from those calculated. The installer would need to drive in
successive rods and measure the earth resistance after each rod and let the user decide on the
number of rods finally installed.
Example 2. An example of lower earth resistance overcome by fewer earth rods.
The installer discovers that it is not possible to drive a 14 mm diameter earth rod longer than 1.2 m
into the ground. The earth resistance meter measures the earth resistance of this rod as 7.3 Ω. This is
higher than 3.0 Ω and further adjacent rods must be added.
Calculate the soil resistivity:
𝜌=
𝜌=
𝑅2𝜋𝐿
(log 𝑒 �8𝐿�𝑑 � − 1)
7.3 𝑥 2𝜋 𝑥 1.2
8
(log 𝑒 � 𝑥 1.2�0.014� − 1)
𝜌=
55.04
(log 𝑒 (685.714 ) − 1)
𝜌=
55.04
(6.531 − 1)
𝝆 = 𝟗. 𝟗𝟓 𝜴. 𝒎
Attempt to bring the earth resistance down to less than 3.0 Ω by calculating the required number of
earth rods necessary to meet this resistance.
where
1 + 𝜆𝛼
�
𝑛
𝑅𝑛 = 𝑅 �
𝛼=
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The earth resistance of 3 earth rods 1.2 m apart and arranged in a straight line would be as follows.
where
𝑅3 = 7.3 �
𝛼=
1 + 1.66𝛼
�
3
9.95
2𝜋 𝑥 7.3 𝑥 1.2
𝛼 = 0.18
1 + (1.66 𝑥 0.18)
�
3
𝑅3 = 7.3 �
𝑹𝟑 = 𝟑. 𝟐 𝜴
This is slightly higher than the target of 3.0 Ω.
The earth resistance of 4 earth rods 1.2 m apart and arranged in a straight line would be as follows:
𝑅4 = 7.3 �
Where α remains the same as with 3 rods above
1 + 2.15𝛼
�
4
𝛼 = 0.18
1 + (2.15 𝑥 0.18)
�
4
𝑅4 = 7.3 �
𝑹𝟒 = 𝟐. 𝟓 𝜴
These calculations would put the user in the ballpark of how many rods theoretically would be
required to achieve the desired earth resistance. 3 rods would give a resistance of 3.2 Ω and 4 rods a
resistance of 2.5 Ω. As with example 1, the actual measured results would vary from those calculated.
The installer would need to drive in successive rods and measure the earth resistance after each rod
and let the user decide on the number of rods finally installed.
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Calculate soil resistivity and earth resistance in MS Excel
The above calculations can be done using MS Excel.
FIGURE 15. SOIL RESISTIVITY AND EARTH RESISTANCE EXAMPLE IN AN MS EXCEL SPREADSHEET.
Following FIGURE 15, enter the relevant values into cells B5, B6, B7, B15, B16 and B17
Insert the following formula into cell B3
=(B5*2*PI()*B6)/(LN(8*B6/B7)-1)
Insert the following formula into cell B11
=B5*((1+(B15*B13))/B16)
Insert the following formula into cell B13
=B3/(2*PI()*B5*B17
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1.6.3 SURGE PROTECTION
Once an adequate earthing system has been established, the means to shunt surges to earth through
that earthing system can be planned. This is a layered approach. Surge protection devices (SPD) are
installed at three basic layers. (14).
•
•
•
The first layer is at the main electric supply board.
The second layer is at the distribution board and irrigation switchboard.
The third layer is at the motors and controllers that feed from the switchboard.
The third layer to a controller would generally be an uninterruptable power supply (UPS) with
surge protection. (2) (14).
A separate possible source of a surge is through the ‘backdoor’ in the form of a telephone supply or
though data cables entering the site from another source, which would also need their own
protective devices.
1.6.3.1 SURGE PROTECTION – FIRST LAYER
The consumer may not have much control of the surge protection at the point of supply – the first
layer - but from thereon, efficient surge protection devices must be installed at the second and third
layers.
1.6.3.2 SURGE PROTECTION – SECOND LAYER - DISTRIBUTION BOARD AND IRRIGATION
SWITCH BOARD
A switch board will normally contain a magnetic circuit breaker (MCB) to each electric device as well
as a thermal overload. Both react to an increase in current. With an MCB, an increase in amps pulls in
a solenoid (coil) to break the circuit and with a thermal overload, the heat generated by the increased
current will bend a bi-metal strip to beak the circuit.
However, neither will react to a voltage surge. For this, the incoming supply to the board must be
fitted with a 3-phase surge protection device. This SPD has open connections from each of the three
phases to earth. The potential between each live phase and earth is 230 V. This connection is
immediately closed at 275 V. Unless specified to the switch board manufacturer, this SPD would not
normally be provided. (3)
1.6.3.3 SURGE PROTECTION – THIRD LAYER
1.6.3.3.1 UPS – GENERAL DESCRIPTION
An uninterruptable power supply unit (UPS) enables IT equipment to tolerate a wide range of
voltages without any processing circuits malfunctioning. They will assist in flattening-out voltage
surges. They are generally over-current protected and will shunt surges to earth.
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There are two main types of UPS.
•
•
Line-interactive UPS. This is a relatively lower-cost type, usually bought to keep IT equipment
going in the event of a power cut and provide a degree of voltage regulation. They are sold by
Netafim.
Double Conversion On-line UPS. The type is more sophisticated and provides a much tighter
degree of protection against voltage fluctuations.
While a UPS enables a controller to tolerate a wide range of voltages without any processing circuits
malfunctioning, even without a UPS, a controller is very tolerant of voltage fluctuations.
The voltage from a 50Hz AC supply passes through zero volts 100 times per second (twice per cycle),
at which moments there is no power to an electric device. This poses no problem to a device such as
an electric light or a motor but a controller’s processing circuits cannot function under such a
condition. All computers and controllers therefore operate under a constant flat DC supply. Where
the electricity supplied to the controller is AC, this DC power comes from a switch-mode power
supply (SMPS), which converts low voltage AC to extra low voltage DC.
Electricity
provider
SMPS
LV AC
ELV DC
Controller
FIGURE 16. THE CURRENT FROM THE PROVIDER TO THE CONTROLLER
The SMPS uses capacitors that are charged at voltage cycle peaks and which then constantly release
electricity that bridges this ‘gap’ when there are zero volts. In the same manner, the SPMS can also
overcome an interruption in power supply, albeit very brief. Thus to a small degree, the SMPS is its
own mini-UPS.
The SMPS can generally achieve these functions providing its own incoming voltage does not vary by
more than 10% higher than nominal voltage and 20% lower. In other words the nominal 230V supply
should not exceed 253V and should not reduce below 184V. (4).
Netafim recommends that the power to its own NMC power supply does not vary by more than 253V
or lower than 207V (i.e. ± 10%).
A UPS widens this voltage range. It regulates the AC supply to the SMPS but its ability regulate
depends on the type chosen.
Electricity
provider
LV AC
UPS
LV AC
SMPS
ELV DC
Controller
FIGURE 17. THE CURRENT FROM THE PROVIDER TO THE CONTROLLER VIA A UPS
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1.6.3.3.2 LINE-INTERACTIVE UPS
Apart from the batteries, there are two main parts to a line-interactive UPS: a power interface and an
inverter. The power interface filters the AC power, suppresses voltage spikes and provides some
regulation to the voltage output. The inverter redirects some of the input AC power to keep its
batteries fully charged. When the AC input voltage falls outside of the input range of the power
interface, the inverter supplies AC power from the batteries.
The output voltage varies with the input voltage, although not to the same degree. This output
variation is acceptable though, provided it remains within the permissible input voltage range of the
SMPS.
AC input
AC output to SMPS
Power
interface
Inverter
Normal mode
Stored energy mode
Battery
FIGURE 18. BASIC COMPONENTS OF A LINE-INTERACTIVE UPS
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The line-interactive UPS supplied by Netafim has an input range from 173V to 276V and an output to
the power supply of between 211V to 250V, which is within the power supply’s input range of 207V
to 253V.
276 VAC
253 VAC
NMC Power
supply input
NMC
power
supply
output
Lineinteractive
UPS input
Line interactive UPS output
250 VAC
12 VDC
207 VAC
211 VAC
173 VAC
FIGURE 19. LINE-INTERACTIVE UPS OUTPUT WITHOUT GOING TO BATTERIES
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If the voltage goes outside this range, the UPS goes to battery and produces an output in a narrower
range of 219 V to 242 V.
Lineinteractive
UPS input
greater than
276V
276 VAC
253 VAC
NMC Power
supply input
NMC
power
supply
output
Lineinteractive
UPS input
Line interactive
UPS output
241 VAC
12 VDC
219 VAC
207 VAC
Lineinteractive
UPS input
less than
173V
FIGURE 20. LINE-INTERACTIVE UPS OUTPUT, WHEN GOING TO BATTERIES
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1.6.3.3.3 DOUBLE CONVERSION ON-LINE UPS
The on-line UPS has about three times more components than a line-interactive UPS, but these
components are generally smaller. As the name suggests, it converts power twice. First, it is
converted from AC to DC using capacitors, just like an SMPS. Then second, it is converted back into
tightly regulated AC. When the AC input goes out of its specified range, the on-line UPS draws power
from its batteries so that the UPS output is not affected.
AC input
Bypass (prime or standby)
AC input
Bypass
switch
DC Link
AC output to SMPS
Inverter
Rectifier
Battery to
DC
converter
Normal mode
Stored energy mode
Bypass mode
Battery
FIGURE 21. DOUBLE CONVERSION ON-LINE UPS – BASIC COMPONENTS
Unlike a line-interactive UPS, an on-line UPS typically has a bypass, which comes into action when
there is either a fault with the UPS or the load exceeds what the UPS is designed to handle
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An on-line UPS will operate at a wider input voltage range than a line-interactive UPS and will
regulate output much more tightly. An example of this range is shown in FIGURE 22.
300 VAC
253 VAC
NMC Power
supply input
NMC
power
supply
output
Online
UPS
output
235 VAC
12 VDC
225 VAC
Double
conversion
on-line UPS
input
207 VAC
110 VAC
FIGURE 22. DOUBLE CONVERSION ON-LINE UPS OUTPUT WITHOUT GOING TO BATTERIES
An on-line UPS is more frequency tolerant than a line-interactive UPS. Even though the voltage might
be within a line-interactive UPS’s range, if the frequency deviates from 50 Hz, the line-interactive UPS
may go to its batteries when the on-line UPS will not necessarily do so. This is because the on-line
UPS’s AC output comes from the DC that the input was initially converted to.
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A few line-interactive UPSs do have a frequency tolerance and others have no tolerance. All on-line
UPSs have a frequency tolerance. This frequency tolerance is important where the main supply is
subject to frequency variation and particularly from generator sets. For instance, a genset may give
out a frequency of 48 Hz, in which case a line-interactive UPS with 0% frequency tolerance will go to
battery when an on-line UPS will not do so, providing its tolerance will allow the frequency to drop
the 4% from 50 Hz to 48 Hz.
1.6.3.3.4 SURGE PROTECTION - POWERLINE PROTECTOR
Netafim provides the NMC Power Line Protector as a surge protection device. As well as being fused,
it contains several components such as varistors that divert surges to its earth terminal.
Like the UPS, this is a third layer SPD. It is provided as additional protection and is usually installed at
the incoming supply to the UPS.
1.6.3.3.5 SURGE PROTECTION – COMMUNICATION
As mentioned in Earthing – Communication Wiring above, the NMC RS485 PC Communication Unit at
the PC has its own surge protection with its communication cable to the controller earthed to the
ground of its incoming 230 VAC power.
The communication card for the NMC 64 / Pro is surge protected and has an earth terminal that will
ground through the NMC 64 / Pro controller to which it is fitted.
The communication card for the NMC Junior is not surge protected and has no earth terminal. It is
necessary to use an additional device, an NMC Junior COMS lightning protector at each NMC Junior in
the communication network. This device is earthed at the incoming power to the NMC Junior.
1.6.3.3.6 LIGHTNING PROTECTION – BUILDING
Lightning protection of a building is specialised and is best undertaken by an LPS designer or an LPS
installer. They are able to issue an LPS installation safety report and maintenance certificate
according to SANS 10313.
1.6.3.3.7 LIGHTNING PROTECTION – WEATHER STATION
The weather station is mounted at the top of a pole / mast, normally of 50 mm diameter aluminium.
Its protection from lightning may not necessarily be done by the LPS designer, who would mainly be
concerned with the building mentioned above.
The weather station must be protected at the top with a lighting rod that is earthed at the mast’s
base as well as to the central earthing busbar. A suitable lightening rod is the standard 1.2 m long x
14 mm diameter earth rod supplied by Netafim, described above in Vertical or Horizontal Earth
Electrodes. The height of its tip above the weather station should be such that it provides a cone of
protection where the angle of the cone’s apex is 70⁰. Effectively this means that one 1.2 m rod will
suffice for a weather station without any wind sensor and a 2.4 m rod would be needed if there is a
wind sensor. (9)(11). See FIGURE 23.
The bottom end of the lightning rod is connected to a small length of 10 mm² earth wire, which wire
in turn is clamped to the mast. This wire is necessary as the clamps holding the rod to the mast give
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too much resistance by themselves. The mast itself will act as a conductor. It must be earthed at its
base in a similar manner as described in The Earth Terminal above. It should also be earthed at the
central earthing busbar. (12)
The lightning rod must be offset from the pole in order to clear the weather station mounting arm.
See FIGURE 24.
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Top of a 2.4m lightning rod
70°
Minimum height for weather station with a wind sensor
70°
1380
1700
Cone of protection
Top of a 1.2m lightning rod.
Minimum height for weather station without wind sensor
430
700
210
500
70°
Ø 50mm aluminium pole
Connect the rod to the pole with 10mm ² earth wire
FIGURE 23. A LIGHTNING ROD PROTECTING THE WEATHER STATION
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130
30°
15
210
4
430
500
77
Lightning rod
10mm² earth wire
Ø 50mm aluminium pole
U bolt and clamps
50
FIGURE 24. FIXING THE LIGHTNING ROD TO THE WEATHER STATION POLE
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REFERENCES
Some references specify wire sizes that are not used in South Africa, in which case the text specifies
the next highest commercially available size. For example, should a reference specify an earth wire
size of 14 mm², which is not used in SA, the text will specify 16 mm².
1. Buijs, Machiel. 2010. Buys Electrical, Krugersdorp. Personal communication.
2. Cloete, Johannes. 2009. Lightning Protection and Earthing (Grounding). Input Output Agricultural
Controls CC. Brakenfell
3. Colussi, Renato. 2010. Electroparts, Cape Town. Personal communication.
4. Samstad, Jeffrey. Hoff, Michael. 2004. Technical comparison of on-line vs. line interactive UPS
designs. American Power Corporation.
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