EDC3 (5238) P 3 
 
  
       
 
 
 
 
 
 
 
 
            
DRAFT TANZANIA STANDARD  
(Draft for comments only)  
 
Mini-Grid Systems –  
 
 
Part 4:  Low voltage (LV) underground distribution network  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
TANZANIA BUREAU OF STANDARDS  
 
 
TBS 2017                                                                                                  First Edition 2017  
 
 
i 
 Table of contents  
0 Foreword ................................ ................................ ................................ ......................  ii 
1 Scope  ................................ ................................ ................................ ............................  3 
2 References  ................................ ................................ ................................ ...................  3 
3 Definitions  ................................ ................................ ................................ ....................  3 
3.1 Classification of Minigrid Networks  ................................ ................................ ................................ ...... 3 
3.2 Voltage Drop  ................................ ................................ ................................ ................................ ....................  4 
4 Requirements  ................................ ................................ ................................ ...............  4 
4.1 Service Conditions  ................................ ................................ ................................ ................................ ........  4 
4.2 Electrical System Parameters  ................................ ................................ ................................ ..................  4 
5 Underground Distribution Layout  ................................ ................................ ...............  4 
6 Right of Way  ................................ ................................ ................................ .................  5 
7 Low Voltage Cables  ................................ ................................ ................................ ..... 5 
7.1 Cable Installation  ................................ ................................ ................................ ................................ ...........  5 
7.2 Trench Specification  ................................ ................................ ................................ ................................ .... 6 
7.3 Excavation  ................................ ................................ ................................ ................................ ........................  6 
7.4 Cable Placing  ................................ ................................ ................................ ................................ ...................  6 
7.5 Backfillig and Reinstatement ................................ ................................ ................................ ....................  7 
8 Connections  ................................ ................................ ................................ .................  8 
9 Grounding of LV System  ................................ ................................ .............................  8 
10 Installation of LV Distribution Pillars  ................................ ................................ ...... 8 
11 Customer Service Drops  ................................ ................................ ..........................  9 
12 Annex A  ................................ ................................ ................................ ...................  11 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
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0 Foreword 
 
This Draft Tanzania Standard is part of an integrated set of specifications and standards issued to 
govern design and construction of mini -grid power systems in Tanzania.  The mini -grid standards have 
been developed to support improved residential, commerc ial and public services for rural communities 
of Tanzania. The mini -grid energy systems, when properly designed, will support affordable and reliable 
energy supply for remote households, community services, commercial and economic activities 
including shop s, workshops, micro -industry, fresh water pumping, secondary schools, health services, 
public lighting, places of worship or cultural activities, agro -processing activities and other activities.   
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
3 
 1 Scope  
 
This standard covers minimum technical requirements for the design and construction of 
underground low voltage, single phase and three -phase , power distribution networks for mini -grid 
systems.  
 
Note 1 This standard shall be read in conjunction with other relevant  Tanzania  Standards,  
applicable standards and specifications to have uniformity, compatibility and standardization in the 
distribution system.  
 
Note 2 This standard ensures safety and reliability of mini -grid power system providing low -cost 
solutions for small customers without compromising performance requirements. Materials or 
approaches that are higher in quality or provide better performance than th ose specified shall be 
acceptable . 
2 Reference s 
 
For the purpose of this Tanzania standard, the following references shall apply:  
 
TANESCO – Specification S01 Supply and Installation of Plant and Equipment  
 
TANESCO Engineering Manuals  
 
IEC 62257 -9-2 Recommendations for renewable energy and hybrid systems for rural 
Electrification -Part 9 -2: Integrated systems - Microgrids  
 
IEC 62257 (all parts)  Recommendations for renewable energy and hybrid systems for rural 
electrification  
 
IEC TS 62257 -5 Recommenda tions for renewable energy and hybrid systems for rural 
electrification – Part 5: Protection against electrical hazards  
3 Definitions  
 
For the purpose of this Tanzania standard, the following definitions apply : 
 
3.1 Classification of Minigrid Networks  
3.1.1  Distribution networks are classified by the amount of power they are intended to deliver and by 
the voltages employed.   
 
3.1.2  Mini-grids with AC delivery systems and an installed capacity of 100 kW or less would likely 
deliver power at EWURA standardized user voltages, that is, at 230/400V (10%), 50 Hz and would not 
require medium voltages ( MV). Mini -grids in this size range may  also be single phase and deliver power 
at 230V ( 10%), 50Hz.  
 
3.1.3  Minigrids with higher installed power capability from 100kW to 1MW would require medium 
voltage (MV).  
  
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3.2 Voltage Drop  
3.2.1  The primary measure of quality of service during mini -grid operation is user voltage, which shall 
remain within the specified range ( 230/400V 10%, 50Hz).   
 
3.2.2  Determination of voltage quality shall require consideration of length of the distribution feeder, 
the load and the size of the conductor and compariso n of the parameters with the specified range of 
voltage and frequency.  
 
3.2.3  For low voltage (LV) systems, voltage performance shall be determined from specified tables.  
4 Requirements  
4.1 Service Conditions  
4.1.1  The distribution network composed of equipment, material and components for mini -grid system 
shall be suitable for satisfactory operational performance under the local service conditions in Tanzania, 
which are as follows:  
 
a) Altitude above mean sea level (MSL): Up to 3,000 metres  
b) Maximum/record  yearly high ambient temperature: +40°C   
c) High humidity promoting the growth of fungi  
d) Extended periods of intense sun exposure  
 
4.1.2  For some items of equipment such as transformers, IEC standards specify  an altitude not 
exceeding 1000 m as normal service c onditions.  For these types of equipm ent, altitudes higher than 
1000 m are considered as abnormal service conditions and may require special considerations in the 
design, manufacture or application of the materials or equipment.  It is the responsibility of  the proponent 
to recognize the p articular conditions occurring o n any site and call them to the attention of the 
manufacturer.  
4.1.3 Air in coastal areas is frequently salt laden and can result in significant corrosion. It is the 
responsibility of the pro ponent to specify equipment with characteristic that resist such corrosion, 
including provision of additional creep distance on insulators, or specification of stainless steel for 
cabinets and equipment enclosures.  
4.2 Electrical System Parameters  
System param eters  of mini -grids for low voltage underground power distribution network are as follows:  
 
a) Nominal system low voltage (U): 230/400 V (±10%)  
b) Maximum permissible system low voltage (U m):  253/440 V 
c) Minimum permissible sy stem low voltage : 215/374  V  
d) System frequency: 50 Hz  
e) Neutral grounding  arrangement: multi- grounded  
f) Power frequency withstand voltage for 5 minutes: 3kV p 
5 Underground Distribution Layout  
 
This specifies the design  and installation  practices to be applied in the construction of LV cable s for the 
underground distribution system.  
 
5.1 The underground distribution network should be laid out in a radial fashion, with main feeders 
serving branches without interconnections between feeders.  Sufficient reliability can be achieved with 
the radial syst em, if it is properly designed and installed and uses high quality equipment.  In the event 
that greater reliability is desired than can be offered by a purely radial network, normally open 
interconnection points between feeders can be created.  Such inter connections require additional 
feeder construction.   
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5.2 All connections to the feeders and branches shall be made above ground in pedestals.  It is 
recommended that the use of breakers within the network be minimized to avoid spurious tripping and 
that connections between cables be made with removable links so that open points can established for 
fault finding.  
 
5.3 Service drop connections shall be made at pedestals designed for the purpose.  Each service 
drop shall be protected by a breaker whose trip current is near the available fault current at the location.  
The use of low tripping point breakers is not recommended as they will respond to faults in the consumer 
premises and will require utility attention to reset.  Their purpose is to respond to fau lts occurring in the 
service drop cable, which may result from dig -ins or other mechanical damage to the service drop cable.  
6 Right of Way 
 
If required by local authorities, the right -of-way (ROW) for the proposed route of an underground cable 
shall be obtained from the relevant authorities during planning and design stage of the project by 
developers. G uidelines for obtaining the ROW are given below:  
 
a) ROW drawings shall be prepared showing proposed route of the underground cable, cross -
section of the t rench or duct bank (if required) and location of other utilities (telephone, 
water, sewer etc.) lines along the proposed route.  
 
b) It is recommended that alignments be chosen carefully to avoid potential construction 
activity that could harm the cable.  
 
c) The ROW drawings along with the covering letter and supporting documents shall be 
submitted to the relevant department in the concerned Agency for obtaining necessary 
permission.  
 
d) Actual construction work shall be started once permit is granted from the releva nt 
authorities for execution of job.  
7 Low Voltage Cables   
 
Low voltage cables intended for underground mini -grid application shall comply with the requirements 
of Tanzania Board of Standards,  Standard 5239 : “Technical Specification for Low Voltage Power 
Cables (600/1000) for Underground Network”.  
7.1 Cable Installation  
7.1.1 Low voltage cables may be installed either via direct burial or in conduit. The provisions of this 
standard apply to both direct buried cables and to those that are installed in conduit, except where 
noted.  
 
7.1.2 If cables are installed in conduit, suita ble reductions in the allowable ampacity shall be made, 
in accordance with manufacturer’s recommendations to allow for the reduced cooling capacity of such 
installation. Conduits shall be sized taking into account manufacturers recommendations considering 
that cables will be installed by being pulled into the conduit.  
 
7.1.3 Regardless of how installed, cables or conduit shall be protected where the cable route crosses 
a road, highway or street.  Such protection shall take the form of a concrete cap or tile  the width of the 
trench and not less than 10cm in thickness installed 30cm above the cable or conduit. The purpose of 
this cap is to spread the weight of traffic loads so that they do not crush the cable or conduit. Such a 
protective cap shall be installe d anytime the cable crosses any location where unusually heavy loading 
is anticipated, such as the entrance to an industrial facility.  Conduit by itself is not a suitable protective 
cover for cables, and it is not acceptable that normally direct buried ca ble be installed in conduit under 
roads or other high traffic locations.   
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 7.2 Trench Specification  
 
The figures in Annex A  give details of the trench construction for direct buried cables . In the event it is 
planned to install cable in conduit, the burial depth  of the conduit shall be the same as that of the direct 
burial cable, but the trench depth may be reduced slightly as it is not necessary to provide the sand 
bedding for conduit systems.  
 
Before proceeding with the actual work, the following activities sha ll be completed.  
 
a) Survey of cable routes, preparations of drawings, ROW permissions, digging permits and 
inspection of cable route from end to end.  
 
b) Decision regarding locating cable drums after unloading at site keeping in view the actual cable 
lengths on  the drums.  
 
c) For cables installed in conduit, taking into account the maximum allowable pulling length, 
considering bends.  
 
d) Knowledge about locations of cable joints.  All joints should be made above ground in pedestals 
or equipment.  No underground joints  are allowed in the initial construction.  
 
e) Obtaining necessary permissions from local police/traffic police authorities for work schedule, 
warning and lights and design detours.  
 
f) Keeping necessary number of night warning lamps, stands, wooden/steel bridges  for 
pedestrian  protection . 
 
g) Arrangement of necessary manpower and machines keeping in view the nature of work 
schedule and expiry date of digging permits.  
  
7.3 Excavation  
7.3.1 After the depth of the trench has been decided, excavation work shall be started.  It is necessary 
to take in to consideration the changes being made in the existing level due to any earth cutting or back 
filling work to be carried out later. Such information should be gathered from the relevant authorities, 
so that ultimately the cable  depth in the trench shall be achieved.  
 
7.3.2   Trenches may be dug by hand or if site conditions permit, a mechanical excavator can be used. 
Asphalt shall be cut by a motor driven saw. Manual digging shall be used where existing underground 
facilities are  encountered. Existing facilities uncovered during excavation shall be protected, shored, 
braced and supported, as deemed necessary. All protective covers, warning tapes, position markings 
etc., pertaining to the existing underground utilities shall be pre served during the excavation work and 
restored in actual position during backfilling.  
7.4 Cable Placing  
 
7.4.1  At the time the trench is ready for cable installation, the first task is to clear the bottom of the 
trench and remove rocks, trash, or other materia ls that could harm the cable or conduit.  
 
7.4.2  Prior to placing direct buried cables, an underlayment of fine sand of the depth specified in the 
relevant figure of Annex A shall be placed in the bottom of the trench.    
7 
 7.4.3  For direct buried cable, cable s shall preferably be installed by the layout method, in which a 
vehicle with a cable reel passes the length of  the trench and the cable is pay ed out into the trench.  If 
this is not possible due to obstructions or conflicting underground facilities, the c able may be pulled into 
place over the sand bedding, using appropriate means to prevent damage to the cable from contact 
with the sides of the trench or other obstructions.  
 
7.4.4  For cables installed in conduit, it is not necessary to provide a sand underlayment and the 
conduit may be placed on the bottom of the trench.   
 
7.4.5  Conduit used for cable installations shall be PVC -U, of the type used for water line, with a 
pressure ra ting of 16 bar.   PVC pipe normally used for drain and vent  line is unacceptable.  Because 
PVC is susceptible to degradation from sun exposure, any exposed conduit such as equipment risers 
shall be rated for UV exposure.  Conduit sections should be fully as sembled and glued or sealed 
together to prevent ingress of water prior to installation of the cable.  
 
7.4.6  Cables installed in conduit shall be pulled into place using suitable equipment and lubricants, 
and over a maximum section length in accordance with the recommendations of the cable 
manufacturer.     
7.5 Backfillig and Reinstatement  
7.5.1  After placing the dir ect burial cable, all stone rubble, cans, trash, etc.  that have fallen into the 
trench during the cable laying shall be removed. A cover of clean, fine sand shall be placed over the 
cable. The thickness of sand bedding/cover shall be as per the relevant t rench construction drawing. 
The sand bedding and cover shall be spread by hand tools to maintain an even thickness.  
 
7.5.2   Following completion of the sand bedding, the trench shall be backfilled with clean fill.  There 
shall be no rocks larger than 25mm in diameter in the fill and no trash, broken concrete or other debris. 
The backfill shall be tamped to approximately 90% of original density.  
 
7.5.3  If cable has been pulled into conduit, the trench shall be back filled as in given in clauses 7.5.2. 
There is no need for sand bedding or cover for conduit installed cable.  
 
7.5.4  Reinstatement of the trench shall be in accordance with the requirements of the relevant 
authorities under whose jurisdiction the area falls. Warning tapes shall be placed/laid in the  backfill and 
any pavement cuts shall be restored to their prior condition.  
  
8 
  
8 Connections  
8.1 All connections between cables such as cable splices, taps to sub -feeders and taps for consumer 
services shall be made above ground in pedestals located and designed for the purpose . 
 
8.2 All connections shall be made with terminations that are appropriate for the connection at hand.  For 
instance, bolted studs on equipment shall be mated with terminal lugs compressed onto the cable. 
Cable to cable connections  shall be executed with compression splices.  
 
8.3 Cable entries into equipment or junction boxes shall be sealed with cable glands to prevent entry 
of vermin, dirt or water, and cables under load shall be supported independently of connections.  
9 Grounding of LV System  
9.1 The LV system shall be a multi -grounded system, whic h means that the neutral of the distribution 
system shall be connected to ground in a number of locations as follows:  
 
a) At any source,  such as an inverter feeding the system from a PV or other renewable resource 
or battery, or  the low voltage side of a distribution transformer interconnecting the mini -grid 
with the main grid . 
b) At any piece of equipment on the AC side of the system, such a s an inverter or a distribution 
pedestal.  
c) At any interconnection point between feeders or at any feeder tap . 
d) At the point that a customer service drop is tapped off the feeder line.  
 
9.2  The metal work of all LV network equipment such as LV distribution p illar housings, and the metal 
parts of any racks or support frames shall be bonded to the system neutral conductor and to any ground 
rod or grounding counterpoise using a suitable bonding conductor.  The bonding conductor shall be 
sized to carry the antici pated fault current but in no case, shall be of smaller cross section than 10mm2 
copper -equivalent.  The actual material of the bonding conductor may be copper or such other material 
as may be appropriate considering corrosion and risk of theft.  
 
9.3 The g round connections at the equipment locations mentioned in clause shall  be executed using 
two driven ground rods, each 16mm in diameter and 2,400mm long.  The rods shall be separated by at 
least 4meters and shall be individually interconnected to each other and to the ground bus of the 
equipment being grounded by a conductor of suitable cross section, but not less than 10mm2 copper -
equivalent.  The ground rods shall be either hot dip galvanized or copper coated, depending upon the 
corrosivity of the soil, and the interconnecting wire may be annealed galvanized steel ground wir e or 
copper.  Connections shall take into account the potential for corrosion between dissimilar metals, if 
any are used in the system.  
 
9.4 In rocky ground where it is difficult to install ground rods, a counterpoise may be used consisting 
of a loop of gr ound wire of length not less than 12 meters installed at a depth of 3 00mm in a trench 
around the equipment or pedestal such that the gro und wire is located everywhere at least 1 m distant  
from the equipment . 
 
9.5 It is recommended that the resistance to ground of the system neutral, as measured at any point 
using industry standard methods and with all grounds attached, not exceed 1 ohm.  It is further 
recommended that the resistance to ground of any individual piec e of equipment, such as a distribution 
pedestal, not exceed 10 ohms.  In the event these values are exceeded, the system operator can install 
additional ground rods in parallel with the existing rods.  No rod should be closer to any other than its 
own leng th (2.4m) to prevent undesirable interactions.  
10 Installation of LV Distribution Pillars  
  
9 
 The following points are required to be considered at site during the installation of the distribution pillars: 
  
 
a) The minimum clearance between the distribution pillar  and any other utility service such as 
telephone and water  boxes shall not be less than 2 m.   
 
b) The distribution pillar shall be easily accessible from the front of consumer’s boundary without 
any obstruction.  
 
c) Bottom part of the distribution pillar shall be direct buried to the ground and back filled, after 
fixing the bottom legs with unarmored fine cement mortar.   
 
d) In case of bulk consumers,  it can be allowed to terminate more than one cable feeding the 
same consumer.   
 
e) The distribution pillar shall not  be located on the top of sewerage system.   
 
f) The fixation of the distribution pillar into the frame shall be in such a way that the used tools 
and fasteners will not damage the body of the cabinet.   
 
g) All cable loops between the cabinets shall pass throug h the cable trench.   
 
h) The area assigned for installing the distribution pillar shall be finished clean regardless the 
finishing of the surrounding area.  
11 Customer Service Drops  
 
11.1 Customer service drops shall be derived from feeders or branches in a distribution pedestal and 
shall consist of underground cable to the consumer’s residence.  The service drop shall be metered, 
but this metering function may be met in a number of ways that may or may not involve the installation 
of equipment in either the pedestal or the consumer’s premises. In either case, the pedestal should be 
secured against unauthorized access . 
11.2 The service drop cable shall be protected by a breaker of suitable rating installed in the 
distribution pedestal.  The purpose of the breaker is to protect the system against faults in the service 
drop, so that the rating of the breaker should be only slightly less than the available fault current at the 
location to prevent nuisance tripping resulting from faults in the consumer’s premis es. 
 
 
11.3 The cable of the consumer service drop shall be of a cross section consistent with the anticipated 
consumer demand but shall in no case be less than 2.5mm2. 
 
11.4 The service drop conductor/cable shall be rated for underground, direct buried ins tallation in wet 
locations but need not comply with the Tanzania  Standard TZS 5239 . 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
10 
  
 
 
 
 
 
 
 
 
 
 
  
11 
 12 Annex A  
 
Figures  
 
 
Fig 1: Standard arrangement for ground rod installation  
 
 
 
 
 
 
 
 
All dimensions in mm  
 
 
 
 
 
 
 
 
 
 
 
 
  
 
12 
  
 
 
 
Fig 2: LV Trench Configuration (Single Cable)  
 
 
 
All dimensions in mm  
 
 
 
 
 
 
 
 
 
 
 
  
 
13 
  
 
 
 
 
 
Fig 3: LV Trench Cable Configuration (Two LV cables)  
 
 
All dimensions in mm  
 
 
 
 
 
 
 
 
 
 
 
 
  
 
14 
  
 
 
 
Fig 4: LV Trench Configuration (Three LV Cables side by side)  
 
 
 
 
 
 
 
All dimensions in mm  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
15 
  
 
Fig 5: Service Drop Trench Configuration  
 
 
 
 
 
 
 
All dimensions in mm  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
16 
  
 
 
Fig 6: Bonding and Grounding arrangement - LV Dist. Pillar  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
17 
  
 
 
 
Fig 7: Bonding and Grounding Arrangement of Dist. Pillar in Rocky Soil