Thermal Analysis and Ampacity Calculation for Underground Cable Raceway

Thermal analysis and ampacity calculation for underground cable raceways are crucial for ensuring the safe and efficient operation of electrical systems. Here's a brief overview:

Thermal Analysis Underground Cable Raceway

The thermal analysis involves calculating the temperature rise in cables due to electrical current flow and the heat dissipation to the surrounding environment. Key methods include:

Underground Cable Raceway Ampacity and Thermal Analysis

It sounds like you're interested in the technical aspects of underground cable raceways! This topic involves understanding the ampacity (current-carrying capacity) and thermal analysis of cables within raceways.

What Is The Underground Cable Raceway?

An underground cable raceway is an enclosed conduit system that provides a pathway for electrical cables. It protects cables from environmental factors such as moisture, heat, and physical damage. Here are some key points about underground cable raceways:

Electrical Load Flow Analysis in Power Network System

Load flow (or power flow) in a power network system is an analysis that calculates how electrical power is distributed and flows through an electrical power system from the generation sources to the loads (or consumers). The main objectives are to determine the voltage levels at each bus (node), and the power flowing through each transmission line, and to ensure the system is operating within its limits and constraints.

Electrical Power Cable Short Circuit Current Capacity

How Calculate Cable Short Circuit Current from Short Circuit Current from Table?

You may find the short circuit current for a particular type of cable for a specified period of time from your cable manufacturer’s provided catalogue. For example the below table is showing different sizes from 16 square mili-meter to 2500 square mili-meter of cable in first left column and time duration from 0.1 second to 5 second in next 10 columns.

Type Test on Complete Underground Power Cables

Here in this article, we would like to present a sample type test report on complete underground power cable:

Bending test in accordance with IEC 60840, clause 12.3.3

A cable sample of approximately 40 m was bent six times around a test cylinder with a maximum diameter of (25 x (d+D) +5%) at ambient temperature.

The results are stated in annex A. For a picture of the bending test, a reference is made to Annex G.

 Result: The test was completed successfully.

Partial discharge test at ambient temperature in accordance with IEC 60840, clause 12.3.4.

After the bending test and short duration test, the cable samples were examined for partial discharges in accordance with IEC 60885-3 at ambient temperature. The sensitivity of the measuring circuit was checked with a calibrator, and the noise level was < 2 PC. The voltage was first raised to and held to 133 kV (1, 75 Uo) for 10 s and then lowered to 114 kV (1, 5 Uo). At this voltage, the partial discharges were measured. For the results, reference is made to Annex A.

Result: No visual internal partial discharges were established.

Measurement of the dielectric loss angle in accordance with IEC 60840, clause 12.3.5

After the partial discharge measurement as mentioned under 2.1, the dielectric losses of the test installation were measured at Uo and a conductor temperature of at least 95 °C.  The results are stated in annex A. 

 Result: The test results met the requirements.

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Heating cycle voltage test in accordance with IEC 60840, clause 12.3.6

The test installation was subjected to a power frequency test of 152 kV (2 Uo), 60 Hz for at least 480 h. During this test, the test set-up was subjected to at least 20 heating cycles. Each heating cycle consisted of 8 h heating and 16 h of natural cooling. During the last 2 h of each heating period, the conductor reached a temperature of at least  95 C (5 C above the maximum rated temperature). For the data of the test, a reference is made to Annex A.
Result: The test installation passed the test successfully. 

Partial discharge test in the hot condition in accordance with IEC 60840, clause 12.3.4

After the heating cycle voltage test, the test the set-up was examined for partial discharges in accordance with IEC 60885-3 at a conductor temperature of at least 95 °C The test was carried out as described in 2.1. The results are stated in annex A.
 Result: The test results met the requirements.

Short duration power frequency withstand voltage test in accordance with Tranco specification

After the above-mentioned tests the test installation was subjected to a power frequency test of 275 kV (3,6 Uo), 60 Hz for at least 1 minute. The test was carried out while the test installation was at ambient temperature.
Result: No breakdown occurred.

Impulse withstand voltage test in accordance with IEC 60840,  clause 12.3.7

After the above-mentioned tests, the test set-up was tested with an impulse voltage while the conductor was heated to a temperature of at least 95 C (5 C above the maximum rated temperature). The test was carried out in accordance with IEC 60230 and IEC 60060. The sample was tested with ten positive and ten negative voltage impulses of 650 kV. The oscillograms of these tests are stated in annex B.
Result: No breakdown occurred. 

Power frequency voltage test for 15 minutes in accordance with  IEC 60840, clause 12.3.7

Upon completion of the impulse withstand voltage test, a voltage test was executed with an alternating voltage of 50 Hz. The test installation was tested with a voltage of at least 190 kV  (2,5 Uo) for 15 minutes in accordance with the specification. The test was carried out while the test installation was at ambient temperature.
  Result: No breakdown occurred.

Examination of the cable and the accessories after the tests in accordance with IEC 60840, clause 12.3.8

After completion of the electrical tests as mentioned above the cable and the accessories were dismantled and inspected for electrical degradation of the insulation. For pictures of the dismantling, a reference is made to Annex I.

 Result: No signs of electrical degradation were detected. The construction of the accessories complied with the construction drawing

Tests on semi-conducting layers

Electrical resistivity before and after aging in accordance with IEC 60840, clause 12.3.9

The resistivity of the extruded semi-conducting layers was measured and calculated according to annex B of the IEC 60840. The aging treatments were carried out in accordance with IEC 60811-1-2, clause 8 at a temperature of 100 C for 7 x 24 h. The measurement was carried out at a temperature of 90 C. The test results are stated in annex F.

Result: The test results met the requirements as stated in the specification

Conditions for Current Carrying Capacity of Power Cable

The tabulated current ratings are designed by the conditions below 

Current carrying capacity is defined as the amperage a conductor can carry before melting either the conductor or the insulation. Cables are mainly designed as per requirement. Power cables are mainly used for power transmission and distribution purposes. It is an assembly of one or more individually insulated electrical conductors, usually held together with an overall sheath.

Consider one circuit of a three-phase load to make sense.
Load factor = 1.0

Maximum operating conductor temperature :
70 degrees Celsius ( PVC insulation ) and 90 degrees Celsius ( XLPE insulation );
No other heat sources are installed near the group of cables.


Cable laying :
Cables may lay in the air or on the ground. Cable laying in the air means cables will be placed on a cable tray. Cabl laying in the ground means cable will be laid into the ground using a cable trench or direct burid into the ground.

 Cables In Air : 

• Ambient temperature

• The cable has to protect against heat radiation of the sun as well as sufficiently large and ventilated rooms whose temperature is not perceptibly increased by the heat dissipating from the loaded cable.

Cables In-Ground :

• Soil temperature: 30 degrees Celsius

• Depth of laying: 70 cm
• Specific thermal resistivity of soil: 100 degrees C.m/watt

If the actual installed conditions are different from the above-mentioned condition, the tabulated current ratings should be multiplied by the appropriate derating factors as shown below:

Copper and Aluminum Bus-bar Size Selection Chart

What is Bus-bar?

Bus-bar is the common header where one or multiple sources poor the power also one or more sources can take power simultaneously; means bus-bar is a common bus that receives power and releases power.

Literally, the bus bar is usually a very thick wire or bar that leads from the power source and is the wire that every load in the system is connected to.

Bus-bar is like a city water reserver where water is accumulated and distributed from.

Types of Bus bar

Bus bar mainly classified into two, though the function is same.
1. In outdoor substation the bus bar is a kind of bulk-sized aluminium conductor;
2. In indoor substation copper or Aluminum bar is used as a bus bar.

Why Bus bar is required in an electrical transmission network?

In electrical substations, the bus bar is a common component where all the anchorages that have the same voltage level are connected. In an electrical transmission network, it’s required an electrical node but it is physically impossible to connect all the bays- such as transmission lines, transformers, reactors, shunt capacitors, etc. into a single point. But, the bus-bar system provides the best solution that can connect all equipment together and this node is known as the bus-bar system.

Typical Sectionalized Double Bus bar Arrangement System

The figure shows a typical substation with two main bus-bar with a bus-coupler connection facility, also shows auxiliary bu-bar and outgoing feeder connection with isolator and circuit breakers. Different configurations in substations and multiple bus bars are used to assure a constant operation even if a circuit breaker or any other element requires maintenance.

Copper Bus-bar Selection Chart

Electricity is usually sent over electrically conductive metal wires. The wire is more convenient because we can put insulation on it and bend it around corners, move it around, string it between power poles, that sort of thing. But, sometimes wire isn’t the best option, it may be easier if we used a solid conductor instead of wire which is bus-bar. The chart below shows a copper bus-bar selection procedure.

Aluminum Bus-bar Selection Chart

Using slabs of metal instead of the cable has some advantages- we can bolt them into place using insulators, and just like a welding table, we can attach a connector anywhere on the bus bar. The chart below shows the Aluminum bus-bar selection procedure.

What are the Advantages of Bus-bar Trunking over Cabling System?

1. Bus-bar trunking save the cost compared to a cabling system
2. Bus-bar trunking reduce installation times on site compared to hard-wired systems.
3. Bus-bar trunking provides increased flexibility in design for future modifications.
4. Greater safety and peace of mind for specifiers, contractors and end-users.
5. Bus-bar trunking is easy to design/estimate and the installation stage is also cost-effective.
6. Distribution bus bar distributes power along its length through tap-off points along the bus bar, tap-off units are plugged in along the length of the bus bar to supply a load that could be a sub-distribution board what reducing production downtime.
7. Installed vertically the same systems can be used for rising-mains applications.
8. Certified fire barriers are available at points where the bus bar passes through a floor slab.
9. Very compact so provides space savings.
10. Protection devices such as fuses, switch fuses or circuit breakers are located along the bus bar run, reducing the need for large distribution boards and cables running to and from installed equipment.
11. Bus bar trunking can be installed with a natural galvanized, aluminium, or painted finish.
12. Special colours to match switchboards or a specific colour scheme are also available on request.
13. Uneven distribution of current takes place where multiple runs of cables are used in parallel.

WHAT IS PCD (PITCH CIRCLE DIAMETER)?

How to Calculate PCD (Pitch Circle Diameter)?

PCD is the short form of Pitch Circle Diameter; the Pitch Circle Diameter (PCD) is the diameter of that circle that passes through the center of all the bolt holes or wheel bolts or wheel rim holes or studs.  The best example is Flanges, there are multiple holes in the Flanges, the circle through the center of these holes is known as the pitch circle, and the diameter of this circle is known as Pitch Circle Diameter, in short PCD. Look at the figures and find the PCD of different stud arrangement circles.

Studs: the definition of a stud is “a large-headed piece of metal that pierces and projects from a surface, especially for decoration”, shown in the figure as different forms of stud arrangement, such as 3 studs means 3 holes; 4 stud means 4 holes; 5 stud means 5 series of holes and so on.  

TEST OF THE OUTER PROTECTION OF THE BURRIED JOINT IN ACCORDANCE WITH IEC 60840

Fig-Typical Medium Voltage Cable Joint

Here in this article we will find a case study of type test report for direct buried type cable joint. Hope these text will help us to understand what kind of test actually required for underground power cable direct buried joints. In another article “TYPE TESTS ON COMPLETE UNDERGROUND POWER CABLE” we discussed detail a case study to make sense on type test of an XLPE cupper core underground power cable.

Dry heating cycle test in accordance with IEC 60840

On the joint three heating cycles without voltage were performed. Each heating cycle consisted of 8 h heating and 16 h of natural cooling. During the last 2 h of each heating period the conductor reached a temperature of at least  95 C (5 C above the maximum rated temperature).

Result: The test was performed in a correct way.

Water immersion and heat cycling in accordance with IEC 60840

After the three dry heating cycles, mentioned under 3.1 the joint was immersed in water, at a depth of at least 1 meter at the highest point of the outer protection. Then the joint was subjected to 20 heating cycles. In each heating cycle the water around the joint was heated to a temperature between 70 °C and 75 °C (between 15 °C and 20 °C below the maximum rated temperature of the cable conductor) and kept on this temperature for at least five hours. Hereafter the water was cooled down to a temperature less than the ambient temperature +10 °C.

Result: The water immersion and the heat cycling did not give reason for remarks.

DC voltage test on insulation joint in accordance with IEC 60840

After the water immersion and heat cycling, a voltage of 20 kV DC was applied for 1 minute between the metallic screen and the earthed exterior of the joint outer protection. Hereafter a voltage of 20 kV, DC was applied for 1 minute between the metallic screens on both sides of the joint. The test was carried out while the test installation was at ambient temperature.

Result: No breakdown occurred.

Impulse withstand voltage test on insulation between the screens and water in accordance with IEC 60840

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After the above mentioned tests the test set-up was tested with an impulse voltage of 37.5 kV. The test voltage was applied between the metallic screen and the earthed exterior of the joint outer protection. The test was carried out while the test installation was at ambient temperature.  The test was carried out in accordance with IEC 60230. The joint was tested with ten positive and ten negative voltage impulses.

Result: No breakdown occurred.

Impulse withstand voltage test on sheath sectionalizing insulation in accordance with IEC 60840

After the above mentioned tests the joint was taken out the water and the sheath sectionalizing insulation was tested with an impulse voltage of 75 kV. The test voltage was applied between the metallic screens on both sides of the joint. The test was carried out while the test installation was at ambient temperature.  The test was carried out in accordance with IEC 60230. The joint was tested with ten positive and ten negative voltage impulses.

Result: No breakdown occurred

Examination of the joint in accordance with IEC 60840

The joint was dismantled and inspected for water penetration and cracks.

Result: No signs of cracks or water penetration were detected. The construction of the joint complied with the construction drawing.

Hope this article on TEST OF THE OUTER PROTECTION OF THE BURRIED JOINT IN ACCORDANCE WITH IEC 60840 is helpful for you and your friends. If you think it’s need some add, put your comments below- comment space.

List Of All International Standard

The IEC has now more than 10 000 valid publications in its library.

In 2017, 841 publications were issued, including 507 IEC International Standards, 44 Technical Specifications, 71 Technical Reports, 9 Publicly Available Specifications, and 3 Guides, as well as 207 publications developed outside of the IEC.

Within the four IEC CA Systems, certification bodies issued around 100 000 certificates in 2017, covering a large number of different technologies, products, systems, and services as well as the competence of persons undertaking key roles including maintenance and repair.

What is Standard?

HOW CALCULATE CAPACITANCE OF TWO PARALLEL CABLES

How Calculate Capacitance of Underground Power Cable?

 Capacitor is nothing but a two parallel metal plate separated by a dielectric that store electrical energy; formally we can define capacitor is a passive two terminal electrical component that used to store energy electrostatically in electric field, it also known as condenser. Practically a capacitor usually used a thin films of metal, aluminum foil or disks, etc.

The property of capacitor is known as capacitance, means the capacitance is the ability of a body to store an electric charge. The unit of capacitance is farad, symbol of farad is F, mostly common is micro farad µF.

The formula to calculate the capacitance between two cables is as

C = (Permittivity*Area)/D,

Where, A is the area is that of one cable, D is the distance between the center of the cable and the permitivity is that of the material between the cables.

Capacitance may measure as

C= Q/V= Charge/Potential difference.

How Calculate Single Core Cable

Let,r = radius of the inner conductor and d = 2rR = radius of the sheath and D = 2Rε0 = permittivity of free space = 8.854 x 10-12εr = relative permittivity of the mediumConsider a cylinder of radius x meters and axial length 1 meter. x be such that, r < x < R.Now, electric intensity Ex at any point P on the considered cylinder is given as shown in the following equations.Then, the potential difference between the conductor and sheath is V, as calculated in equations below.After that, capacitance of the cable can be calculated as C= Q/V

Capacitance Between Two Parallel Cables

Practically in electrical transmission lines both in overhead and underground cable system. Simply we can find two wire system both in dc or ac type. If the transmission system so long and voltage is high, charging current drawn by the metallic line conductor due to their capacitance property between two conductors.

If two conductor A & B are placed apart distance between d, consider r is radius of  each conductor, Q is the charge in conductor/meter (if +Q at A & –Q at B); then electric intensity

= Q/ (2π€₀€_rx) V/m

How Calculate Capacitance Of Three Core Cable 

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Consider a three cored symmetric underground cable as shown in the following figure (i). Let Cs be the capacitance between any core and the sheath and Cc be the core to core capacitance (i.e. capacitance between any two conductors).

In the above figure (ii), the three Cc (core to core capacitance) are delta connected and the core to sheath capacitance Cs are star connected due to the sheath forming a single point N. The circuit in figure (ii) can be simplified as shown in figure (iii). Outer points A, B and C represent cable cores and the point N represents the sheath (shown at the middle for simplification of the circuit).

Therefore, the whole three core cable is equivalent to three star connected capacitors each of capacitance Cs + 3Cc as shown in fig. (iii).The charging current can be given as,I_c = 2πf(Cs+3Cc)V_ph      A

Measurement Of Cs And Cc

In order to calculate Cs and Cc we perform various experiments like:

1. First, the three cores are connected together and capacitance between the shorted cores and the sheath is measured. Shorting the three cores eliminates all the three Cc capacitors, leaving the three Cs capacitors in parallel. Therefore, if C₁ is the now measured capacitance, Cs can be calculated as, Cs = C₁/3.
2. In the second measurement, any two cores and the sheath are connected together and the capacitance between them and the remaining core is measured. If C₂ is the measured capacitance, then C₂ = 2Cc+Cs (imagine the above figure (iii) in which points A, B and N are short circuited). Now, as the value of Cs is known from the first measurement, Cc can be calculated.

To know more details about capacitor read or download pdf files

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CAPACITANCE

DRAWINGS DIAGRAMS AND CALCULATIONS FOR UNDERGROUND CABLE PROJECT

General Requirements And Importance Of Drawings-Diagrams For Underground Power Cable Project

The employer may engaged or contract with service-provider to execute an underground power cable project, service-provider actually carry out the whole works as per employer requirements. Here in this article we would like to focus on general requirements and importance of various drawings, designs and calculation sheet to execute an underground power cable project.

The term “drawing” should also included here diagrams, schedules, performance curves, and calculations etc. required for the comprehensive design of the works. The Service Provider should be responsible for the provision of all drawings required for the various stages of the Contract to execute the project.

All drawings, apart from workshop drawings, should be submitted to the nominated Engineer for his approval, in accordance with an approved program. 

The Service Provider should ensure that drawings are submitted for approval in prior enough time such that they may be approved within the specified period by the Engineer, prior to manufacture or construction commencing. 

Further adequate time must be allowed by the Service Provider to permit any comments for revised or modification made by the Engineer to be incorporated.

Any works performed prior to approval of drawings by the Engineer will be entirely at the Service Provider’s own risk including any delays that may result from modifications being found to be necessary by the Engineer.

The numbers of drawings required and the method of issue should be mentioned  in flow diagrams to  specification that keep free from any confusion.

The Service Provider should be fully responsible for obtaining any drawing or data of existing plant and installations that he requires in order to carry out the works, and should also be responsible for verifying that any drawings of existing plant and installations are accurate.

The Service Provider should provide suitable drafting and other staff on site that he requires investigating and producing any drawings that he requires of existing equipment and installations in order to carry out the works.

Where existing installations have been modified or extended the Service Provider may  provide complete new sets of drawings. In this respect the Service Provider should provide drawings detailing both the existing and new works and may not limit the scope of the drawings to the new works only.

What Should be Format of Drawings and Calculation Sheet ?

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Drawings are to be submitted for approval on paper prints, folded to A4 size with the project title-block and drawing numbers clearly visible. Details of the project title-block must be as per prior specified by employer.

All drawings are to be submitted on A series paper to ISO/5457. The maximum size of drawings should be A1 except for site survey and layout drawings which may be submitted as A0 size sheets, if necessary, to accommodate details on a scale of 1:100. Single line diagrams and schematic drawings should preferably be on a maximum sheet size of A2. All dimensional drawings should be the following scales and detailed.

• 1:1, 1:2, 1:5, 1:10 and factors of 10 thereof;
• Drawings symbols should be in accordance with IEC 117.
• All drawings are to be submitted in Auto Cad format in CDR Disk.
• Drawing titles should clearly identify the specific function of the drawings and where appropriate the name of the site(s) to which the drawing applies.

Cable Schematic Line Diagram

How Follow Drawing Numbering and Revisions?

The Service Provider should be responsible for adding the nominated Engineers drawing numbers to all drawings prior to submittal. Following award of the contract the Engineer and Service Provider will review the numbering system, familiarize with each other requirements, and agree on the numbering system to be applied.

Comprehensive cross-reference are to be included on drawings and the Service Provider should include the Engineer’s drawing number in the cross-references.

At each and every issue of a drawing the revision should be raised, and details given in revision boxes on the drawings. Comprehensive details of revisions are to be given and phrases such as “REVISED”, “UPDATED”, “MODIFIED” or similar are not acceptable.

Reference to any drawing in communications should include the Engineer’s drawing number.

Drawing Submittals and Approvals Procedure

The Service Provider should submit drawings for the plant and works for formal approval to the nominated Engineer. A program of drawing submittal should be agreed with the Service Provider following the contract award. 

Drawings issued in accordance with this program should take account of the time periods necessary for postage, and approved by the Engineer, to ensure that approved drawings are available prior to manufacture the materials for the project execute. For site construction works, “Construction Issues” drawings are to be available, on site, at least 21 days prior to the commencement of the works.Where appropriate the drawings should be accompanied by supporting calculations.

Following examination the Engineer should allocate a status on drawings. The subsequent action and distribution of drawings will depend on the status given by the Engineer as detailed below.

“Approved”, the details of the drawing have been checked by the Engineer and appear to comply with the requirements of the specification. Once approved the Service Provider should raise the revision to indicate Approved by the Engineer on Mark the drawing “Construction Issue” and distribute as per employer requirement that mentioned in contract.

“Approved Subject to Comment”. The drawing has been examined by the Engineer, and apart from the minor details can be considered Approved subject to the Service Provider making the amendments required by the Engineer. The Service Provider may issue the drawing as “Approved” as outlined above without resubmitting to the Engineer for formal approval, provided the corrections have been made as required. 

“Examined and Returned with Comments”. The drawing is considered to be revised by the Service Provider and immediately resubmitted to the Engineer for approval. 

“Examination not required”. The drawing not requiring approval by the Engineer has been examined by the Engineer. Examples of typical drawings of this nature are: equipment schedules and diagrams of connections. Drawings returned to the Service Provider of this status should be stamped “For information only” and “Construction Issue” and issued in the same manner as Approved drawings.

Approval of a drawing by the Engineer must in no way to relieve the Service Provider of his responsibilities under the Contract.

Service Provider may revise a drawing for any reason following approval by the Engineer, the revised drawing is to be resubmitted for re-approval by the Engineer, the original approval automatically being void.

The Service Provider must submit,  marked-up copies of the drawing issue programmed indicating the up to date status on drawing submittal and approval on a monthly basis. In addition the Service Provider should submit a schedule of the total number of drawing submitted, together with the total numbers of each of the above categories. An “S” curve is also to be submitted indicating the total number of planned approved drawing together with the actual numbers to date.

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As Built Recorded Drawing and Construction Amendment for Project

During construction of the Works on the site, the Service Provider should ensure that all departures, modifications and corrections to the approved drawings are recorded. All such changes to the drawings are to be marked in red to show on “as-built” stated and one set of “as-built” drawing are to be available on the site at all times.

If Service Provider wish to make modifications as per site requirement on the approved construction drawings which influence the operation of the Plant, he should obtain the approval of the Engineer’s representative prior to instituting the modifications.

Following examination of the “as-built” drawing the Engineer’s representative should return one copy to the Service Provider indicating approval of the construction modifications, or further modifications required to satisfy the requirements of the specifications.

Record Drawing for Cable Line Construction Project

On receipt of Approval of “as-built” drawings, the Service Provider  be responsible for the production of Record Drawings for the complete plant on each section of the Works.

The Service Provider should modify the revision to “As-Built” and “Record Drawing” for all drawings applicable to the section of the works and submit these for approval to the Engineer.

On receipt of the Engineer’s approval the Service Provider will provide sets of paper prints of each drawing as detailed on pre-specification of the contract. Any contract drawing included in the Operation and Maintenance manuals should also be revised in accordance with any “as-built” modifications and re-issued.

These sets of record drawings should be issued within 90 days of the Taking-over of the completed plant.