ElectricityPower Plants and Distribution (year 7) · 144 h · Topic 4 of 7

Substations and distribution lines

Between the generator and the wall socket there is a chain of transformers, switching devices and kilometres of conductors that someone has to design, build, operate and maintain. This page follows the last stretch of that chain —from medium voltage to the user’s door— and also the way of working it demands: planning the job, controlling its quality, meeting deadlines and leaving no fires or spills behind.

Distribution transformer station Single-line diagram Overhead and underground lines Voltage drop Critical path

01What a substation is and what types exist

A substation is the set of equipment where voltage is changed or regulated and from which energy is distributed: transformers, switching and protective devices, busbars and the instruments that measure and watch over all of it. Substations are told apart by what they do:

Step-up, at the power plant: raises the generator voltage so the energy can be transmitted (see Electrical energy).
Transformer substation (ET): steps down from high voltage (132 kV, for example) to medium voltage (33 or 13.2 kV) and feeds several feeders.
Distribution transformer station (CT, from centro de transformación): the last step, where 13.2 kV becomes 380/220 V for the neighbourhood, the building or the factory.
Switching or sectionalizing: does not transform; it only connects, disconnects and reconfigures the grid to isolate a fault or to allow work.

This page deals mostly with the CT, the one most often designed and built in a technical school, and its ideas hold for a large ET, which only adds more equipment. By construction, CTs come in four types:

Type	What it looks like	Typical rating	Where it is used	Pros and cons
Overhead (pole or platform)	The transformer hangs from a pole or rests on a platform between two. Switching is done with fuse cut-outs and lightning is handled by surge arresters. A cable runs down to a low-voltage board.	up to about 160 kVA on one pole; up to about 315 kVA on two	Rural and suburban areas, new housing plots.	Cheap and quick to install. Outdoors, with simple protection, and accessible to anyone who climbs.
Ground-level (in a kiosk or room)	Masonry or concrete room with medium-voltage switchgear, transformer and low-voltage board. Entered from the pavement or the yard.	250 to 1,000 kVA	Factories, neighbourhoods, buildings, large shops.	Protected and easy to operate. Takes up land and needs ventilation and an oil pit.
Underground (in a vault)	Vault under the pavement or in a basement; cables in and out.	250 to 630 kVA	Dense city centres, where there is no room or overhead lines are not allowed.	Out of sight. Expensive, needs forced ventilation and can flood.
Compact (prefabricated)	Metal or concrete enclosure delivered assembled and factory-tested; placed on a base and the cables connected.	100 to 1,000 kVA	Industrial parks, new neighbourhoods, jobs with a short deadline.	Installed in a few days with factory quality. Needs a crane and transport.

The ratings in the table are orders of magnitude: each utility sets the types and limits it accepts in its area. Transformers are made in standard ratings (63, 100, 160, 250, 400, 630, 1,000 kVA…), and the usual distribution transformer is connected delta on the medium-voltage side and star with neutral on the low-voltage side (vector group Dyn11), so the 380/220 V neutral comes straight out of the secondary.

Oil or dry

The mineral-oil transformer is the most common: the oil insulates and also carries heat from the core and windings to the tank walls (natural cooling, ONAN). The cast-resin dry type has no liquid: it is used inside buildings, hospitals or places with many people, because there is no oil to spill or burn. It costs more and tolerates overloads less well.

A CT is not opened “just to look”

Inside a distribution transformer station there are 13,200 V and, even if the door is at street level, access is only for personnel authorized by the utility or by the person in charge of the installation. Medium voltage does not need contact to kill, and work on it follows its own procedures (see Live-line work and Occupational health and safety).

02The single-line diagram and its equipment

A single-line diagram draws with one line what is really three phases. It does not show every cable but the order in which energy passes through the devices and what protects what. It is read from left to right, from the grid to the user. The figure shows a 400 kVA, 13.2/0.4 kV CT and four situations it goes through in its life.

Figure 1. Single-line diagram of a 13.2/0.4 kV distribution transformer station, animated through four situations. Green marks what is at 13.2 kV, orange what is at 0.4 kV and grey what is dead. Note in which one the circuit breaker acts and in which the disconnector is used.

Equipment	What it does	What it can interrupt
Disconnector	Separates a section and leaves a visible isolating gap: you can see it is open.	Nothing under load. It is operated with no current.
Load-break switch	Switches the CT in normal service. Often has fuses in series.	Rated current, but not a short circuit.
Medium-voltage fuse	Protects the transformer and the grid from a short circuit. It blows and is replaced.	Short circuits, once.
Circuit breaker	Makes and breaks under load and clears short circuits, controlled by relays. It is reset and reused.	Load and short-circuit current.
Current transformer (measuring CT) and voltage transformer (VT)	Reduce current and voltage to standard values (for example 5 A or 1 A in the CT) that instruments and relays can measure.	Does not interrupt: measures.
Protective relay	Receives the current-transformer signal, compares it with its setting and orders the breaker to open. ANSI/IEEE device numbers identify it: 50 (instantaneous overcurrent), 51 (time overcurrent), 87 (differential).	Does not interrupt: gives the order.
Surge arrester	Diverts lightning and switching overvoltages to earth before they damage the insulation.	Does not interrupt: protects the insulation.
Busbars	Thick copper or aluminium conductor to which the incoming and outgoing circuits connect: the distribution point.	—
Earthing	Bonds the metal parts and the neutral to ground, to limit touch and step voltages and to give the fault current a path (Earthing, insulation and generator testing).	—

A disconnector is not a circuit breaker

Opening a disconnector with current draws a sustained arc that can spread between phases and injure whoever stands next to it. So the order is always the same: to de-energize, first the circuit breaker (which breaks under load) and only then the disconnector; to energize, the reverse: first the disconnector, then the circuit breaker. Well-built switchgear has an interlock that prevents reversing the sequence.

Transformer protection

Besides fuses or the overcurrent relay, an oil transformer carries its own protections: a thermometer with contacts that alarms and trips on temperature, an oil level indicator, a pressure relief valve and, on those with a conservator tank, a Buchholz relay, which detects the gas formed inside the tank when there is an arc or internal overheating. Large units add differential protection. How they are coordinated with one another is in Electrical protection and the transformer-specific protection is in Supply quality and machine protection.

Example · Currents of a 400 kVA transformer

A 400 kVA transformer, ratio 13.2/0.4 kV, with short-circuit voltage u_k = 4 %. What current flows on each side, and what short-circuit current must the low-voltage breaker withstand? With the three-phase current formula:

I_{n} = \frac{S_{n}}{\sqrt{3} \cdot V_{n}}

Sₙ: rated power (kVA) · Vₙ: rated line voltage of the side being calculated

I_{1 n} = \frac{400 kVA}{\sqrt{3} \cdot 13.2 kV} = 17.5 A

I_{2 n} = \frac{400 kVA}{\sqrt{3} \cdot 0.4 kV} = 577 A

The short-circuit current at the low-voltage terminals, assuming a very strong medium-voltage grid (the worst case for the breaker), is estimated from the transformer’s short-circuit voltage:

I_{cc} \approx \frac{I_{n}}{u_{k}}

uₖ: short-circuit voltage of the transformer, per unit (4 % = 0.04)

I_{cc} \approx \frac{577 A}{0.04} = 14 400 A \approx 14.4 kA

The main low-voltage breaker must have a breaking capacity above 14.4 kA: the next standard value is chosen, for example 25 kA. A 10 kA unit would be destroyed by the first short circuit. On the medium-voltage side the rated current is only 17.5 A; the fuse is chosen from the manufacturer’s table so that it withstands the transformer’s inrush current —several times the rated value for tenths of a second— without blowing.

03Distribution lines: overhead and underground

From the CT leave the low-voltage networks to the users, and to it arrive the medium-voltage ones. Both can run through the air, held up by poles, or underground, in insulated cables. The two solutions coexist in any city.

Overhead lines

Element	Function and variants
Conductors	Carry the current. In rural medium voltage, bare aluminium conductor steel reinforced (ACSR): the aluminium conducts and the steel takes the tension. Where there are trees, covered or protected conductors are used. In low voltage, aerial bundled cable: three phases insulated with cross-linked polyethylene (XLPE), twisted around a bearing neutral.
Insulators	Separate the conductor from the support and withstand the voltage. Pin insulators where the line runs straight; suspension insulators (strings) where the conductor is dead-ended or turns. Made of porcelain, toughened glass or polymer (silicone). Sized by voltage and by creepage distance, which must grow near the coast, industry or dust.
Structures	Reinforced-concrete poles (the most used), treated-wood or steel poles; lattice towers at higher voltages. By function: suspension (tangent), dead-end, angle, terminal and branch structures. Designed for weight, wind and conductor tension.
Crossarms, brackets and fittings	Concrete, wood or galvanized steel: hold the insulators and set the phase spacing.
Guy wires	Steel cables that counter the lateral force on terminal, angle and dead-end poles.
Earthing and arresters	Metal fittings are earthed; arresters are placed at the start of the line, at CTs and at points exposed to lightning.

Between two poles the conductor hangs in a curve (a catenary). The vertical distance between the straight line joining the attachment points and the lowest point is the sag, and the distance between poles, the span. When it is hot the conductor expands and the sag grows, so the clearance above ground falls; when it is cold the conductor contracts and the tension rises. Both extremes are checked at design. Heights and safety clearances to roads, buildings and trees are the subject of Faults and safety in distribution networks.

Underground lines

A medium-voltage cable is not a low-voltage cable with more insulation: it has a layered structure designed to control the electric field, from the inside out:

Layer	Function
Conductor	Copper or aluminium, usually stranded.
Inner semiconducting layer	Smooths the field over the rough surface of the conductor and prevents partial discharges.
Insulation	Cross-linked polyethylene (XLPE) in modern cables; oil-impregnated paper in old ones.
Outer semiconducting layer	Completes the field control on the outside face of the insulation.
Metallic screen	Earthed copper (wires or tape): confines the field inside the cable and carries the fault current.
Outer sheath	Protects against moisture and mechanical damage; if armoured, it has a metal armour.

The cable can be laid directly buried (trench with a sand bed, mechanical protection on top and warning tape) or inside ducts with pull pits, which allow replacing it without opening the street. Joints and terminations are the most delicate points: a high share of faults occurs there, and only qualified personnel make them, with the manufacturer’s kit. The current rating depends on how the cable is laid (buried or in duct), on the soil temperature and thermal resistivity, and on how many cables run together. The minimum bending radius and the maximum pulling tension are given by the manufacturer.

Aspect	Overhead line	Underground line
Initial cost	Lower.	Considerably higher (trenches, cables, joints).
Construction	Fast.	Slower; works on public roads.
Faults	More frequent (wind, branches, birds, collisions), but almost always temporary.	Less frequent, but almost always permanent (insulation damage).
Repair	The fault is visible and is repaired in hours.	It must be located with instruments and dug up; it can take days.
Safety and landscape	Accidental contact is possible and the landscape is affected.	No contact risk except for excavations; no visual impact.
Where it fits	Countryside, suburbs, long runs.	Dense city centres, protected areas, special crossings.

Which standard applies

Distribution transformer stations and medium-voltage supply stations are regulated by AEA 95401, outdoor underground power lines by AEA 95101, and outdoor overhead lines by the AEA regulation for that type of line. Materials are tested to IRAM standards. In Córdoba, the utility also sets its own requirements for connecting a CT to its network, under the supervision of ERSeP. The user’s internal installation, downstream of the meter, is governed by AEA 90364 (Electrical installations).

04Voltage drop along a line

A conductor has resistance and, in AC, also reactance. When current flows, each section “keeps” part of the voltage, and what arrives at the end is lower than at the source. That difference is the voltage drop. It is not a detail: an induction motor develops a torque that depends on the square of the voltage, so with 10 % less voltage it delivers barely 81 % of its torque; lamps give less light and electronic equipment resets. For a three-phase line with the load concentrated at the end:

Δ V = \sqrt{3} \cdot I \cdot L \cdot (R′ \cdot \cos φ + X′ \cdot \sin φ)

R′ and X′: resistance and reactance per kilometre (Ω/km) · L: length (km) · I: line current (A)

ε = \frac{Δ V}{V_{n}} \cdot 100 %

Reactance only matters in medium voltage and with thick conductors; for small-section low-voltage cables the simplified form is used (in a single-phase circuit the current goes out and back):

Δ V \approx \sqrt{3} \cdot I \cdot L \cdot R′ \cdot \cos φ

Approximation for low voltage with small-reactance cable (X′ ≪ R′).

Δ V = 2 \cdot I \cdot L \cdot R′ \cdot \cos φ

Single-phase circuit: the current goes out and back (hence the 2).

The admissible value is set by the regulations or by the utility for each type of network: the example uses 5 % as a design criterion, and internal installations have their own limits in AEA 90364. To correct an excessive drop there are four ways: increase the cross-section, shorten the run, raise the line voltage or reduce the current (for example by correcting the power factor, see AC power). At the transformer, the tap can also be changed, normally in 2.5 % steps.

Example · A 13.2 kV, 8 km line

A three-phase 13.2 kV, 8 km line feeds a 1.5 MW load with cos φ = 0.9 at the end. Two ACSR conductors are compared, with assumed catalogue data: 50/8 mm² (R′ = 0.59 Ω/km, X′ = 0.38 Ω/km) and 95/15 mm² (R′ = 0.31 Ω/km, X′ = 0.35 Ω/km). Do they comply with a maximum drop of 5 %?

Current and sine of the angle:

I = \frac{1.5 MW}{\sqrt{3} \cdot 13.2 kV \cdot 0.9} = 72.9 A

\sin φ = \sqrt{1 - {0.9}^{2}} = 0.436

50/8 mm² conductor:

Δ V = \sqrt{3} \cdot 72.9 A \cdot 8 km \cdot (0.59 \cdot 0.9 + 0.38 \cdot 0.436) Ω/km = 704 V

ε = \frac{704 V}{13 200 V} \cdot 100 = 5.33 %

95/15 mm² conductor:

Δ V = \sqrt{3} \cdot 72.9 A \cdot 8 km \cdot (0.31 \cdot 0.9 + 0.35 \cdot 0.436) Ω/km = 436 V \to ε = 3.30 %

The 50/8 mm² conductor fails: the drop (5.33 %) exceeds the limit and the voltage at the load falls to 12,496 V. The 95/15 mm² passes with 3.30 %. Besides the drop, the smaller section wastes more energy as heat:

Δ P = 3 \cdot R \cdot I^{2}

R: resistance of one conductor over the whole line (R′ · L)

Δ P_{50/8} = 3 \cdot (0.59 \cdot 8 Ω) \cdot {(72.9 A)}^{2} \approx 75 kW

Δ P_{95/15} = 3 \cdot (0.31 \cdot 8 Ω) \cdot {(72.9 A)}^{2} \approx 39.5 kW

The thinner conductor wastes 75 kW, 5 % of the power carried, against 39.5 kW (2.6 %) for the thicker one. The thick conductor costs more to buy, but the difference in losses is paid every hour of service for decades.

Figure 2. Voltage profile of the example line with two conductors, animated. The voltage falls in a straight line with distance; the 50/8 mm² conductor crosses the 5 % limit at about 7.5 km and the 95/15 mm² reaches 8 km with margin.

05Planning the job: quality, productivity and critical path

A substation or a line is built with crews, trucks, materials that arrive from outside and permits that depend on third parties. A good work process always goes through the same stages: survey and design, planning, execution with control, testing and commissioning and close-out, with the as-built drawings and the final report.

Quality: doing well what was asked

Quality is not decoration but a list of checks agreed before starting. An inspection and test plan sets when each thing is checked, with which instrument, what value is accepted and who signs it; some checks are hold points: without approval work does not continue (for example, depth and verticality of the poles before concreting). What does not comply is a nonconformity: it is recorded, corrected, and its cause is sought.

Check	When	What is verified
Receiving materials	On arrival at site	That they match the specification and IRAM standards, the nameplate and the factory test report.
Setting out and foundations	Before concreting or erecting	Location, depth and verticality of the poles; design distances and heights.
Cable insulation	Before energizing	Insulation resistance with a megohmmeter of suitable voltage, against the value in the cable specification.
Earthing	Before energizing	Earth resistance, measured with an earth tester, less than or equal to the design value.
Transformer and protection	At commissioning	Turns ratio, oil level and dielectric strength, relay settings and breaker tripping.
As-built drawings	At close-out	That the drawing shows what was actually built, not what was designed.

Productivity: how well the work performs

Productivity relates what is produced to the resources used to produce it. In a line job it is measured in metres of line strung, poles erected or connections made per person-hour.

P = \frac{quantity produced}{resource used}

Example · A crew of four

A crew of 4 people strings and sags 400 m of line in 3 working days of 8 hours. The budget assumed 5 m per person-hour. How did it perform?

P = \frac{400 m}{4 workers \cdot 3 d \cdot 8 h} = \frac{400 m}{96 h·worker} = 4.17 m/(h·worker)

\frac{4.17}{5} \cdot 100 = 83 %

It delivered 83 % of what was planned. Before demanding more from the crew it is worth asking why: was material missing at the work front? Did a section have to be redone? Did it rain? Rework, waiting and travel take more hours than the task itself, and quality and productivity go together: what is done badly twice performs at half.

Schedule and critical times

The schedule arranges tasks in time. It starts from the work breakdown (which tasks there are), estimates the duration of each from the expected productivity, defines the precedences (what must finish before what) and assigns resources. From this a Gantt chart or a task network is drawn and the critical path is calculated: the longest sequence of tasks, which sets the duration of the whole job. Every task has an earliest start and finish (ES, EF) and a latest start and finish (LS, LF), which are those that still do not delay the job; their difference is the float.

{ES}_{j} = \max ({EF}_{i}) for every task i that precedes j

{EF}_{j} = {ES}_{j} + d_{j} float = {LS}_{j} - {ES}_{j}

ES: earliest start · EF: earliest finish · LS: latest start · d: duration

Example · Critical path of an overhead CT with 400 m of line

The job has eight tasks. The transformer is purchased and takes 7 days to arrive. Durations are in working days.

Task	Duration	Precedes	ES	EF	LS	LF	Float
A · Survey and permits	2	—	0	2	1	3	1
B · Pole holes and poles	4	A	2	6	3	7	1
C · Conductor stringing	3	B	6	9	8	11	2
D · Transformer delivery	7	—	0	7	0	7	0
E · CT assembly	2	B, D	7	9	7	9	0
F · Earthing system	2	B	6	8	9	11	3
G · LV connection and board	2	E	9	11	9	11	0
H · Testing and commissioning	2	C, F, G	11	13	11	13	0

Forward pass: task E needs B and D to finish, and D finishes later, so E starts on day 7. And H waits for the three tasks that precede it:

{ES}_{E} = \max ({EF}_{B}, {EF}_{D}) = \max (6, 7) = 7 \to {EF}_{E} = 7 + 2 = 9

{ES}_{H} = \max (9, 8, 11) = 11 \to {EF}_{H} = 11 + 2 = 13 d

The duration is 13 days. The backward pass, from day 13, gives the latest starts and the float of each task. Those with zero float (D, E, G and H) form the critical path: 7 + 2 + 2 + 2 = 13. The other routes are shorter: A, B, C and H add up to 2 + 4 + 3 + 2 = 11 days (2 of float) and A, B, F and H add up to 10 (3 of float). The conclusion surprises whoever does not plan: what sets the schedule is not the field work but the delivery of the transformer.

Figure 3. Critical path of the example job, animated. First the tasks and their precedences; then the critical path (in red); and finally what happens when a critical task slips and when a task with float slips.

What to do with the critical path

To shorten the job, only shortening the critical tasks works: order the transformer sooner, add resources to the assembly, do in parallel what can be done in parallel. Speeding up tasks with float changes nothing. To avoid falling behind, watch the critical ones every day and those with short float every so often. And float is managed: it is margin for the unexpected, and a task that uses it up becomes critical.

06Efficiency, fire prevention and environmental care

Efficiency: losses paid for every day

A network is not perfect: part of the energy is lost as heat, in the transformer and in the conductors. The transformer losses are of two kinds (see Transformers): the iron losses, which are always there while it is connected, even with no load, and the copper losses, which grow with the square of the current. That is why efficiency has a maximum, reached when the two are equal:

x_{opt} = \sqrt{\frac{P_{0}}{P_{k}}}

x: fraction of rated power at which efficiency peaks · P₀: no-load (iron) loss · Pₖ: full-load (copper) loss

Example · At what load is a 400 kVA transformer most efficient?

A 400 kVA transformer has (catalogue data, indicative) P₀ = 0.93 kW of no-load loss and P_k = 4.6 kW at rated load. With power factor 0.9:

x_{opt} = \sqrt{\frac{0.93 kW}{4.6 kW}} = 0.45

Load	Output power	Total losses	Efficiency
20 %	72 kW	1.11 kW	98.48 %
45 %	162 kW	1.86 kW	98.86 %
100 %	360 kW	5.53 kW	98.49 %

The difference in percentage looks small, but the transformer is connected all 8,760 hours of the year: choosing one too large “to be safe” makes it work far from its best point and extra no-load losses are paid; choosing one too small overheats it and shortens its life. The criterion is to size for the real demand and leave a reasonable reserve.

The other efficiency levers are those of the previous section’s example —adequate cross-sections, short runs and higher voltage— and keeping the power factor high so that less current flows. In the work process, efficiency also means planning outages so they last the minimum and not repeating switching operations.

Fire prevention

The mineral oil in a transformer does not ignite easily, but when an internal arc heats it, it can burn, and a tank with hundreds of litres feeds a large fire. The usual causes are an internal fault, sustained overload, a loose terminal that heats up and oil contamination. It is prevented with:

Location and separation: distance to buildings, other equipment and combustible materials, with fire walls between transformers or toward neighbouring rooms, as the design requires.
An oil pit under the transformer, filled with stone, which retains the oil and puts out the flame by cooling it, connected to a separator sump.
Protection that disconnects quickly (Buchholz, temperature, overcurrent) and smoke or temperature detection in the rooms.
Indoors, dry transformers or ones with high-fire-point liquids.
Maintenance: re-tightening terminals, thermography, oil analysis (Maintenance management).
Extinguishers suited to fires in live equipment (carbon dioxide or ABC powder), marked and in date, and an emergency plan known to everyone.

Fire in electrical equipment

Equipment that may be live is not put out with water or foam. First the supply is cut —if it can be done safely— and the utility and the fire brigade are called. A CO₂ or ABC powder extinguisher is used from the distance on its label, and nobody enters a room full of smoke: there is arc flash and toxic gas. The fire-protection requirements for the workplace are in chapter 18 and Annex VII of Decree 351/79 (Law 19.587).

Environmental care

Oil spills: contaminate soil and water. The oil pit, the separator sump and periodic checks of gaskets and valves are part of the design, not an add-on. Used oil and soaked materials are hazardous waste (Law 24.051): they are removed by a licensed carrier and treated by a licensed operator, with records.
PCBs (polychlorinated biphenyls): some old transformers and capacitors, from before the 1980s, used oils that contain them. They are persistent organic pollutants: they do not degrade, they accumulate in living things and they are toxic. Law 25.670 sets the minimum requirements for their management and elimination. Nothing is assumed about an old unit: an oil sample is taken and a laboratory analyses it; if it contains PCBs, the unit is labelled, isolated and managed as hazardous waste, without trying to recycle the oil.
SF₆ gas: some medium-voltage switchgear uses it as an insulator. It is a greenhouse gas thousands of times stronger than CO₂: it is not vented, it is recovered.
Lines and surroundings: the route and the right-of-way strip (Law 19.552, administrative easement for power lines) are negotiated with landowners; pruning is controlled and covered conductors or bird diverters reduce the impact on birds. Every project also complies with the General Environmental Law (25,675).
Debris and packaging: drums, wood, iron and concrete waste are sorted and removed, not buried at the site.

07In the workshop and on site

Lab 1 · Reading the single-line diagram of a distribution transformer station

Materials: single-line diagram of a real CT (from the school, a cooperative or the utility) and, if a guided visit is authorized, the matching CT for observation only, from where the person in charge indicates. Steps: 1) identify and name every symbol (disconnector, breaker, CT, relay, arrester, busbars); 2) note the rating, the turns ratio and the protection of each outgoing feeder; 3) calculate the rated current on both sides and the short-circuit current on the low-voltage side; 4) write on a sheet the sequence for leaving the CT dead. What is measured: nothing with instruments. Acceptance criterion: the calculated currents match the diagram or the nameplate within 5 % and the sequence follows the order breaker–disconnector–verification–earthing.

Lab 2 · Measuring a voltage drop

Materials: 50 m of two-core 2.5 mm² cable, fully unrolled, a 1,000 W resistive load (heater or iron), a socket with residual-current and overcurrent protection, a voltmeter and a clamp meter. Steps: 1) with the load connected, measure the voltage at the origin of the cable and at the far end; 2) measure the current; 3) calculate the resistance of the whole (out and back, 100 m) and the expected drop:

R = ρ \cdot \frac{2 \cdot L}{S} = 0.0175 Ω·mm²/m \cdot \frac{100 m}{2.5 mm²} = 0.70 Ω

I = \frac{1 000 W}{220 V} = 4.55 A \to Δ V = 4.55 A \cdot 0.70 Ω = 3.18 V \approx 1.4 %

Acceptance criterion: the measured drop differs from the calculated one by less than 15 % (hot copper has slightly more resistance than at 20 °C) and the voltage at the far end is above 95 % of that at the origin.

Working at 220 V

Lab 2 is done at mains voltage: unrolled cable (a coiled reel under load overheats), insulated connections, protected board, no bare conductor in sight, and the teacher present. The 4.5 A current is not dangerous for the cable, but the voltage is for people.

Lab 3 · Schedule for a CT job

Materials: the task table of section 05 without the ES-to-float columns, graph paper, pencil and ruler. Steps: 1) draw the task network with its precedences; 2) calculate by hand ES and EF of each task (forward pass) and LS and LF (backward pass); 3) mark the critical path and draw the bar chart; 4) repeat the calculation with the transformer delivery delayed 3 days and with the stringing delayed 2 days. Acceptance criterion: duration of 13 days with path D-E-G-H; with the 3-day delay in D, duration of 16; with the 2-day delay in C, duration of 13 and zero float in A, B and C.

Lab 4 · Inspecting an overhead line from the ground

Materials: checklist, camera and binoculars. Steps: walk along 200 m of a low- or medium-voltage line, without going closer than the person in charge indicates and without touching anything; record on the checklist the type of poles and insulators, pole lean, guy wire condition, nearby vegetation, apparent height of the conductors above roads and signage. Acceptance criterion: the checklist identifies each element of the table in section 03 with a photo and the anomalies to report to the utility are listed.

08Common mistakes

Mistake	Consequence
Opening a disconnector under load	A sustained arc that can spread; risk of severe burns. The circuit breaker is opened first.
Choosing the low-voltage breaker by rated current only	If its breaking capacity is lower than the short-circuit current, it is destroyed clearing the first fault.
Calculating the voltage drop with the current of a single phase or the wrong voltage (phase instead of line)	The result is off by a factor of √3 and a conductor that fails looks fine.
Forgetting the “2” (out and back) in a single-phase circuit	The calculated drop is half the real one.
Planning with all the tasks chained one after another	The duration comes out too long and it is not seen which tasks can be done in parallel.
Speeding up tasks with float to recover the delay	Not a single day is gained: what rules is the critical path.
Starting the job without having ordered the long-lead material	The transformer, as in the example, ends up setting the duration of the whole job.
Assuming an old transformer has no PCBs	Hazardous waste mishandled, fines and lasting environmental damage. The oil is analysed.
Throwing water at a burning transformer or board	Electrocution of the person fighting the fire and spread of the fire. The power is cut and the correct extinguisher is used.
Handing over the job without as-built drawings	The next person to work there does not know what is buried or how it is connected.

09Self-assessment

What is the difference between a disconnector and a circuit breaker? Which one is opened first to de-energize?

The breaker interrupts under load and short-circuit current; the disconnector cannot break current and serves to leave a visible isolating gap. To de-energize, the breaker is opened first and then the disconnector; to energize, the reverse. After opening, the absence of voltage is verified and the equipment is earthed.

What is the rated current on the 0.4 kV side of a 160 kVA transformer?

I_{n} = \frac{160 kVA}{\sqrt{3} \cdot 0.4 kV} = 231 A

With that value the main low-voltage protection and the cross-section of the outgoing conductors are chosen.

Name four elements of an overhead line and the function of each.

Conductors (carry the current), insulators (separate the conductor from the support), poles or structures (hold everything up, with mechanical design) and guy wires (counter lateral forces). Also: crossarms and fittings, arresters and earthing.

A 380 V three-phase line feeds 40 kW at cos φ = 0.9 over 150 m, with a cable of R′ = 0.87 Ω/km and X′ = 0.08 Ω/km. What is the voltage drop?

First the current:

I = \frac{40 kW}{\sqrt{3} \cdot 380 V \cdot 0.9} = 67.5 A

Then the drop, with sin φ = 0.436:

Δ V = \sqrt{3} \cdot 67.5 A \cdot 0.15 km \cdot (0.87 \cdot 0.9 + 0.08 \cdot 0.436) Ω/km = 14.3 V \to ε = \frac{14.3 V}{380 V} = 3.8 %

It is 3.8 %: it meets a 5 % limit, and would not meet a 3 % one.

In the example job, what happens if the transformer delivery slips one day? And if the earthing slips one day?

With the transformer one day late (D = 8 d), task E starts on day 8 and the whole job moves one day:

{ES}_{E} = 8, {EF}_{G} = 12, {EF}_{H} = 14 d

The earthing, however, has 3 days of float: a one-day delay does not change the duration.

When is an underground line better and when an overhead one?

The overhead line is cheaper, quicker and easier to repair: it suits the countryside and long runs. The underground line costs more but is out of sight and protected from wind and collisions: it suits dense city centres and protected areas. Its faults are less frequent but almost always permanent, and costlier to repair.

A 1970 oil transformer is going to be taken out of service. What must be found out before moving it?

Whether the oil contains PCBs: it is not assumed, a sample is taken and analysed in a laboratory. If it does, the unit is labelled, any spill is avoided and it is managed as hazardous waste with licensed operators (Law 25.670). In any case, used oil is not thrown away either: it is hazardous waste (Law 24.051).

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