Execution of the electrical works
A well-calculated design is not yet an installation. Between the drawing and the energized switchboard you have to set out, run the raceways, pull, connect, identify and test, and in the end put down in writing what was really built. This page follows that path: the papers that rule the job, the order of the work, cable and tray, the trench, the motor picked from the catalogue, and the acceptance.
01The works file and how to read it
A job is not done “from memory” or “the way it has always been done”: it is done according to a set of documents, the works file, which is both the instruction for whoever builds and the commitment to whoever pays. How the design is calculated is explained in The industrial electrical project and the general method in Installation project; what matters here is what happens when those documents reach the site.
| Document | What it contains | What it is used for on site |
|---|---|---|
| Design drawings | Single-line diagram, raceway and equipment layout, switchboard diagrams, installation details and sections. | Knowing what is installed and where. This is what gets set out. |
| Technical specifications | Materials and accepted makes, standards to meet, installation method, required tests and acceptance criteria. | Knowing how to install and with what quality. This is what gets inspected. |
| Calculation report | Demand, cross-sections, voltage drop, short circuit, protection and its coordination, with the assumptions used. | Understanding why a cross-section or setting is what it is, before changing it. |
| Bill of quantities and budget | Quantity of each material by item, and prices. | Ordering and receiving materials; certifying progress. |
| Schedules | Load, cable (origin, destination, size, length), terminal and label schedules. | Pulling, connecting and identifying without guessing. |
| Catalogues and data sheets | Dimensions, weights, curves and limits of each device, and the manufacturer’s installation instructions. | Checking that what arrived is what was specified, and mounting it properly. |
| Applicable standards | AEA 90364 for installations in buildings, AEA 95101 and 95401 for networks and transformer stations, IRAM material standards. | Settling what neither the drawing nor the specification says. |
How to read a drawing
Before looking at the drawing, read the title block: title, drawing number, scale, date, author and above all the revision table, which says which edition is current. Then the list of legends (what each symbol means, see Component symbols), the general notes —where conditions valid for the whole drawing usually sit— and only then the lines. Dimensions are taken from the dimension figures, never with a ruler on the paper: the printed drawing may be reduced or distorted (the language of drawing is explained in Technical drawing and CAD). A raceway drawing is always cross-checked against the cable schedule and the single-line diagram: every line on the drawing must match a circuit in the schedule.
Working from an old revision is the most expensive way to be wrong: the tray is mounted, the cable pulled, and only then does someone notice the drawing had changed. Before each task, confirm that the copy in hand is the current one. And when two documents contradict each other —a drawing says 4 mm² and the schedule 6 mm²— do not settle it by your own judgement: the conditions of contract set the order of precedence among documents, and the question is raised in writing with the inspector or the designer (in public works, through a request note and a service order). What is agreed by word of mouth does not exist when a cost has to be justified.
Specifications often name a make and add “or equivalent”. A material is equivalent if its technical data —voltage, current, breaking capacity, degree of protection, standards met— are equal or better and the inspector approves it. It is not equivalent because it “looks alike” or because it is “what the hardware store had”. The comparison is made with the data sheets of both manufacturers, side by side.
02Setting out and work sequence
Setting out is taking the drawing to the place: marking on the floor, walls and structure the centreline of each tray, the position of each switchboard, the penetrations and the drops to each machine. It is not copying the drawing at 1:1; it is the last check of the design against reality. While setting out you discover interferences —a compressed-air pipe where the tray was going, a column that is not on the drawing, a machine that was moved— and clearance heights, and you check that there is room to work and operate in front of the switchboards. Every deviation is noted and agreed with the inspector before mounting, because afterwards it has to be dismantled.
The order of work is not arbitrary: whatever ends up buried or embedded is done first, because later it is costly to reach; supports and trays before cables, and switchboards before pulling, so each cable arrives at a terminal that is already in place.
| Stage | What is done | Checked before moving on |
|---|---|---|
| 1 | Setting out and stocking. Mark the job and receive materials. | Materials against specification; interferences resolved. |
| 2 | Earthing and buried raceways. | Electrode resistance; ducts clean and with a pull wire. |
| 3 | Supports and trays (or conduits), with joints and earth bonding jumpers. | Alignment, fixing, continuity between sections, burr-free edges. |
| 4 | Mounting switchboards and the motor control centre. | Levelling, fixing, cable entry with the specified IP rating. |
| 5 | Cable pulling in schedule order. | Bending radius, tension, single layer, power separated from control. |
| 6 | Termination and identification of cables, terminals and boards. | Tightening torque, lugs, labels matching the diagram. |
| 7 | Testing and commissioning. | Complete, signed report (section 07). |
| 8 | As-built drawings and handover. | The drawings show what was built. |
The bars in figure 1 overlap, but not at random: termination does not start until the switchboards are fixed and the cables pulled, and testing does not start until everything is connected. That dependency among tasks, and what to do when one slips, is the critical path method, worked out with numbers in Substations and distribution lines; safety during installation is in Safety, hygiene and quality in installation work.
03Trays, pulling, switchboards and identification
Raceways
The cable tray (perforated or ladder type, in galvanized steel, aluminium or fibreglass-reinforced plastic in corrosive environments) is the solution for many cables over long, accessible runs: it is wired quickly, everything is visible and it is easy to extend. Conduit (rigid metal or PVC) is preferred where there is risk of impact, moisture or dust, or where it must be embedded; trunking, in switchboards and short runs. In every case the supports are placed at the spacing given by the manufacturer for the load, for example every 1.5 m:
Metal trays are bonded to the earthing network and continuity between sections is ensured with copper joints and jumpers; a tray that floats between badly joined sections is not a protective conductor, it is a hazard. Power and control or signal cables are kept apart, in separate trays or with a divider, because power conductors induce noise in signal ones; where they must cross, they cross at 90°, and analogue and communication signal cables are shielded (see Variable speed drives).
Pulling conductors
Pulling begins before touching the cable: the tray must be complete, burr-free, and the route checked against the schedule. Then:
| Rule | Why |
|---|---|
| The reel turns and the cable is paid out; it is never pulled off “the coil” in a spiral. | A cable twisted during pulling keeps internal stresses and can damage the insulation. |
| Rollers at short intervals and at every bend; do not drag the cable on the floor or over edges. | Friction increases tension and scrapes the sheath. |
| Do not exceed the maximum pulling tension or the manufacturer’s minimum bending radius. | Conductors deform and the insulation cracks, and the damage cannot be seen from outside. |
| Pulling temperature above the manufacturer’s minimum (for PVC-insulated cables, around 0 °C). | Cold PVC stiffens and breaks when bent. |
| A single layer in the tray, thickest cables below or at the side, no needless crossings. | Stacked cables do not dissipate heat and force a lower current rating. |
| Slack at both ends, and label before cutting. | Without slack you cannot re-terminate; without a label, the cable is lost among twenty identical ones. |
The minimum radius is a multiple of the cable’s outer diameter, and that multiple changes with the type of cable and is given by the manufacturer in the data sheet:
It is the radius of the cable, not of the tray: the tray bend is chosen with an equal or larger radius.
Switchboards, termination and identification
The enclosure is fixed level and plumb, with room in front to operate and maintain, and cables enter through glands or gland plates that keep the IP rating. Flexible cables end in ferrules or compression lugs of the right size, one conductor per terminal unless the terminal is designed for two, and are tightened with a torque wrench to the torque given by the manufacturer: loose, the terminal overheats (detected by the thermography in Maintenance of electrical components); over-tightened, the thread strips. The AEA 90364 colours are light blue for neutral, green-yellow for the protective conductor and brown, black and red for the phases, always in the same phase order from one end to the other.
Identification is part of the work, not a final detail: each cable carries the same label at both ends as in the schedule; each terminal, the number in the diagram; each switchboard, a plate with its name, voltage, short-circuit current and manufacturer, and the single-line diagram kept inside, up to date. A switchboard without labels forces whoever maintains it to measure everything again.
Earthing
It is done early, together with the buried raceways: electrodes (rods) are driven, joined with welded connections or suitable clamps, inspection pits are left and earth conductors are run to the switchboards. To that network are bonded the switchboards, trays, machine frames and metal structures. It is measured with the earth tester before energizing (method in Earthing, insulation and generator testing) and the value is compared with the design value; what the regulations require is in Regulation of electrical installations.
Four three-phase circuits leave a motor control centre along the same perforated tray. The data come from the cable schedule (cross-sections already calculated) and from the manufacturer’s catalogue (outer diameters, values assumed for the example). Each circuit adds the route measured on the drawing, 4 m of rises and drops and 3 m of slack to terminate at both ends, plus 5 % allowance for cuts:
| Circuit | Cable | Outer D | Drawing | With rises and slack | To buy |
|---|---|---|---|---|---|
| C1 · pump 5.5 kW | 4 × 2.5 mm² | 14 mm | 18 m | 25 m | 27 m |
| C2 · fan 7.5 kW | 4 × 4 mm² | 16 mm | 26 m | 33 m | 35 m |
| C3 · conveyor 11 kW | 4 × 6 mm² | 18 mm | 34 m | 41 m | 44 m |
| C4 · compressor 15 kW | 4 × 10 mm² | 21 mm | 41 m | 48 m | 51 m |
The tray carries the four cables in a single layer. Its usable width is obtained by adding the diameters and adding a 30 % reserve for additions and clearance:
The next commercial width is 100 mm. The longest run is 41 m: with 3 m sections, 14 sections are ordered (14 × 3 = 42 m), and with supports every 1.5 m the calculation above gives 29 supports. Add 2 horizontal bends, 3 branches (to C1, C2 and C3; C4 continues to the end) and the covers. The control cables of each motor (a push-button station, an interlock) do not go here: they are pulled in a separate tray or compartment.
When extending operating plants, the new tray often runs beside energized cables and busbars. The work is planned so as not to open live switchboards: the outage is coordinated with production, the lock-out and tagging procedure is applied, and absence of voltage is verified with an instrument. Tools and long lengths of metal tray can touch live parts with one false move. Work criteria are in Safety, hygiene and quality in installation work and, for live work, in Live-line work.
04Underground lines, distribution and transformer stations
What an underground cable is like, which standard governs (AEA 95101 for underground lines, AEA 95401 for transformer stations) and why a line may or may not be buried is in Substations and distribution lines; how to locate a fault in a buried cable, in Faults and safety in distribution networks. Here what matters is the order of the work, which is the same in low and medium voltage, and which is watched more closely the higher the voltage:
| Step | What is done |
|---|---|
| 1 | Checking for interferences. Drawings of the other services (water, gas, communications, other lines) are requested and hand-dug trial pits are made where crossings will occur. A medium-voltage cable pierced by a shovel or a backhoe is a serious accident. |
| 2 | Opening the trench with the width and depth set by the design, shored if the soil requires it, fenced and signposted for pedestrians and vehicles. |
| 3 | Levelled bottom and sand bed of fine, clean sand without stones, which supports the cable evenly and protects it from rubbing against the soil. |
| 4 | Pulling the cable from the reel, over rollers and without dragging it, respecting the manufacturer’s maximum tension and minimum bending radius. Each cable and phase is identified before being covered. |
| 5 | Sand over the cables and mechanical protection on top (slabs or bricks), which warns whoever digs later and spreads the blow of a shovel. |
| 6 | Warning tape some distance above the protection, and backfill compacted in layers. |
| 7 | Surface marking (markers) at bends, crossings and joints, and testing of the cable sheath and insulation before energizing, with the instrument and value given by the manufacturer and the specification. |
| 8 | Route record: actual depth, distances to fixed points, crossings and joint locations, for the as-built drawing. |
Where the line crosses a roadway or the cable must be replaceable without breaking the pavement, ducts (PVC or concrete) with pull pits are installed, and a pull wire is left in each. Joints and terminations —the point where cables fail— are made by qualified personnel, in a clean, dry environment, with the manufacturer’s kit and following its instructions step by step; a joint made in a hurry or with moisture inside is a postponed fault.
In the low-voltage distribution leaving a transformer station, the work shares the circuits so that the three phases carry similar loads, and when energizing the current of each phase and of the neutral is measured to check it. A large imbalance is paid for in voltage drop and neutral losses.
The transformer station on site
A transformer station is built in this order: civil works (foundation, pit or bund to collect the transformer oil, ventilation and enclosures), earthing grid with its measurement, mounting of the transformer and the medium-voltage cubicles, connections with medium- and low-voltage terminations, protection settings, labelling and danger signs. Before connecting it to the grid, the utility inspects and tests it; in Córdoba, connection and service are under the control of the ERSeP. The station is a medium-voltage installation: it is built and operated by authorized personnel following the corresponding safety procedures (how it is made up and protected, in Substations and distribution lines; the transformer itself, in Transformers).
05Manufacturer catalogues, motor selection and motor control centre
The catalogue is the source of the data the design needs and the job has to verify. It is worth distinguishing the product’s data sheet (guaranteed values for that model), the family’s general catalogue (to compare and choose), and the installation manual (how to mount, tighten, connect and maintain it). From the data sheet the same things are always read: rated voltage and current, breaking or making capacity, degree of protection (IP), working temperature and derating factor for temperature or altitude, dimensions and weight (for the support and space), standards met and order code. What the sheet says is compared with what the specification asks, line by line.
Reading a motor data sheet
The motor nameplate is explained in AC motors; the catalogue adds the data used to choose and to verify starting. A typical sheet (values of an example three-phase motor):
| Catalogue item | Example value | What it is for |
|---|---|---|
| Rated power (at the shaft) | 11 kW | It is what it delivers, not what it draws. Compared with what the load requires. |
| Poles · speed | 4 · 1,465 rpm | Sets the load speed and, with the power, the torque. |
| Voltage · connection | 380 V · Δ (380/660 V Δ/Y) | Which supply it connects to and how; conditions star-delta starting. |
| Efficiency · IE class | 91.4 % · IE3 | How much energy is lost. Sets the cost of running the motor for years. |
| Power factor (cos φ) | 0.84 | Enters the current and the compensation (AC power). |
| Rated current In | 21.8 A | Sets the overload protection and sizes the cable. |
| Starting current Ia/In | 7.5 | Sizes the contactor and short-circuit protection, and decides whether the starting current must be reduced. |
| Rated torque Mn | 71.7 N·m | Reference for the other torques. |
| Starting torque Ma/Mn | 2.2 | Must exceed the torque the load demands at zero speed. |
| Breakdown torque Mmax/Mn | 2.6 | Margin against momentary overloads before the motor stalls. |
| IP · insulation · duty | IP55 · class F · S1 | Environment it can work in, temperature the winding withstands and continuous duty. |
Three relations let you move from one item to another and detect a wrongly copied value. Rated current comes from the shaft power, efficiency and power factor (with the line voltage):
and rated torque from power and speed:
The efficiency classes IE1 to IE4 (IEC 60034-30-1) rank motors from lowest to highest efficiency; the minimum values required in the country are set by the regulations in force, which should be checked for each project.
The centrifugal pump of a process needs 9.3 kW at the shaft at about 1,470 rpm, on a 380 V, 50 Hz supply. The motor must be chosen from the catalogue and checked for starting.
Power. A 15 % margin is added over the load power, covering the pump-curve tolerances and wear, and the next standard value is taken:
Speed. 1,470 rpm corresponds to a 4-pole motor (synchronous speed 1,500 rpm). Current. With the sheet above (efficiency 0.914 and cos φ 0.84):
Torque. The chosen motor delivers 71.7 N·m rated, and the pump demands, at 1,470 rpm:
The pump runs at 84 % of rated torque: the motor has headroom and runs with good efficiency. A much larger motor would run lightly loaded, with worse power factor and efficiency. Starting. With direct-on-line starting, the starting current is:
The utility or the plant may limit that current, and then it is reduced with star-delta starting (the current drops to a third, 54.5 A) or with a soft starter. In star the starting torque also drops to a third:
0.73 · Mn is enough for a centrifugal pump, which demands little torque at low speed, but not for a loaded conveyor: figure 3 shows it. Protection. The thermal overload relay (class 10 for a pump) is set at 21.8 A; the other switching and protection components are chosen according to Electrical installations.
The motor control centre
When a plant has several motors, their feeders are grouped into a motor control centre (MCC): a modular switchboard made of columns with main busbars, where each motor has its own module or drawer with disconnector or circuit breaker, short-circuit protection, contactor and overload protection (or, where applicable, a soft starter or a drive). Drawers may be fixed or withdrawable: with a withdrawable drawer a starter is changed without leaving the whole plant in the dark. The MCC is built and tested to IEC 61439 (low-voltage switchgear and controlgear assemblies), and the starter is chosen with a coordination the manufacturer guarantees (IEC 60947-4-1): with type 2, after a short circuit the starter remains fit for service, except for slight contact welding. On site they are mounted level on a plinth, with service space and front access; check that each drawer has the protection the calculation report asks, that each feeder is labelled with the machine name, and that the busbars are tightened to the specified torque.
06Installed and contracted power, lighting and rational use of energy
Installed power is the sum of the power of everything connected. Not all of it is drawn at once: maximum demand is what the installation really asks for at its busiest moment, and it is calculated by applying to each group of loads its demand factor (how it is determined, in The industrial electrical project). Contracted power is what is agreed with the utility for the supply, and it defines how much fixed charge is paid and what equipment the service requires: if too little is contracted, real demand exceeds it and the tariff may penalize it; if too much, you pay for power that is never used. For an own transformer station, the transformer rating is chosen from the demand in kVA, with a reserve for growth.
A workshop has four motors in the MCC (5.5 + 7.5 + 11 + 15 = 39 kW, nameplate power), 6 kW of lighting, 8 kW of sockets and services, and 10 kW of air conditioning: 63 kW installed. With demand factors of 0.65 for the motors, 0.9 for lighting, 0.4 for sockets and 0.8 for air conditioning:
| Group | Installed | Demand factor | Demand |
|---|---|---|---|
| MCC motors | 39 kW | 0.65 | 25.35 kW |
| Lighting | 6 kW | 0.90 | 5.40 kW |
| Sockets and services | 8 kW | 0.40 | 3.20 kW |
| Air conditioning | 10 kW | 0.80 | 8.00 kW |
| Total | 63 kW | 41.95 kW |
Demand is about 42 kW: that is the order of the power to contract, not the 63 kW installed. In kVA, with an average power factor of 0.85:
A standard 63 kVA transformer is loaded to 78 % (49.4 / 63) and allows 20 % growth (49.4 · 1.2 = 59.2 kVA, less than 63). If the power factor is compensated to 0.95 with capacitors (see AC power):
the transformer load drops to 70 % (44.2 / 63). The standard ratings available are given by the manufacturer.
Lighting
The lighting calculation (lumen method, lamp types) is in Lighting installations. On site the luminaires are set out on the ceiling or structure —aligned with trays and air ducts, the usual interferences—, the design mounting heights are respected, circuits are zoned so areas can be switched off separately and emergency lighting is tested. At acceptance, illuminance is measured with a lux meter on a grid of points over the working plane and compared with the design.
Rational use of energy
The works determine what it will cost to operate the installation over decades, far more than what it cost to build. The decisions that weigh most:
| Measure | Why it saves |
|---|---|
| High-efficiency (IE3), properly sized motors | Lower losses; a much oversized motor runs lightly loaded, with worse efficiency and power factor. |
| Drives on pumps and fans | Reducing flow by lowering speed saves far more than throttling a valve (Variable speed drives). |
| Power factor correction | Less current in cables and transformer, and no utility penalty. |
| Efficient lighting and zone control | LED lamps, sensors and switching off by areas. |
| Sub-metering | What is not measured cannot be improved: one meter per line or area shows where consumption is. |
| Maintenance | Loose terminals, worn bearings and air leaks waste energy every day (Maintenance of electrical components). |
A motor delivers 9.3 kW at the shaft for 6,000 hours a year. If it is IE1 (efficiency 0.876 at 11 kW) instead of IE3 (0.914), the extra energy bought each year is:
This uses nominal efficiencies; with the real load the number changes somewhat, but the order of magnitude is enough to decide. And for a pump or fan, lowering the speed to 80 % with a drive leaves:
That is, a little over half the power at 80 % of the flow.
07Acceptance tests, commissioning and as-built drawings
A job is not finished when the last cable is connected, but when measurements have shown that it works and is safe, and when whoever will maintain it receives the documentation of what was actually built. Acceptance tests verify the whole installation; those done on each machine (no-load, load, temperature rise) are in Motor and transformer testing, and the method and instruments for earthing and insulation, in Earthing, insulation and generator testing.
| Test | What is verified and with what | Reference criterion |
|---|---|---|
| Visual and mechanical inspection | Installation against drawing: cross-sections, labels, tightening, supports, IP ratings, burr-free edges. | No open non-conformities. |
| Continuity of protective conductor and trays | Low-resistance ohmmeter between each exposed part and the originating switchboard. | Continuity at every point; values recorded. |
| Insulation resistance | 500 V megohmmeter between conductors and earth, loads disconnected. | Reference: at least 1 MΩ on circuits up to 500 V (IEC 60364-6, the basis of part 6 of AEA 90364). |
| Earthing | Earth tester on the electrode and the network. | Less than or equal to the design value. |
| Phase sequence and direction of rotation | Phase sequence meter at the board; motor rotation uncoupled from the load. | Phase order equal to the whole system; correct rotation. |
| Protection | Thermal relay and motor breaker settings at nameplate current; RCD trip tested with an instrument. | Values from the calculation report and the regulations. |
| Interlocks and emergency stops | Functional test of emergency stops, limit switches and sequences. | Each command does what the diagram says, and only that. |
| Motors running | Current in each phase at no load and under load, noise and vibration, bearing temperature. | Currents similar to each other and below the nameplate rating. |
| Thermography under load | Thermal camera over boards, terminals and busbars, after a few hours of operation. | Differences against the counterpart point (Maintenance of electrical components). |
| Illuminance | Lux meter on the grid of points. | Not lower than the design value. |
The order of first energization (protection open, measure before closing, circuits one at a time, progressive loads) is described in Installation project. Everything goes into a test report with the measured values, the instrument and its calibration, the date and the signature of the tester and of the inspector. With it, authorization is requested; the role of the licensed installer and of the certificate of fit installation is in Regulation of electrical installations.
As-built drawings
During the works almost everything changes a little: a route corrected because of an interference, a cross-section increased, a cable added, a switchboard that arrived from another manufacturer, a setting changed at commissioning. The as-built drawings record what was actually built. They are made during the job —marking changes on a copy of the current drawings and cleaning them up at the end— and not from memory in handover week.
| What is recorded | Why it matters |
|---|---|
| Actual route of trays, conduits and cables, with dimensions | The next extension starts by knowing where what exists runs. |
| Buried route: depth, distances to fixed points, crossings and joints | Buried work is invisible: without this drawing, the next person to dig breaks it, or the fault cannot be found. |
| Cross-sections and cable types actually installed | Changing a size changes voltage drop and protection; if it is not recorded, nobody knows. |
| Updated switchboard diagrams, with terminal labels | It is the only thing that allows diagnosis without taking everything apart. |
| Final protection settings | They are the reference configuration for maintenance. |
| Makes, models and data sheets of installed equipment | To order spare parts and for the maintenance plan. |
| Test report | It is the initial health status: the first measurement against which the trend is compared. |
When a design drawing is handed over as if it were as-built, people trust it. Whoever designs an extension on a drawing that does not match reality calculates with false data, and whoever cuts “the cable the drawing says is unused” may cut one that is live. Delivering as-built drawings is part of the task and of professional responsibility; the job is not considered handed over without them.
08On site
Materials: a short file (a raceway drawing with single-line diagram, its cable schedule and a two-page specification), prepared by the teacher with three deliberate inconsistencies (a different cross-section between drawing and schedule, a label that does not exist in the diagram, a make without “or equivalent”). Steps: 1) note the current revision of each drawing; 2) build the list of materials and quantities; 3) cross-check drawing, schedule and specification; 4) write a query note for each doubt. Acceptance criterion: the three inconsistencies are detected and each note states which document contradicts which and what is being asked.
Materials: 8 m of perforated tray with supports, joints, copper jumpers, level, tape measure, drill and a low-resistance ohmmeter. Steps: 1) set out the centreline with chalk or string; 2) mount the supports at the spacing given by the catalogue; 3) join the sections with their jumpers; 4) measure continuity between the two ends. Measured: support spacing, levelness and resistance between ends. Acceptance criterion: spacing no larger than the catalogue’s, tray without visible slope or twist, burr-free edges and continuity measured and recorded at every joint.
Materials: a training switchboard with motor breaker and contactor, 10 m of 4 × 1.5 mm² cable, a small three-phase motor, ferrules, labels, 500 V megohmmeter, clamp meter and phase sequence meter. Steps: 1) with the board de-energized and locked out, label the cable ends according to the diagram; 2) pull respecting the catalogue bending radius; 3) terminate with ferrules and tighten to torque; 4) measure insulation with loads disconnected; 5) energize with the motor uncoupled, check sequence and direction of rotation; 6) measure the current in each phase. Measured: insulation, sequence and current per phase. Acceptance criterion: insulation of 1 MΩ or more, identical labels at both ends, correct rotation and currents similar to each other and lower than the nameplate.
Materials: a motor manufacturer’s catalogue, the curves of a load (a pump or conveyor) and a calculator. Steps: 1) calculate the power with the given margin; 2) choose poles, voltage and IE class; 3) calculate rated current from the catalogue data and compare it with the sheet value; 4) check starting torque against the load, direct and in star; 5) justify the relay and its setting. Acceptance criterion: the chosen motor starts with the chosen method and each decision rests on a catalogue figure.
Materials: the original drawing of the installation of exercises 2 and 3 and a red pencil. Steps: 1) measure what was actually built (routes, dimensions, cross-sections, labels); 2) mark each difference in red on a copy; 3) clean up the changes. Acceptance criterion: another person can locate each cable and each board using only the drawing.
09Common mistakes
| Mistake | Consequence |
|---|---|
| Installing from a drawing of an earlier revision | Rework and lost materials; usually discovered when it is already installed. |
| Settling a contradiction between drawings “by eye” | If it turns out wrong, the responsibility lies with whoever decided without asking. |
| Measuring on the printed drawing instead of using the dimensions | Cables too short or too long: the paper may be reduced or distorted. |
| Bending the cable tighter than the manufacturer’s radius | Internal insulation damage that shows up as a fault months later. |
| Pulling without leaving slack or labelling | It cannot be re-terminated and the cable is lost among identical ones. |
| Stacking cables in several layers in the tray | Poor heat dissipation: the current rating drops and the cable overheats. |
| Covering a trench without recording the route | The next person to dig breaks it; the fault cannot be located. |
| Starting under load a motor chosen only by power | Starting torque is not enough (for example with star-delta) and the motor does not accelerate. |
| Contracting installed power instead of demand | You pay for power that is never used, and the transformer ends up oversized. |
| Leaving as-built drawings for the end | They are made from memory, with errors; the job is handed over with drawings that do not match reality. |
10Self-assessment
Which documents make up the works file and what is looked up in each?
Drawings (what and where), specifications (how and with what quality), calculation report (why), bill of quantities (how much), schedules (cables, terminals, labels), catalogues and standards (limits and installation rules). If they contradict each other, ask in writing; do not decide by your own judgement.
Why are earthing and buried raceways done first, and switchboards before pulling?
Because buried or embedded work is costly to reach afterwards, and because each cable has to arrive at a terminal that is already in place. The order avoids rework.
A cable with an outer diameter of 18 mm has a minimum bending radius of 12 times its diameter. How much is it?
12 · 18 mm = 216 mm. It is the cable’s minimum radius; the tray bend is chosen with an equal or larger radius.
Four cables of 12, 14, 16 and 20 mm go in a single layer in a tray. What minimum usable width is needed with a 30 % reserve?
The sum is 62 mm and with the reserve, 1.3 · 62 = 80.6 mm. The next commercial width is 100 mm.
How is the rated current of a 7.5 kW, 380 V motor with efficiency 0.90 and cos φ 0.85 calculated?
I = 7,500 / (√3 · 380 · 0.90 · 0.85) = 7,500 / 503.5 = 14.9 A.
Why can a motor be right for steady running and still fail to start the load?
Because in steady running only power matters, but when starting the motor torque must exceed the load torque at every speed. With star-delta the torque drops to a third: fine for a pump, but it may not be enough for a loaded conveyor.
What goes in a trench, from the bottom up, for an underground line?
Levelled bottom, sand bed, cables, sand on top, mechanical protection (slabs or bricks), warning tape and backfill compacted in layers, with surface marking where appropriate. Depth and distances are set by the standard.
A workshop has 63 kW installed and a demand of 42 kW. What power should be contracted?
Contracted power is fitted to the demand (about 42 kW), not to the installed power. Contracting the installed power means paying for power that is never used.
What is an as-built drawing and why is it made during the job?
It is the drawing showing what was actually built: real routes, cross-sections, buried route, settings. It is made during the job so as not to depend on memory, and it is what allows maintenance and extension without mistakes.