Power Carrying Devices

A 3.5 core armoured power cable has three full-size phase conductors (R, Y, B) and one reduced-size neutral (about half the phase area, hence "0.5 core"). It is the standard cable for 400 V three-phase, four-wire distribution in industries, laid underground or on trays.

Cross-section sketch

        ┌───────────────────────┐ ← outer PVC sheath
        │ ░░░░ armour ░░░░░░░░░ │ ← steel wire / strip
        │ ┌───────────────────┐ │ ← inner sheath
        │ │  ( R )     ( Y )  │ │   (bedding)
        │ │    filler         │ │
        │ │  ( B )     (n)    │ │ ← n = half-size
        │ └───────────────────┘ │   neutral
        └───────────────────────┘
 Each core: conductor + insulation

Parts and materials

1. Conductors: stranded copper or aluminium, often sector-shaped to make the cable compact. Copper has higher conductivity; aluminium is lighter and cheaper and is common for large sizes. Stranding gives flexibility.
2. Insulation: each core is insulated with PVC (rated 70 °C) or XLPE (cross-linked polyethylene, rated 90 °C). XLPE has better thermal and electrical properties and allows higher current. Cores are colour-coded red, yellow, blue and black.
3. Fillers: non-hygroscopic PVC or polypropylene material that fills the gaps between the laid-up cores to make the cable round.
4. Inner sheath (bedding): extruded PVC layer over the laid-up cores. It holds the cores together and acts as a cushion so the armour does not damage the insulation.
5. Armour: galvanised steel round wires (for larger cables) or flat strips (for multi-core cables), wound helically. It gives mechanical protection against crushing, digging and rodents, and also acts as an earth continuity path. Single-core AC cables use non-magnetic (aluminium) armour to avoid eddy-current heating.
6. Outer sheath (serving): tough extruded PVC (or FRLS/LSZH for fire-safe areas). It protects the armour from moisture, corrosion and chemicals and carries the cable markings (size, voltage, maker, standard).

Standard: IS 1554 (PVC) or IS 7098 (XLPE) for 1.1 kV cables. Designation example: "3.5 × 95 mm² Al, XLPE, armoured, 1.1 kV".

Difference between fuse and MCB

Operating principle of MCB

An MCB (miniature circuit breaker) has two trip elements acting on one latch mechanism:

 Line ─┬─[contacts]──[bimetal]──[coil]── Load
       │      ▲          │         │
       │      └── latch ─┴─────────┘
       └ arc chute (splits the arc)

1. Thermal trip (overload): the load current passes through a bimetal strip. On a sustained overload the strip heats, bends and releases the latch. The time delay is inversely related to the current, so small overloads take longer to trip.
2. Magnetic trip (short circuit): a solenoid coil carries the current. A large short-circuit current (for example 3–5 × In for B curve, 5–10 × In for C curve, 10–20 × In for D curve) pulls the plunger instantly, which trips the latch and also pushes the moving contact open.
3. Arc quenching: the arc formed between opening contacts is driven into an arc chute of steel plates, where it is split, cooled and extinguished.

Rupturing capacity of HRC fuse

The rupturing (breaking) capacity of a fuse is the largest prospective fault current (r.m.s.) that it can safely interrupt at rated voltage without damage or danger. It is expressed in kA (or MVA).

• HRC (high rupturing capacity) fuses have a silver element inside a ceramic body filled with pure quartz sand. When the element melts, the sand absorbs the arc energy and forms a high-resistance glass-like substance (fulgurite), which quenches the arc quickly.
• Because the fuse melts before the fault current reaches its peak, it limits the cut-off current, so the actual current is much lower than the prospective current.
• Typical breaking capacity of LV HRC fuses is 80 kA to 120 kA (some up to 200 kA), far higher than MCBs (6–10 kA) and comparable to large MCCBs.
• This makes HRC fuses suitable near transformers and in main panels where fault levels are very high, and as back-up protection for MCBs.

How proper cable selection reduces fire accidents

Most electrical fires start when a cable overheats or a short circuit produces an arc that ignites the insulation or nearby material. Correct cable selection helps by:

1. Correct current rating: choosing a size whose current-carrying capacity (after derating for temperature, grouping and installation method) is more than the load current, so the cable does not overheat.
2. Matching protection: the fuse/MCB rating must be below the cable's capacity, so an overload trips the device before the cable overheats.
3. Short-circuit withstand: the conductor area must survive the fault current until protection clears it (A ≥ I√t/k). An undersized cable can melt and burn before the breaker trips.
4. Good-quality, standard cable: cables meeting IS/NS standards have the full copper/aluminium area and proper insulation. Cheap, under-sized "local" cables are a main cause of fires in Nepal.
5. Higher temperature insulation: XLPE (90 °C) instead of PVC (70 °C) where cables run hot.
6. Fire-retardant cables (FR / FRLS): the sheath does not spread flame along cable runs.
7. Voltage drop and loose-joint control: correct size and proper lugs avoid hot joints, which often start fires.
8. Proper installation: conduits, trays and fire barriers where cables pass through walls and floors.

Why halogen-free cable is important for fire fighting

Ordinary PVC contains chlorine, a halogen. When PVC burns it gives off:

• Thick black smoke, which blocks vision so people cannot find exits and fire fighters cannot see the fire.
• Hydrogen chloride gas, which is toxic, forms hydrochloric acid in the lungs and eyes, and corrodes electronic equipment.

Halogen-free (LSZH / LS0H, Low Smoke Zero Halogen) cables use halogen-free compounds. In a fire they:

• Produce little and light smoke, so escape routes stay visible.
• Release no toxic or corrosive halogen gases, so people can breathe longer and fire fighters can work safely.
• Cause less damage to equipment and the building.

So LSZH cables are used in hospitals, high-rise buildings, tunnels, shopping malls, data centres and other crowded or enclosed places. Fire-survival (FS) cables are used for fire alarms, emergency lighting and fire pumps, which must keep working during the fire.

Why splicing cannot be avoided

Splicing (jointing) means joining two lengths of cable so that the joint is electrically and mechanically as good as the cable. It cannot be avoided because:

• Cables are made and transported on drums of limited length (a few hundred metres for HV cables), while routes are often longer.
• A damaged section after a fault or digging accident must be cut out and a new piece joined in.
• Tee/branch joints are needed to tap supply to new loads.
• Cable routes may need changes when roads and buildings are extended.

Splicing techniques of HV cable (e.g. 11 kV XLPE)

Common types of joints:

1. Heat-shrink joint: tubes of cross-linked polymer that shrink tightly when heated with a gas torch. Most common in Nepal and India.
2. Cold-shrink joint: pre-expanded silicone rubber tubes held on a removable core; they shrink when the core is pulled out. No flame needed.
3. Pre-moulded (push-on) joint: factory-made rubber body pushed over the prepared cable.
4. Resin (cast) joint: a mould is filled with epoxy or polyurethane resin.
5. Taped joint: layers of insulating and semiconducting tapes applied by hand (older method).

Steps for an 11 kV XLPE heat-shrink straight-through joint

Layers built over each core joint (inside out):
 conductor + crimped ferrule
 stress-relief mastic over ferrule
 stress-control tube (over screen cut-back)
 insulating tube(s)
 semiconducting tube
 tinned copper mesh (screen continuity)
Over all three cores:
 armour bond (braid) + outer sheath tube

1. Prepare cables: check for moisture, overlap the ends, cut to length. Slide on the outer sheath tube and other tubes beforehand.
2. Remove outer sheath, armour and inner sheath to the dimensions given by the kit. Clamp the armour ends.
3. Remove the copper tape screen and the semiconducting layer back to the marked length, leaving a clean, smooth cut on the insulation.
4. Remove insulation from the conductor ends equal to half the ferrule length.
5. Join conductors with a mechanical or crimped (compression) connector/ferrule; file off any sharp edges.
6. Fill voids over the connector with stress-relief mastic.
7. Stress control: shrink a stress-control tube over the semicon cut-back to spread the electric field evenly; otherwise the field concentrates at the screen edge and causes breakdown.
8. Insulation: shrink one or more insulating tubes over the joint, then a semiconducting tube (or a dual-wall tube) over them.
9. Screen continuity: wrap tinned copper mesh over the joint and solder/clamp it to the copper screens of both cables.
10. Repeat for all three cores, then bind the cores together.
11. Armour continuity and earth: bridge the armour across the joint with a braided earth connector.
12. Outer protection: shrink the outer heavy-wall sheath tube (or a joint box) over the whole joint to keep out water and give mechanical strength.
13. Test: insulation resistance test and, for HV, an HV/VLF withstand test before energising.

Operating principle of MCB

A miniature circuit breaker trips automatically on overload and short circuit:

• Thermal trip: load current heats a bimetal strip. On a sustained overload it bends and releases the trip latch after a time inversely related to the current.
• Magnetic trip: a solenoid coil carries the current. A short-circuit current (several times rated, e.g. 5–10 × In for C curve) pulls the plunger instantly and trips the latch.
• The contacts open and the arc is driven into an arc chute, where it is split and extinguished. The MCB can be reset by its lever.

Differences between MCB, MCCB and RCCB

Cut-off current characteristic of HRC fuse

When a short circuit occurs, the prospective current would rise to a very high peak. An HRC fuse melts during the first rising part of the current wave, before the first peak, so the current is cut off at a much lower value.

 i  prospective current
 ^      .-.
 |     /   \
 |    / .   \
 |   / /\    \      cut-off
 |  / /  \    \ ← current (actual)
 | / /    \    \
 |/_/______\____\______> t
   |←pre-arcing→|←arcing→|
     total operating time

• Pre-arcing time: from fault start until the element melts.
• Arcing time: from melting until the arc is quenched by the quartz sand.
• Cut-off current: the peak current actually reached, much less than the prospective peak.
• The cut-off characteristic is a graph of cut-off current against prospective current for each fuse rating. It shows that the HRC fuse is current limiting: the let-through energy (I²t) and electromagnetic forces are very small, which protects cables and switchgear.

An HRC (high rupturing capacity) fuse is current limiting: on a heavy short circuit it melts and clears the fault within the first quarter-cycle, before the current can reach its full prospective peak. The cut-off current is the highest instantaneous current reached before the fuse interrupts the circuit.

 i
 ^         prospective (would-be)
 |        .--.    current
 |      /      \
 |    /    .     \
 |   /   /  \     \
 |  /  /  ^  \     \
 | /  /   |   \     \
 |/  / cut-off  \     \
 +--+--------+-----------> t
 0  t1      t2
 0–t1 : pre-arcing (melting) time
 t1–t2: arcing time
 0–t2 : total operating time

Explanation

1. Pre-arcing time: the silver element heats and melts at its narrow sections. For a large fault this takes only a few milliseconds.
2. Arcing time: arcs form at the breaks. The arc energy melts the surrounding quartz sand, which absorbs heat and forms a high-resistance glassy material; the arc voltage rises and the current is forced to zero.
3. Cut-off current: the current at the instant of melting/peak is much lower than the prospective peak (for example, prospective peak 50 kA, cut-off about 10 kA).

Cut-off characteristic curve

It is drawn on log-log axes: cut-off current (peak) vs prospective current (r.m.s.), with one line per fuse rating. For small faults the fuse does not cut off (the line follows the prospective peak); above a certain fault level the line flattens, showing the current-limiting action.

Importance

• The let-through energy (I²t) and electromagnetic forces are greatly reduced, so cables, busbars and switchgear downstream do not need to withstand the full fault current.
• Allows use of HRC fuses as back-up protection for MCBs and contactors.
• Gives very high breaking capacity (80–120 kA) at low cost.

Current carrying capacity

The current carrying capacity (ampacity) of a cable is the maximum continuous current it can carry under given installation conditions without the conductor exceeding its permissible temperature (70 °C for PVC, 90 °C for XLPE insulation).

Factors affecting current carrying capacity

1. Conductor material and size: copper carries about 1.3 times more current than aluminium of the same size; larger area gives lower resistance.
2. Type of insulation: XLPE (90 °C) allows more current than PVC (70 °C).
3. Ambient temperature (air or ground): a higher ambient leaves less temperature margin, so the rating is reduced.
4. Grouping: cables laid close together heat each other; derating factors apply for the number of cables and their spacing.
5. Method of installation: in air, on trays, in conduit, in ducts or buried directly; each method dissipates heat differently.
6. Depth of laying: deeper cables dissipate heat less well.
7. Thermal resistivity of soil: dry or sandy soil conducts heat poorly, lowering the rating.
8. Number of loaded cores and harmonics: harmonic currents (e.g. from drives) load the neutral and increase heating.
9. Load cycle: cables with cyclic loads can be rated higher than continuous ones.

Method of sizing a 3.5 core cable

1. Find the full-load current:

I = P / (√3 × V × cos φ × η)

2. Find the total derating factor (product of temperature, grouping, depth and soil factors from the standard/maker's tables).
3. Required tabulated rating = I / derating factor. Select the smallest size whose rating is equal or more.
4. Check voltage drop:

VD = √3 × I × L × (R cos φ + X sin φ)

It should normally be within about 5% (often 3% for motor feeders). 5. Check short-circuit withstand: A ≥ I_sc × √t / k (k = 115 for Cu PVC, 76 for Al PVC, 94 for Al XLPE). 6. Choose the size that meets all three conditions; the neutral is half size (3.5 core).

Example

A 75 kW, 400 V motor, cos φ = 0.85, η = 0.92, fed by a 100 m Al PVC armoured cable buried in ground. Derating: temperature 0.89, grouping 0.8. Fault level 10 kA for 0.2 s.

I = 75000 / (1.732×400×0.85×0.92) = 138.4 A
Total derating = 0.89 × 0.8      = 0.712
Required rating = 138.4 / 0.712  = 194.4 A

From the manufacturer's table, a 3.5 × 120 mm² Al armoured cable (rating about 215 A in ground, R ≈ 0.32 Ω/km, X ≈ 0.07 Ω/km) is selected. (Exact ratings vary by maker.)

VD = 1.732 × 138.4 × 0.1 × (0.32×0.85 + 0.07×0.527)
   = 7.41 V  = 1.85 % of 400 V   (OK, < 5%)
Short circuit: A ≥ 10000 × √0.2 / 76 = 58.8 mm²  (OK)

Answer: 3.5 × 120 mm² Al armoured cable satisfies current, voltage drop and short-circuit conditions.

A fuse is a protective device with a thin metal element that melts and opens the circuit when the current exceeds a safe value for enough time.

Types of fuse

A. Low-voltage fuses

1. Rewirable (kit-kat) fuse: a porcelain base and carrier with a replaceable tinned copper wire. Cheap and common in old domestic wiring, but low breaking capacity, inexact operation and easily over-wired.
2. Cartridge fuse: the element is sealed inside a glass or ceramic tube with end caps. Used in instruments and small appliances.
3. HRC (high rupturing capacity) fuse: silver element in a ceramic body filled with quartz sand; very high breaking capacity (80–120 kA) and current limiting. Types include knife-blade (link) type and bolted type. Used in industrial panels and as back-up for MCBs.
4. D-type (Diazed) fuse: cartridge fuse with a screw cap and indicator, used in distribution boards.
5. Semiconductor (high-speed) fuse: very fast acting, to protect thyristors and diodes.

B. High-voltage fuses

1. Drop-out (expulsion) fuse: used on 11 kV lines and distribution transformers. When the element melts the gases expel the arc and the fuse carrier drops down, giving visible indication.
2. HV HRC cartridge fuse: used in ring main units and for transformer protection.
3. Liquid-filled fuse: the element is in a liquid (carbon tetrachloride) that quenches the arc.

Why a fuse is placed in phase and not in neutral

1. Safety after fuse operation: if the fuse is in the phase wire, when it blows the equipment is disconnected from the live supply and becomes dead. If it were in the neutral, the equipment would stop working but stay connected to the live phase, and a person touching it could get a shock.
2. Earth faults: a fault between phase and earth (body) does not pass through the neutral. A neutral fuse would not see this fault current and would not blow, while the body stays live.
3. Rules: electrical regulations require fuses and single-pole switches to be in the phase conductor only.

Operating principle of MCB

An MCB has a thermal and a magnetic trip acting on a common latch:

• Thermal (overload): load current heats a bimetal strip; on sustained overload it bends and trips the latch after an inverse-time delay.
• Magnetic (short circuit): a solenoid in series pulls its plunger instantly when current exceeds a set multiple of rated current (B: 3–5 × In, C: 5–10 × In, D: 10–20 × In), tripping the latch and helping to open the contacts.
• The arc is drawn into an arc chute of metal plates, split and cooled until it goes out. The MCB is then reset by its lever.

MCB vs MCCB

HRC fuse vs MCCB

Rupturing (breaking) capacity

The rupturing capacity is the maximum prospective short-circuit current a device can interrupt safely at rated voltage. Typical values:

• MCB: 6 kA or 10 kA (IEC 60898), up to about 25 kA.
• MCCB: 16 kA to about 100 kA, some up to 200 kA (IEC 60947-2).
• HRC fuse: 80 kA to 120 kA, some up to 200 kA.

The device's breaking capacity must be higher than the fault level at the point where it is installed.

Wires and cables are classified by conductor, insulation, construction and use. The common types in industrial buildings are:

For power distribution

1. PVC insulated armoured cables (1.1 kV), Cu or Al, IS 1554: 3.5 core or 4 core cables from the main panel to sub-panels and motors, laid on trays, in trenches or buried. Steel armour gives mechanical protection.
2. XLPE insulated armoured cables, IS 7098: rated 90 °C, so they carry more current than PVC; used for large feeders and HV (11 kV, 33 kV) incoming cables.
3. Unarmoured PVC/XLPE cables: for runs inside panels, conduits and protected cable trays.
4. Flexible cables (multi-strand, rubber or PVC sheathed): for portable tools, cranes, hoists, welding sets and moving machines.
5. Bus ducts / bus trunking: for very high currents (from transformer to main panel), as an alternative to many parallel cables.

For lighting and small power

1. PVC insulated single-core copper wires (IS 694), 1.0–6 mm²: run in PVC or metal conduit for lighting, fans and socket circuits. Colours: red/yellow/blue (phase), black (neutral), green (earth).
2. FR / FRLS (fire-retardant, low-smoke) wires: where fire safety matters.
3. LSZH (low smoke zero halogen) cables: for crowded or enclosed areas; give little smoke and no toxic gas in a fire.

Special-purpose cables

1. Fire-survival cables: keep working during a fire; for fire alarm, emergency lighting and fire pumps.
2. Heat-resistant cables (silicone, PTFE insulation): near furnaces and ovens.
3. Control cables (multi-core, small size, screened): for control circuits, interlocks and PLC wiring.
4. Instrumentation and data cables (twisted, screened): for sensors and communication, to reduce noise.
5. Bare or PVC-covered earth conductors (GI strip, copper): for earthing.

Selection points

Current rating with derating, voltage drop, short-circuit withstand, installation method, mechanical protection, fire performance and cost.

Underground cables in an industry are laid so that they are protected from mechanical damage, can dissipate heat and can be inspected or replaced. The three classical systems are direct laying, draw-in (duct) system and solid system. Inside plants, cable trenches and trays are also used.

1. Direct laying

 ground ════════════════════
        warning tape / route marker
        backfill soil
        ▭▭▭▭▭ protective bricks/tiles
        ░ sand ░  ●  ●  ●  ░ cables
        ░░░░ sand bed (~8 cm) ░░░
   trench about 0.75–1.2 m deep, 0.45 m wide

• A trench about 0.75 m (LV) to about 1 m or more (HV) deep is dug.
• A layer of sand is laid at the bottom, the armoured cable is laid on it and covered with more sand.
• Bricks or concrete tiles are placed on top for protection, then the trench is back-filled; a warning tape and route markers are placed.
• Advantages: simple, cheap, good heat dissipation, so higher current rating.
• Disadvantages: changes or extra cables need fresh digging; fault location and repair are costly; cables can be damaged by later excavation.

2. Draw-in (duct) system

   manhole ──── ducts (PVC/RCC/HDPE) ──── manhole
   [  ]═════════════════════════════════[  ]
        cables pulled through ducts

• Pipes or ducts of PVC, HDPE, RCC or stoneware are laid underground with manholes (pull pits) at intervals.
• Cables are pulled into the ducts after laying.
• Advantages: cables can be added, removed or repaired without digging; good mechanical protection; neat. Ideal under roads and plant areas with traffic.
• Disadvantages: high initial cost; poor heat dissipation, so cables are derated; difficult on bends.

3. Solid system

• Cables are laid in troughs of cast iron, stoneware or wood, which are then filled with bitumen or asphalt.
• Advantages: good mechanical protection.
• Disadvantages: costly, poor heat dissipation, hard to repair; now rarely used.

4. Cable trenches and trays (inside the plant)

• Concrete trenches with covers, with cables on brackets or trays along the walls, run between substation, panels and machines.
• Advantages: easy inspection, addition and maintenance; good ventilation.
• Disadvantages: must be kept free of water, oil and rodents; need fire barriers.

Comparison

Cables in an industrial plant carry power from the transformer to the MDB, from the MDB to SDBs, and from SDBs to motors, lighting and sockets. They are chosen by voltage grade, insulation, armouring and number of cores, and are laid by a method that suits the route and the risk of damage.

Types of cable used in an industrial plant

1. By voltage grade
   • LT cables (1.1 kV grade) for 400/230 V circuits: main feeders, sub-feeders, motor and lighting circuits.
   • HT cables (11 kV, 33 kV grade, XLPE) for the incoming supply from the utility pole to an indoor substation or ring main unit.
2. By insulation
   • PVC insulated, PVC sheathed: cheap, used for 1.1 kV general wiring and control; max conductor temperature 70 °C.
   • XLPE insulated: higher rating (90 °C continuous, 250 °C short circuit), lower losses; used for main feeders and HT cables.
   • PILC (paper insulated lead covered): older HT installations.
3. By mechanical protection
   • Armoured (steel wire or steel strip armour): laid direct in ground or in places with risk of damage.
   • Unarmoured: inside conduits, trays or trunking where they are protected.
4. By number of cores: single core (large currents, DC, HT), 2-core and 3-core (single-phase and 3-phase without neutral), 3½-core (3-phase 4-wire with reduced neutral), 4-core.
5. Special cables: flexible cables (portable equipment, cranes), control and instrumentation cables (multi-core, screened), fire-resistant/FRLS cables (fire pumps, emergency lighting), trailing cables for mobile machines.
6. Conductor: copper (small sizes, motor and building wiring) or aluminium (large feeders, cheaper and lighter).

Methods of installation

Good practice during installation

• Keep HT, LT power and control cables on separate trays or with spacing, to avoid interference and limit fire spread.
• Respect the minimum bending radius (about 12–15 × cable diameter for armoured cable).
• Apply derating factors for grouping, ambient temperature and depth of laying.
• Earth the armour at both ends, use proper glands and lugs, and mark the cable route with markers.
• Test insulation resistance (megger) before energising.

The conductor size of a cable is chosen so that it carries the load current without overheating, keeps the voltage drop within limits and survives the short-circuit current until the protection clears the fault.

Factors for deciding conductor size

1. Load (design) current: I = P / (√3 · V_L · cosφ · η) for 3-phase loads; the cable rating must be ≥ the design current and ≥ the rating of the protective device.
2. Derating factors: ambient temperature, grouping of cables, depth of laying, soil thermal resistivity and method of installation (air, duct, ground) reduce the rating given in the tables.
3. Voltage drop: the drop from supply to load should be within the permitted value (about 2.5–5 % of rated voltage). Long cables are often sized by voltage drop, not current.
4. Short-circuit withstand: A ≥ I_sc · √t / k, where k depends on conductor and insulation (about 115 for Cu-PVC, 143 for Cu-XLPE).
5. Type of load: motor starting current, harmonics (neutral current), duty cycle.
6. Future expansion: a margin of about 20–25 %.
7. Economy and material: copper vs aluminium, cost of energy losses over the cable life.
8. Mechanical strength: minimum sizes (e.g. 1.5 mm² Cu for lighting, 2.5 mm² Cu for power sockets).

Construction of 3½-core armoured cable

A 3½-core cable has three full-size phase conductors and one reduced-size neutral (about half the phase area), used for 3-phase 4-wire LT supply where neutral current is smaller than phase current.

        outer PVC sheath
      /  steel wire / strip armour
     /  /  bedding (inner sheath)
    /  /  /   filler
   (  (  (  ___   ___  )  )  )
          ( R )  ( Y )
             \    /
          ( B )  (n)  <- reduced neutral
   each core: conductor + PVC/XLPE insulation

Layers from inside to outside:

1. Conductors: stranded copper or aluminium, often sector shaped to reduce the overall diameter; three phase cores plus one half-size neutral.
2. Insulation: PVC or XLPE over each conductor, colour coded (red, yellow, blue, black).
3. Fillers: non-hygroscopic PVC or polypropylene fillers that make the laid-up cores round.
4. Inner sheath (bedding): extruded PVC layer that protects the cores and gives a bed for the armour.
5. Armour: galvanised steel wires (round) or steel strips laid helically; gives mechanical protection and acts as an earth-continuity path. Single-core cables use non-magnetic (aluminium) armour.
6. Outer sheath (serving): tough PVC (or FRLS) sheath against moisture, chemicals and corrosion, with voltage grade and size printed on it.

Such cables (e.g. 1.1 kV, 3½ × 95 mm² Al XLPE armoured) are laid directly in the ground or on trays between the transformer, MDB and SDBs.

A cable is one or more insulated conductors enclosed in a protective covering. Cables are classified by voltage, insulation, construction and number of cores.

Classification of cables

1. By voltage rating
   • Low tension (LT): up to 1 kV (1.1 kV grade, used for 400/230 V)
   • High tension (HT): up to 11 kV
   • Super tension (ST): 22–33 kV
   • Extra high tension (EHT): 33–66 kV
   • Extra super voltage: above 132 kV (oil-filled, gas-pressure cables)
2. By insulation: PVC, XLPE, EPR/rubber, paper (PILC), mineral insulated (MICC, fire survival).
3. By mechanical protection: armoured (steel wire/strip) and unarmoured.
4. By number of cores: single core, 2-core, 3-core, 3½-core, 4-core and multi-core control cables.
5. By conductor material: copper and aluminium.
6. By construction of HT cables: belted, screened (H-type, S.L. type), pressure (oil-filled, gas-pressure) cables.
7. By use: power, control, instrumentation, flexible/trailing, fire-resistant cables.

General construction of 3½-core armoured cable

It has three phase conductors of full section and a neutral of about half section, and is used for LT 3-phase 4-wire distribution in industries.

  outer sheath (PVC)
  armour (GS wire / strip)
  inner sheath / bedding
  fillers
     ____________________
    /   ( R )   ( Y )    \
   |                      |
   |    ( B )    (N½)     |
    \____________________/
  cores: Cu/Al conductor + insulation

1. Conductor: stranded, compacted Cu or Al; sector-shaped phase cores reduce cable diameter. Neutral is smaller (e.g. 3½ × 50 mm² = 3 × 50 + 1 × 25 mm²).
2. Core insulation: PVC (70 °C) or XLPE (90 °C), coloured red, yellow, blue and black for identification.
3. Fillers: PVC or polypropylene to fill the gaps and keep the cable round.
4. Inner sheath / bedding: extruded PVC binding all cores; protects them from the armour.
5. Armour: galvanised steel round wires or flat strips wound helically; protects against mechanical damage (digging, crushing, rodents) and gives an earth path. It is earthed at both ends.
6. Outer sheath: black PVC or FRLS compound; resists water, chemicals and sunlight, and carries the printed marking (voltage grade, size, manufacturer).

Advantages: suitable for direct burial, mechanically strong, saves copper/aluminium in the neutral, single cable gives a complete 3-phase 4-wire circuit.

An HRC (high rupturing capacity) fuse is a cartridge fuse that can safely interrupt very large fault currents (up to 80–100 kA or more) because its element is enclosed in a ceramic body filled with quartz sand.

Why HRC fuses are filled with quartz sand

• When the silver element melts, it vaporises and an arc forms. The silver vapour reacts with the quartz (silica) sand and forms a high-resistance glass-like substance (fulgurite).
• The sand absorbs the arc heat, cools and de-ionises the arc path, and quickly raises the arc resistance, so the arc is extinguished within a few milliseconds.
• The sand also stops the arc and hot metal vapour from bursting out of the cartridge, so the operation is silent, without flame or explosion.
• Because the arc is quenched before the current reaches its prospective peak, the fuse limits the fault current.

Cut-off characteristics of HRC fuse

 i
 |        prospective current (no fuse)
 |       .-'''-.
 |     .'       '.
 |    /  Ic       \
 |   /  /\ cut-off  \
 |  /  /  \          \
 | /  /    \          \
 |/__/______\__________\___ t
   tp  ta
 tp = pre-arcing time, ta = arcing time
 total operating time = tp + ta

• Prospective current: the current that would flow if the fuse were replaced by a link of negligible impedance.
• Cut-off current (Ic): the maximum instantaneous current actually reached before the fuse element melts. For large faults the HRC fuse melts in the first quarter cycle, so Ic is much smaller than the prospective peak.
• Pre-arcing time: from fault start to element melting. Arcing time: from melting to final arc extinction. Their sum is the total operating time (often < 5 ms for heavy faults).
• Cut-off current rises with the prospective current but much more slowly; manufacturers give cut-off charts (Ic vs prospective current) for each fuse rating. This reduces I²t and the electromagnetic and thermal stress on cables, busbars and switchgear.

Advantages of HRC fuse over Kit-Kat (rewirable) fuse

Kit-Kat fuses are cheaper and easy to renew, but HRC fuses are preferred for motor circuits, transformers and main feeders in industry.

A miniature circuit breaker (MCB) is a small, automatically operated switch that protects LT circuits against overload and short circuit, and can be reset after tripping. SP, DP and TP refer to the number of poles switched together.

Construction

   ON/OFF toggle
        |
  [latch & trip mechanism]
     |                 |
 bimetal strip    solenoid coil
 (overload)       + plunger (short circuit)
     |                 |
  fixed & moving contacts -> arc chute
  (arc splitter plates)
  moulded insulating case (DIN rail mount)

• Moulded case of insulating material, mounted on a 35 mm DIN rail.
• Operating toggle with a trip-free mechanism: it trips even if the knob is held ON.
• Contacts: fixed and moving contacts of silver alloy.
• Thermal element: bimetallic strip carrying load current.
• Magnetic element: coil with a plunger (armature).
• Arc chute: stack of steel splitter plates that divides and cools the arc.

Operation

1. Overload: current above rating heats the bimetal strip; it bends with time (inverse-time characteristic) and releases the latch, opening the contacts.
2. Short circuit: high current creates a strong magnetic field in the coil; the plunger strikes the latch and opens the contacts in a few milliseconds.
3. The arc formed between the contacts is driven into the arc chute, split into small arcs, cooled and extinguished.
4. Tripping curves: B (trips at 3–5 In, lighting/resistive), C (5–10 In, general and small motors), D (10–20 In, high inrush loads like transformers and motors).
5. Typical ratings: 0.5–125 A, breaking capacity 6 kA or 10 kA.

SP, DP and TP MCBs

In DP and TP MCBs, the poles are mechanically linked by a common trip bar, so a fault on any one pole trips all poles. This avoids single phasing of 3-phase motors and ensures full isolation.

Advantages: resettable (no fuse renewal), quick and reliable tripping, indicates faulty circuit, can be used as a switch, tamper-proof, compact.

A fuse is the simplest protective device: a short piece of metal wire or strip that melts when the current exceeds a safe value for a sufficient time, and so opens the circuit.

Types of fuse

             Fuses
            /     \
       LT fuses    HT fuses
      /    |   \       |   \
 rewirable cartridge  drop-out  HRC HT
 (Kit-Kat)   |        (expulsion) cartridge
          /  |  \
       D-type  link-type HRC
               (blade/bolted)

1. Rewirable (Kit-Kat) fuse: porcelain base and carrier with a tinned copper wire; cheap, used in old domestic installations.
2. Cartridge fuse: element enclosed in a glass or ceramic tube; used in small equipment.
3. D-type cartridge fuse: base, cap, adapter ring and cartridge; non-interchangeable sizes.
4. HRC (high rupturing capacity) fuse: silver element in a ceramic body filled with quartz sand; link type, blade type or bolted type.
5. HT fuses: HRC HT cartridge fuses, drop-out (expulsion) fuses on 11 kV distribution transformer poles, liquid-filled fuses.
6. Semiconductor fuses: very fast fuses for thyristors and diodes.
7. Thermal fuses / resettable (PTC) in appliances.

Why fuse in the phase wire and not in the neutral

• If the fuse is in the neutral and it blows, the circuit stops working, but the appliance and wiring stay connected to the live phase. A person touching the equipment can get a shock even though it appears "off".
• With the fuse in the phase wire, a blown fuse disconnects the live conductor, so everything beyond it is dead and safe.
• An earth fault (phase to body) current flows through phase and earth, not through the neutral; a fuse in the neutral would not see it and would not clear the fault.
• The neutral is kept continuous and earthed; opening it can cause voltage unbalance in 3-phase 4-wire systems.

Why HRC fuses are used

• High breaking capacity (up to 80–120 kA), suitable near large transformers where fault levels are high.
• Current limiting: they melt in the first quarter cycle, so the actual peak (cut-off) current is far below the prospective current, reducing damage to cables and switchgear.
• Quick and silent operation without flame or external arc, because of quartz sand quenching.
• Consistent inverse-time characteristic that does not change with age.
• Good discrimination with downstream fuses and MCBs, and good back-up protection for contactors and breakers.
• No maintenance, low cost compared with a circuit breaker of the same breaking capacity.
• Used for transformer LT side, motor feeders (with contactor and overload relay), capacitor banks and main switch fuse units.