[Class 1] High-Voltage Receiving Equipment

Article 2 of the Ordinance defines AC high voltage as over 600 V up to 7,000 V. "Over 300 V up to 6,600 V" mixes up the 300 V used in indoor wiring rules with the 6,600 V nominal distribution voltage. "Over 750 V" confuses it with the DC low-voltage limit (750 V). "Up to 10,000 V" is wrong because anything above 7,000 V is extra-high voltage (特別高圧).

Article 21(2) of the Interpretation requires the sum of fence height and distance to live parts to be at least 5 m. 3 m and 4 m are too short. 6 m is the value for extra-high-voltage equipment over 35,000 V up to 160,000 V (Article 22).

Article 21(3) of the Interpretation requires at least 4.5 m above ground (4 m outside urban areas). 4 m is the value outside urban areas, so it is not enough here. 3.5 m does not appear in the rule. 5 m confuses it with the fence-height-plus-distance value or the rule for extra-high-voltage equipment (Article 22).

Article 21(4) of the Interpretation says the equipment may be housed in "a concrete box or a metal box with Class D earthing". Class A applies to the metal frames of high-voltage equipment itself (Article 29) and to surge arresters, not to this box rule. Class B is applied on the low-voltage side of a transformer linking high and low voltage. Class C is for frames of low-voltage equipment over 300 V.

Under Article 23 of the Interpretation, high-voltage switchgear that produces arcs must be at least 1 m from combustibles. 2 m is the value for extra-high voltage. 0.5 m and 1.5 m do not appear in the rule.

Article 37(1)(iii) of the Interpretation requires an arrester at the service entrance of customers receiving 500 kW or more from a high-voltage overhead line. 50 kW, 300 kW and 2,000 kW are not the threshold in this rule.

Article 37(3) of the Interpretation requires Class A earthing for arresters on high- and extra-high-voltage circuits. Class B is for the low-voltage side of transformers, and Classes C and D are for frames of low-voltage equipment; none of these is the rule for arresters that discharge lightning current to earth.

Article 29 of the Interpretation requires Class A earthing for metal cases of high-voltage equipment, and Article 17 sets Class A resistance at 10 Ω or less. "Class A, 100 Ω" mixes in the Class D value. "Class C, 10 Ω" is for low-voltage equipment over 300 V. "Class D, 100 Ω" is for low-voltage equipment of 300 V or less.

In the PF-S type, a current-limiting fuse (PF) and a load-break switch (S) together form the main protective device. A breaker with an overcurrent relay is the CB type. A disconnector cannot switch load current and a voltage transformer only measures, so they are not a main breaker. An arrester and a ZCT are for overvoltage and earth-fault detection.

A sectionalizing switch with an earth-fault relay opens when an earth fault occurs on the premises, so the fault does not black out the whole distribution line. Power factor is improved by capacitors, overvoltage is handled by arresters, and voltage is reduced by transformers.

A disconnector has no arc-quenching ability, so it must not be opened while load current flows. Interrupting short-circuit current and opening automatically on a relay signal describe a circuit breaker. "Switches load current but not short-circuit current" describes a load-break switch (LBS).

A disconnector cannot break load current, so open the breaker first to stop the current, then open the disconnector to isolate the circuit. Opening the disconnector first, or both together, or only the disconnector, would draw an arc at the disconnector while current flows. To re-energize, close the disconnector first and then the breaker.

A load-break switch can switch load current but cannot interrupt short-circuit current, so the current-limiting fuse clears short circuits. "Cannot switch load current" describes a disconnector. A fuse does not improve power factor, and earth faults are detected by a ZCT and earth-fault relay.

A vacuum circuit breaker opens its contacts in a vacuum vessel to quench the arc. Blowing compressed air is how an air-blast breaker works. Operating only without load current describes a disconnector, and melting and needing replacement describes a fuse.

Article 34(2) of the Interpretation requires an enclosed fuse to withstand 1.3 times rated current and melt within 120 minutes at 2 times (or be a compliant current-limiting fuse). "1.25 times / 2 minutes" is the rule for non-enclosed fuses (Article 34(3)). "1.1 times / 1.6 times / 60 minutes" is the low-voltage fuse rule for 30 A or less (Article 33), not the high-voltage rule. "1.5 times / 3 times / 10 minutes" does not appear.

Opening the secondary of an energized CT produces a high voltage across it and can cause insulation breakdown or burning, so short the secondary first, then remove the meter. Leaving it open is exactly what must be avoided. Removing the Class D earth required by Article 28 increases danger if insulation between primary and secondary fails. Raising the primary current does not reduce the danger.

The ratio is 150 ÷ 5 = 30, so the primary current is 3.2 × 30 = 96 A. 480 A is 3.2 × 150, forgetting to divide by 5. 30 A is just the ratio. 0.11 A is 3.2 × 5 ÷ 150, dividing the wrong way.

The ratio is 6,600 ÷ 110 = 60, so the primary voltage is 105 × 60 = 6,300 V. 6,600 V is the rated value, for a secondary reading of 110 V. 6,930 V is 6,600 × 105 ÷ 100, dividing by 100 instead of 110. 11,550 V is 105 × 110, multiplying without using the ratio.

An open-delta bank can deliver √3 times the rating of one unit: √3 × 50 ≈ 86.6 kVA. 100 kVA is simply the sum of two units, which a V connection cannot deliver. 150 kVA is the capacity of the full delta bank. 75 kVA is half of 150 kVA.

Rated current = 300,000 ÷ (√3 × 210) ≈ 824.8 A. Short-circuit current = rated current × 100 ÷ %Z = 824.8 × 100 ÷ 5 ≈ 16,496 A ≈ 16.5 kA. 28.6 kA forgets the √3 (300,000 ÷ 210 × 20). 4.1 kA multiplies by %Z (824.8 × 5) instead of 100 ÷ %Z. 0.82 kA is just the rated current.

A ZCT passes all three conductors through its core and detects the difference in current (zero-phase current) caused by an earth fault, sending it to the earth-fault relay. A VT measures voltage, a series reactor protects capacitors from harmonics and inrush, and a disconnector isolates circuits.

An inverse-time characteristic means the larger the current, the faster the relay operates: quickly for short circuits, slowly for slight overloads. "Larger current, longer time" is the reverse. A fixed time regardless of current is a definite-time characteristic. A relay that operates when current falls below the setting is not an overcurrent relay.

Long cables have large capacitance to earth, so during an earth fault outside the premises, zero-phase current can flow through them and trip a plain earth-fault relay unnecessarily. A directional relay also uses zero-phase voltage to judge the direction and operates only for faults on the premises. Interrupting capacity depends on the breaker, not the relay type. Switching capacitors and handling lightning are not its role.

Protection coordination adjusts pickup currents and times so that only the device nearest the fault on the supply side trips. Opening all breakers at once widens the outage. If the utility breaker tripped first, the whole distribution line would go dark, so the customer side is set to act first. Overcurrent and earth faults are different, so one relay type alone is not enough.

The capacitor's leading reactive power cancels the load's lagging reactive power, so the apparent power and current from the source fall, reducing losses (proportional to current squared) and voltage drop. Active power and motor output do not change. Current rising is the reverse. The load still needs the same reactive power; the capacitor simply supplies it locally.

tanθ1 = 0.8 ÷ 0.6 = 4/3 and tanθ2 = 0.6 ÷ 0.8 = 0.75. Q = 300 × (4/3 − 0.75) = 300 × 0.583… = 175 kvar. 225 kvar is the reactive power remaining after correction (300 × 0.75). 400 kvar is the reactive power before correction (300 × 4/3); cancelling all of it would give a power factor of 1. 60 kvar wrongly multiplies by the difference of power factors, 300 × (0.8 − 0.6).

At PF 0.8, tanθ = 0.6 ÷ 0.8 = 0.75, so reactive power is 240 × 0.75 = 180 kvar. Correcting to 1.0 cancels all of it: 180 kvar. 144 kvar uses sinθ instead of tanθ (240 × 0.6). 300 kvar confuses it with the apparent power (240 ÷ 0.8 = 300 kVA). 320 kvar uses the tanθ for PF 0.6 (240 × 4/3).

The JEMA guide explains that a series reactor reduces harmonic voltage distortion and limits capacitor inrush current (in HV receiving equipment, typically 6 % of the capacitor reactance). It does not increase active power. Detecting zero-phase current is the ZCT's job, and discharging residual charge is done by a discharge coil or resistor.

At light load the lagging reactive power is small, so the capacitor's leading reactive power exceeds it, the power factor becomes leading, and the receiving-end voltage can rise. A worse lagging power factor is the reverse. Capacitors do not add active power. Core saturation and earth faults are unrelated.

Article 28 of the Interpretation requires Class D earthing on the secondary circuit of high-voltage instrument transformers. Class A applies to extra-high-voltage instrument transformer secondaries (Article 28(2)) and to frames of HV equipment. Class B is for the low-voltage side of HV/LV transformers, and Class C for frames of low-voltage equipment over 300 V.