[Class 1] Installation and Inspection of Private-Use Electrical Facilities

Article 168 allows high-voltage indoor wiring only by insulator (knob) wiring in dry, exposed places, or by cable wiring. Metal conduit, PVC conduit and metal duct wiring are low-voltage indoor wiring methods and are not listed for high voltage.

Article 168 requires Class A grounding for metal parts of protective conduit, metal junction boxes and metal cable sheaths in high-voltage cable wiring. Class D is allowed only when contact-protection measures are taken. Class C is for enclosures of low-voltage equipment over 300 V, and Class B is for the low-voltage side of a transformer that couples high and low voltage, so they are wrong here.

Article 168(2) requires at least 15 cm between high-voltage indoor wiring and other indoor wires, water pipes or gas pipes (30 cm if the other is a bare low-voltage wire on insulators). 5 cm is the clearance between a high-voltage wire and the building in insulator wiring, 30 cm applies to bare wires, and 1 m is the clearance of arcing high-voltage switches from combustibles, so none fits here.

Article 168(1)(ii) requires at least 8 cm between wires and at least 5 cm between a wire and the building structure. The pairs 6 cm/2.5 cm, 10 cm/10 cm and 15 cm/8 cm do not match (15 cm is the separation from other wires and pipes).

Article 120(4) requires at least 1.2 m where vehicles or heavy objects may press on the line, and at least 0.6 m elsewhere. 0.6 m is the value for other places; 0.3 m and 2.0 m are not burial depths in Article 120.

Article 21 requires the fence height plus the distance from the fence to the live parts to be at least 5 m. 5 m − 2 m = 3 m. With 1 m the sum is 3 m, with 2 m it is 4 m, and with 2.5 m it is 4.5 m, all short of 5 m.

Article 23 requires arcing switches and similar devices to be at least 1 m from combustibles for high voltage, and as a rule 2 m for extra-high voltage up to 35,000 V. 2 m is the extra-high-voltage value; 0.5 m and 3 m are not in Table 23-1.

Article 29 (Table 29-1) requires Class D for low-voltage equipment up to 300 V, Class C for low voltage over 300 V, and Class A for high or extra-high voltage. Class B is not for enclosures; it is applied to the low-voltage neutral (or a terminal) of a transformer coupling high and low voltage. Classes C and D are for low-voltage equipment enclosures.

Article 17(1) sets Class A grounding at 10 Ω or less, with an annealed copper earthing conductor of at least 2.6 mm diameter (or a corrosion-resistant metal wire with tensile strength of at least 1.04 kN). 100 Ω is the Class D value and 1.6 mm is the conductor size for Classes C and D, so pairs containing them are wrong.

Article 17(1)(iii) requires the earth electrode to be buried at least 75 cm deep and the conductor from 75 cm below ground to 2 m above ground to be covered with plastic conduit (excluding PVC conduit thinner than 2 mm and CD pipe) or similar. The other ranges do not match (60 cm is easily confused with the '60 cm above ground' in the rule on using insulated wire for the conductor).

Article 28 requires Class D grounding on the secondary circuit of a high-voltage instrument transformer, and Class A for an extra-high-voltage one. Class A is the extra-high-voltage case, Class B is for the low-voltage side of a transformer coupling high and low voltage, and Class C is for enclosures of low-voltage equipment over 300 V.

With no cutoff device, Article 17(2) (Table 17-1) uses 150/Ig: 150 ÷ 5 = 30 Ω. 60 Ω would come from 300/Ig (cutoff in more than 1 s up to 2 s) and 120 Ω from 600/Ig (1 s or less). 10 Ω is the Class A value and has nothing to do with this calculation.

Because the cutoff time is 1 s or less, use 600/Ig from Article 17(2) (Table 17-1): 600 ÷ 4 = 150 Ω. 37.5 Ω comes from 150/Ig (no cutoff device) and 75 Ω from 300/Ig (more than 1 s up to 2 s). 600 Ω is just the numerator, not divided by Ig.

Article 37 requires a surge arrester at the service entrance of a customer receiving 500 kW or more from a high-voltage overhead line, and requires Class A grounding for arresters on high-voltage and extra-high-voltage circuits. 50 kW is not in the rule, and Classes B, C and D are the wrong classes for an arrester.

Article 117 says a cable service line follows Article 67, and Article 67 limits hanger spacing to 50 cm for high-voltage overhead cable. 20 cm is the pitch for spirally wrapping metal tape, and 2 m is the support spacing for low-voltage cable fixed along a building. 1 m is not in Article 67 (in Article 164 it is the support spacing for cabtyre cable).

Under Article 1 (Table 1-1), for circuits over 1,000 V and under 500,000 V the maximum operating voltage is the nominal voltage × 1.15/1.1: 6,600 × 1.15 ÷ 1.1 = 6,900 V. 6,600 V has no factor applied, 7,590 V uses only 1.15 (the factor for 1,000 V or less), and 9,900 V is 1.5 times the nominal voltage.

The maximum operating voltage is 6,600 × 1.15 ÷ 1.1 = 6,900 V. Under Article 15 (Table 15-1), an AC circuit with a maximum operating voltage of 7,000 V or less is tested at 1.5 times that: 6,900 × 1.5 = 10,350 V. 9,900 V is 1.5 times the nominal voltage, 8,625 V uses 1.25 times, and 20,700 V is the DC value for cable.

Article 15(ii) allows a cable AC circuit to be tested with a DC voltage twice the Table 15-1 value for 10 minutes. The AC test voltage is 6,900 × 1.5 = 10,350 V, so 10,350 × 2 = 20,700 V. 10,350 V is the AC value unchanged, 13,800 V is twice the maximum operating voltage, and 16,560 V uses 1.6 (the factor for DC testing of rotating machines).

Article 15 requires the circuit to withstand the test voltage applied between the circuit and earth 'continuously for 10 minutes'. 1 minute and 30 minutes are not in the rule, and splitting the time into parts does not meet the 'continuously' condition.

The low-voltage winding's maximum operating voltage is 200 × 1.15 = 230 V. Article 16 (Table 16-1) uses 1.5 times for windings of 7,000 V or less, but not less than 500 V. 230 × 1.5 = 345 V, which is below 500 V, so the test voltage is 500 V. 230 V is just the maximum operating voltage, 345 V forgets the 500 V minimum, and 300 V is 1.5 times the nominal voltage.

The maximum operating voltage is 3,300 × 1.15 ÷ 1.1 = 3,450 V. Article 16(2) (Table 16-2) tests rotating machines of 7,000 V or less at 1.5 times: 3,450 × 1.5 = 5,175 V. 3,450 V is the maximum operating voltage itself, 4,950 V is 1.5 times the nominal voltage, and 8,280 V is the DC test value (1.6 × 5,175 V).

The charging current is I = 2πfCV: I = 2 × 3.14 × 60 × 0.3 × 10⁻⁶ × 10,350 ≈ 1.17 A. 0.98 A is the result for 50 Hz, 0.19 A forgets the 2π factor (60 × 0.3 × 10⁻⁶ × 10,350 ≈ 0.19), and 2.34 A wrongly uses the DC test voltage of 20,700 V. The test transformer needs enough capacity to supply this current (roughly 10,350 V × 1.17 A ≈ 12.1 kVA).

Insulation resistance is measured before the test so that high voltage is not applied to obviously bad insulation, and after the test to check that the test did not damage it. Cable has large capacitance and keeps a charge, so it must be discharged (for example, with a grounding rod) before touching. Measuring only before cannot find damage from the test, touching without discharging risks electric shock, and Article 15 requires the voltage to be held continuously for 10 minutes, so removing it immediately is wrong.

A ground relay is combined with a zero-phase current transformer (ZCT), which detects the sum of the currents in the three wires (zero-phase current). Without a ground fault the sum is almost zero; with one, the current flowing to earth appears. A VT transforms voltage for meters, a CT detects load current for devices such as overcurrent relays, and a surge arrester sends abnormal voltage to earth, so none of them detect ground-fault current.

Relay testing checks whether the relay operates at its set value (operating current) and how long it takes (operating time), and then an interlock test checks that the breaker actually opens when the relay operates. A temperature rise test runs equipment to see how hot it gets, a continuity test checks that wiring is connected, and a withstand test checks insulation under high voltage; none of them checks the breaker's operation.