Wiring Diagrams 3: Integrated Questions

Lamp a is switched from two places with 3-way switches (② and ③). The live (non-earthed) wire goes to the common terminal of the living-room switch, so the hallway switch needs the two travellers plus the wire from its common terminal back to lamp a: 3 wires. Two is not enough for the travellers; 4 or 5 wrongly adds the circuit of switch b, which is wired separately by cable ⑤.

The live wire comes to the common terminal of the living-room 3-way switch, and two travellers leave from terminals 1 and 3. Section ② needs 3 wires, so the cable is VVF1.6-3C (1.6 mm, 3 cores). A 2C cable has only 2 cores. The notes use 2.0 mm only for the 200 V circuit ⑦; this lighting circuit is 1.6 mm.

Section ⑥ only carries the supply for lamp b, switch b and the indicator lamp. Sending one live and one earthed (neutral) wire to J2 is enough, because J2 splits them to the switch group (⑤) and lamp b: 2 wires. The travellers for lamp a go directly to the hallway switch via ③, so they are not in ⑥. Three or more wrongly counts wires of lamp a or the switched return.

Switch b needs one live wire and one switched return to lamp b. The indicator lamp is connected in parallel with lamp b, so it also needs an earthed (neutral) wire: 3 wires in total. Two would be a switch without the indicator lamp. Four or five wrongly adds the wires of 3-way switch a, which arrive separately through ③.

A locator lamp is connected across the switch contacts; while the switch is off, a small current flows through lamp b and makes it glow. No earthed (neutral) wire is needed at the switch, so ⑤ needs only the live wire and the switched return: 2 wires. Three is the count for an indicator lamp in parallel with the load; four has no basis; one wire cannot form a switch circuit.

Wires entering J1: ① (live, earthed), lamp a (earthed, return), ② (common, two travellers), ③ (two travellers, return) and ⑥ (live, earthed). The connections are: live (①, common of ②, ⑥), earthed (①, lamp a, ⑥), traveller 1, traveller 2, and the return (common of ③ to lamp a): 5 points. Three or four misses the traveller or return joints; six counts a joint twice.

In the ring-sleeve table published by the examination centre (based on JIS C 2806), two 1.6 mm wires use a small sleeve with the crimp mark "○". Marks are fixed for each combination in the table, and "小" does not apply to this one. Medium sleeves are for more or thicker wires; on two 1.6 mm wires they are too large to crimp firmly.

Ring sleeves are crimped with a ring-sleeve crimping tool complying with JIS, which has yellow handles (examination centre sample question and skills-test notes). A tool with red handles is for crimp terminals and must not be used on ring sleeves. Pliers cut and bend wire and cannot apply the specified crimping force. A wire stripper removes insulation.

J1 has 5 connection points. The live joint (①, ②, ⑥) and the earthed joint (①, lamp a, ⑥) each join 3 wires, so they need 3-wire connectors; the two traveller joints and the return joint join 2 wires each, so they need 2-wire connectors. That is three 2-wire and two 3-wire connectors. Two-wire connectors alone cannot join 3 wires; using 3-wire connectors for 2-wire joints does not match the joint.

Crimping over the insulation does not give firm metal-to-metal contact, so the joint resistance rises and it can overheat; the examination centre treats this as a defect. For push-in connectors, pushing the conductor fully in so it is visible at the tip is correct. Checking the crimp mark after crimping is the correct procedure. In cable work, joints are made inside a box.

A VVF stripper removes the sheath and insulation of VVF cable without nicking the conductor. A ring-sleeve crimping tool is for joints, not stripping. A pipe cutter cuts conduit, and a knockout punch makes holes in metal boxes for conduit.

Flush switches and indicator lamps are mounted side by side on a multi-gang mounting frame, which is fixed to a switch box in the wall. A VVF joint box is for joining wires, not for mounting devices. Normal bends and couplings bend and join conduit; locknuts and insulating bushings fix conduit to boxes and protect the wires.

VVF is 600 V vinyl-insulated vinyl-sheathed flat cable. "2.0" is the diameter (mm) of the solid conductor and "2C" is the number of cores (2). Cross-sectional area (mm²) is how stranded conductors are sized, so reading 2.0 as an area is wrong. "2C" is not the number of cables. Allowable current is not written in the cable code; it depends on the size and other factors.

Under JIS C 0303, a receptacle symbol is annotated with its rated current when it is 20 A or more, and with its rated voltage when it is 250 V or more. "20A250V" is the rating of the receptacle itself. It does not show the breaker rating (shown at the panel), the air conditioner's power, or the cable's performance.

Under JIS C 0303, 15 A receptacles carry no current annotation; the current is annotated only at 20 A or more. The voltage is annotated only at 250 V or more, so an unmarked receptacle is the ordinary 15 A, 125 V type. The number of outlets is annotated when there are 2 or more, so "2-gang" means one device with two sockets, not two devices in different places.

Equipment on low voltage of 300 V or less uses Class D earthing (the examination centre's sample question also treats the earth pole of a receptacle on a circuit of 300 V or less as Class D). ⑦ is 200 V, so Class D. Class A is for high-voltage equipment, Class B for the low-voltage side of transformers (such as the neutral), and Class C for low-voltage equipment above 300 V.

Under Article 58 of the ministerial ordinance, a circuit of 300 V or less with a voltage to earth of 150 V or less needs at least 0.1 MΩ. A 200 V circuit from a single-phase 3-wire 100/200 V supply has 200 V between wires but only 100 V from each wire to earth, so 0.1 MΩ applies. 0.2 MΩ is for circuits of 300 V or less with more than 150 V to earth (such as 3-phase 200 V); 0.4 MΩ is for circuits above 300 V; 1 MΩ is not in the ordinance table.

Insulation resistance is measured with the power off. When measuring between wires, connected appliances would be measured through their internal circuits, hiding the wiring's insulation. Measuring with the breaker on applies the tester's voltage to a live circuit, which is dangerous and gives wrong readings. Measuring between the earth pole and earth is an earth-resistance test, not wire-to-wire insulation.

Before switching on, the work is visually inspected and the insulation and earth resistances are measured to make sure there is no leakage or earthing fault. Only after safety is confirmed is power applied, last, to check lamps, switches and receptacle voltage. Visual inspection comes first, and insulation and earth resistance are measured before energising.

A reamer trims the inside of the cut end of a metal conduit so that wire insulation is not damaged when pulled in (examination centre sample explanation). A pipe bender bends conduit; a threader cuts threads on the pipe end and is not used on threadless conduit; a knockout punch makes holes in metal boxes.

Steel conduit is bent with a pipe bender (examination centre sample question). A blow torch is used, for example, to heat rigid PVC conduit for bending, not for steel conduit. A reamer finishes the inside of a cut; a pipe vice holds the pipe while cutting.

An insulating bushing is fitted on the conduit end to protect the insulation where wires enter and leave. A coupling joins two conduits, a normal bend is a ready-made right-angle bend, and a saddle fixes the conduit to a wall or ceiling.

A fish tape is pushed through the conduit first; the wires are tied to its end and pulled in. A brace (click-ball) turns a reamer or bit by hand, a wire stripper removes insulation, and a pipe vice holds pipe; none of them pulls wires.

In metal-conduit work, wires must not be joined inside the conduit; joints there cannot be inspected and may be damaged when pulling, so joints are made in boxes. Threadless conduit is connected to boxes with threadless box connectors and fixed with saddles. Insulated wire such as IV is pulled into the conduit.

200 V is 300 V or less, so Class D earthing applies. Class D is 100 Ω or less in principle, but 500 Ω or less is allowed when a device automatically disconnects the circuit within 0.5 s on an earth fault (Interpretation (解釈) Art. 17(4)); note 4 says RCDs with an operating time of 0.1 s or less are installed, so the maximum is 500 Ω (the same reasoning as the examination centre sample). 100 Ω is the value without such a device. Class C is for low-voltage equipment above 300 V and does not apply to this motor.

Under Article 58 of the ministerial ordinance, for circuits of 300 V or less the minimum is 0.1 MΩ when the voltage to earth is 150 V or less and 0.2 MΩ otherwise; circuits above 300 V need 0.4 MΩ. A 3-phase 3-wire 200 V circuit has more than 150 V to earth, so 0.2 MΩ. 0.1 MΩ is for 100 V circuits or the 200 V side of a single-phase 3-wire supply, 0.4 MΩ is for above 300 V, and 0.3 MΩ is not in the table.

An earth resistance tester uses the electrode under test (E) plus two auxiliary electrodes (P and C) driven at a distance, all three connected. Measuring between the frame and the electrode only measures the connecting conductor, not the resistance to earth. If the auxiliary electrodes are too close, the soil regions around the electrodes overlap and the reading is wrong. An insulation tester measures insulation, not earth resistance.

The rotation direction of a 3-phase induction motor depends on the phase sequence, so a phase-sequence indicator is used. A light meter measures brightness, an earth resistance tester measures the resistance between an electrode and earth, and an insulation tester measures circuit insulation; none shows phase sequence.

Swapping any two of the three wires reverses the phase sequence and the rotation. Shifting all three by one position keeps the same sequence, so the direction does not change. Removing the earth wire is dangerous and unrelated to direction; the RCD sensitivity is also unrelated.

A clamp meter measures current just by clamping around a wire, without breaking the circuit. A circuit tester must be connected in series by breaking the circuit, which is unsuitable for a running power circuit. A voltage detector only checks whether a conductor is live, and an insulation tester measures insulation on a de-energised circuit.

When all circuit wires are clamped together, the outgoing and returning currents cancel and only the leakage current remains. The examination centre sample also says leakage current is measured with a clamp-type leakage meter. Clamping one wire measures load current. A circuit tester cannot measure leakage; its AC voltage range only measures voltage. An insulation tester is for de-energised circuits and is dangerous on a live one.

PF (flexible synthetic-resin) conduit is connected to boxes with a PF conduit box connector. A threadless box connector is for threadless steel conduit, a TS coupling joins rigid PVC conduits with adhesive, and an entrance cap is fitted on an outdoor conduit end to keep rain out.

A plastic conduit cutter cuts synthetic-resin conduit such as PF cleanly. A pipe bender bends steel conduit, a threader cuts threads on steel pipe, and a knockout punch makes holes in metal boxes; none of them cuts conduit.

Under JIS C 0303, an automatic switch used for outdoor lamps is annotated with A and its capacity. It switches on and off automatically according to the surrounding brightness (examination centre sample). A timer switch is marked T, a switch with an indicator lamp L, and a pull switch P.

A hammer drill adds hammering to rotation to drill concrete. A knockout punch is for metal boxes, a brace (click-ball) is turned by hand, and an auger bit drills wood; none is used on concrete.