Inspection Methods

Article 96 of the Ordinance requires the investigation at installation and when alteration work is completed, plus at least once every 4 years as a rule (once every 5 years when a registered inspection-service corporation is entrusted; as of October 2026). "Only at installation", "every 10 years" and "not at installation" do not match the text.

First check the work visually, then measure insulation resistance, earth resistance and continuity with the power off, and only when no fault is found apply power (energizing test). Doing the energizing test earlier could cause electric shock, fire or damage if there is a wiring error or poor insulation, so the other three orders are not appropriate.

In a single-phase 3-wire 100/200 V system, the voltage between each wire and earth (voltage to earth, 対地電圧) is 100 V even for a 200 V circuit. Since the circuit is 300 V or less with a voltage to earth of 150 V or less, the minimum is 0.1 MΩ. 0.2 MΩ is for circuits of 300 V or less with more than 150 V to earth (such as three-phase 3-wire 200 V), and 0.4 MΩ is for circuits over 300 V. 1 MΩ is not a value in the Ordinance.

A three-phase 3-wire 200 V circuit is 300 V or less, but its voltage to earth exceeds 150 V, so Article 58 of the Ordinance requires at least 0.2 MΩ. 0.1 MΩ applies when the voltage to earth is 150 V or less, and 0.4 MΩ applies when the circuit voltage exceeds 300 V. 1 MΩ is not a value in the Ordinance.

Article 58 of the Ordinance requires at least 0.4 MΩ for low-voltage circuits whose voltage exceeds 300 V. 0.2 MΩ and 0.1 MΩ are for circuits of 300 V or less, so they do not apply to a 400 V circuit. 1 MΩ is not a value in the Ordinance.

Article 14 of the Interpretation says that when insulation resistance is hard to measure, the leakage current with the operating voltage applied must be 1 mA or less. With 100 V to earth and 0.1 MΩ, 100 ÷ 100,000 = 0.001 A = 1 mA, which matches the insulation resistance limit. 0.1 mA and 0.5 mA are stricter than the rule, and 10 mA is larger than the rule, so they are wrong.

Use Ohm's law, R = V ÷ I. 0.5 mA = 0.0005 A, so R = 100 ÷ 0.0005 = 200,000 Ω = 0.2 MΩ. 0.1 MΩ would correspond to 1 mA, 0.5 MΩ to 0.2 mA, and 2 MΩ to 0.05 mA; these come from unit-conversion mistakes.

An insulation resistance tester applies a high DC voltage and calculates the insulation resistance from the tiny current. Electronic devices left connected can be damaged, so they are disconnected first. It does not apply 100 V AC. Insulation resistance is measured with the power off. A circuit tester's resistance range uses a low voltage and is not used for megohm-level insulation measurement.

Between conductors, turn off the power (open the branch switch) and remove the loads. If loads stay connected, you measure the load itself (such as a lamp filament) and get a very small value. Keep the wall switch (点滅器) ON so the wiring beyond it is included. Measuring with the branch switch closed is dangerous, and with the wall switch OFF the wiring beyond it is not measured.

Between the circuit and earth, open the branch switch to cut the power, turn the wall switch ON, and measure all wires together against earth. The load sits between the wires, not between the wires and earth, so loads may stay connected. Measuring while energized is dangerous. Measuring only the grounded wire, or with the wall switch OFF, leaves parts unmeasured.

Frames of equipment of 300 V or less need Class D grounding (D種接地工事, Article 29), whose earth resistance must be 100 Ω or less (Article 17). 10 Ω is the Class C value, and 500 Ω applies only when a device breaks the circuit within 0.5 s. 1000 Ω is not a value for any class.

Class D grounding is 100 Ω or less as a rule, but 500 Ω or less is allowed when a device automatically breaks the circuit within 0.5 s on a ground fault (Article 17), so 350 Ω passes. A breaker does not change it into Class C. Grounding may be omitted only with a current-operated breaker of 15 mA or less and 0.1 s or less in a place that is not wet (Article 29), so it cannot be omitted here.

Frames of low-voltage equipment over 300 V need Class C grounding (C種接地工事, Article 29), with an earth resistance of 10 Ω or less (Article 17). Class D is for equipment of 300 V or less. 500 Ω applies only with an automatic breaking device within 0.5 s, which is not the case here. The basic Class C value is not 100 Ω.

Article 29, paragraph 2 of the Interpretation allows omission when, for example, equipment with 150 V AC or less to earth is installed in a dry place. 200 V to earth in a wet place does not qualify. Omission with a breaker needs a current-operated breaker of 15 mA or less and 0.1 s or less in a place that is not wet. 400 V equipment needs Class C grounding, and being in a dry place alone does not allow omission (as of October 2026).

An earth resistance tester sends current through C and measures voltage at P to find the resistance of E. So E, P and C are placed almost in a straight line with P in the middle, spaced as the manual says (often about 10 m each). Putting E in the center, putting P and C in the same spot, or driving them right next to E will not give a correct value.

A voltage detector checks whether voltage is present on a circuit (whether it is live), for example as a safety check before work. Insulation resistance is measured with an insulation resistance tester, earth resistance with an earth resistance tester, and phase sequence with a phase sequence indicator (検相器).

In a continuity test, the resistance range of a circuit tester (回路計) is used to check that current can pass between the two ends of a wire. An illuminance meter measures brightness, a phase sequence indicator checks three-phase phase order, and an earth resistance tester measures resistance between an electrode and earth; none checks wiring continuity.

When all the wires of the circuit are clamped together, outgoing and returning currents cancel to zero if there is no leakage, and what remains is shown as leakage current. For single-phase 3-wire, that means all three wires. Clamping one wire measures load current, and clamping only the two live wires leaves the neutral current uncancelled.

The phase sequence of three-phase AC (the order in which each phase reaches its peak) is checked with a phase sequence indicator (検相器). If it is reversed, the motor turns the other way. A circuit tester measures resistance and voltage, an illuminance meter measures brightness, and a clamp meter measures current; none shows the phase sequence.

Article 11 of the Ordinance says such an office must keep an insulation resistance tester, an earth resistance tester, and a circuit tester that can measure resistance and AC voltage (as of October 2026). A tachometer, an illuminance meter and a phase sequence indicator are not among these three.