Lesson page
Lesson 4: AC, DC, Frequency, and Phase Basics
Read the lesson, then answer the multiple-choice practice set below. You get instant right or wrong feedback with explanation.
Electrical Theory Symbols and Diagrams
Lesson Content
Frequency and phase are easier to understand if you connect them to real equipment instead of memorizing words alone.
Japanese Key Term
交流（こうりゅう / alternating current）
Hiragana
こうりゅう
English Meaning
Alternating Current
Simple Explanation
AC changes direction repeatedly. DC flows in one direction only. Frequency tells you how many cycles happen each second.
Detailed Explanation
Japanese residential wiring uses AC. The written exam may ask about frequency, phase, or why certain equipment needs AC or DC. Use concrete examples: household power is AC, batteries are DC, and industrial equipment may use three-phase power for motors.
Key Vocabulary
- 交流（こうりゅう / alternating current）
- 直流（ちょくりゅう / direct current）
- 周波数（しゅうはすう / frequency）
- 単相（たんそう / single-phase）
- 三相（さんそう / three-phase）
Formula
Frequency is measured in Hz. Power systems in Japan commonly use 50 Hz or 60 Hz depending on region.
Worked Example
A battery-powered flashlight uses DC. A home outlet supplies AC, so the wiring and equipment must be suitable for AC power.
Exam Tips
If the problem mentions homes, outlets, or common Japanese wiring, expect AC unless the problem says otherwise.
Common Exam Trap
Do not assume frequency changes the voltage value. Frequency describes how often the waveform repeats.
Site Reality in Japan
On Japanese job sites, electricians often distinguish AC distribution, DC control circuits, and equipment ratings carefully.
Practice Question
Which power type does a battery provide?
Answer
DC
Explanation
A battery supplies direct current, which flows in one direction.
1. Why do both L and N pass through the RCD/ELCB block?
L ---[MCB]---[RCD/ELCB]--- Branches N --------------[RCD/ELCB]--- Branches
Leakage devices compare line and neutral current balance; imbalance indicates leakage path.
2. What exam skill does this pair of notations test?
[Single-line] DB -> SW -> Lamp [Multi-line] L -> SW -> Lamp -> N
The candidate must translate between simplified and detailed conductor representations correctly.
3. If Outlet 1 fails open at its load, Outlet 2 is usually:
L ---+---(Outlet 1)
+---(Outlet 2)
N ---+---(Outlet 1/2)
In a parallel branch arrangement, one load open-circuit does not necessarily interrupt other branches.
4. Primary purpose of the PE line here is:
PE ------------------------(Metal case) L,N -----------------------(Appliance)
PE is for protective earthing and fault-current safety, not normal operating current.
5. With SW1 open, the expected state is:
L --[SW1]--+--(Lamp A)--N
+--(Lamp B)--N
SW1 is upstream of both branch lamps, so opening it removes supply to both.
6. If the node above has no dot at crossing in exam notation, you should assume:
L ----(Node)----[SW]----(Lamp)
N ------------------------(Lamp)
Standard exam diagram logic treats crossings as non-connections unless explicit junction indication is given.