What a neutral wire is and how it is derived at the transformer.
Also how part of a circuit is a neutral, how it isn’t once disconnected and possible dangers of disconnecting wires which have apparently no voltage on them.
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Other videos on transformers and electrical systems:
Isolating transformers: https://youtu.be/sfzy5IQMzyc
Why normal supplies are not isolated: https://youtu.be/Wx_v1-T1gE4
Earth connections: https://youtu.be/fRhofcMyAyk
Many other electrical basics: https://www.youtube.com/playlist?list=PLVsHvs2SuqmohPoL8Cumvqg2DmGcrZF8R