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Why do households need to have TNCS system (ground electrode near house) rather than TNC? Usually household system needs RCD to protect humans. Why do you need separate ground electrode? An RCD will work without it. Circuit breakers will also trip using a TNC system.

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Each of these pieces protects a different thing:

  • RCD (GFCI in the US) watches for a difference between hot(s) and neutral (or between hots) and breaks the circuit if there is a significant difference, as the assumption is that the difference could be going through a person
  • Ground wires between receptacles or appliances and the breaker panels allow for certain types of faults to trip the circuit breakers, and also provide a path for surge protection devices to send current back to "ground"
  • Ground rods provide a path for lightning or other natural electricity to get back to the earth.

All electricity wants to go in a complete path. If that path includes a person, RCD/GFCI helps. If that path includes the metal case of an appliance, ground wires help. If that path includes a starting point of "outside the utility-generated power" (e.g., lightning), the ground rod helps.

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  • \$\begingroup\$ i know how rcd works, i know how circuit braker work. question was why house need its own metal rod for grounding why it cant use PEN (one wire as neutral and as ground) from the transformer. \$\endgroup\$ Aug 23, 2022 at 9:35
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Usually household system needs RCD to protect humans.

For households, by law, the primary protection device is the fuse. This means that when live comes into contact with an exposed area of an earthed appliance, the fuse immediately blows.

RCDs are secondary protection.

From comments: -

why it cant use PEN (one wire as neutral and as ground) from the transformer

Earth is the node which is used by many modern appliances to shunt away common-mode high-frequency noise from switch mode power supplies. This requires a separate earth wire thus allowing RCDs to be able to operate correctly.

See also my answer to this question: Is ground connection in home electrical system really necessary?.

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The grounding electrode isn't the fundamental feature of TNCS, versus TNC.

Here in the UK, TNCS is common, but not universal. The split between ground an neutral is done at the supply cut-out (main fuse). The householder is not required to provide a grounding rod, but may do so if they wish. We use a PME (protective multiple earth) system where the supplier must ground the CNE (combined neutral/earth) at multiple places along the cable run.

In TNCS, the ground and neutral are split at the origin of the supply. Any RCDs are installed after that split. In TNC, there is no separate ground in the installation.

With TNC, not all dangerous faults are detectable by the RCD. If a person standing on the Earth touches a live wire, the RCD will trip. But if a person leaning on a metal-cased appliance touches a live wire, the RCD will see nothing wrong. The metal case of the appliance will be connected to neutral.

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