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A TN-S earthing system is one where where earth and neutral run separately right back to the supply.
Pros
- Can have low earth loop impedance.
Cons
- Earth can be dragged up slightly by faults in other people's installs.
- Sometimes, significant current can flow down neutral-earth faults, but this is fairly rare.
- Most expensive system for the supply company to provide.
Mitigation of specific risks
Bonding of pipework to the earthing system is required because of the possibility of earth voltage differences in different areas, but it is less important with TN-S than with the other types of earthing system.
Other earthing systems
- TT earth is rodded down in the consumer's install and has no direct connection to the neutral of the supply transformer.
- TN-C-S earth and neutral joined in the supply network but separated in install.
- TN-C earth and neutral are combined right through the install.
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