John Ward has an interesting video about the British TN-C-S grounding system. This is where the utility delivers Line and Neutral, and the dwelling does not have local ground rods and takes ground off of neutral. It's very similar to North American dryers and ranges pre-1996, or pre-2008 subpanels inside the same building. Now outlawed.

At the end John comments that this would never be used outdoors or at dockside / marinas.

So how is the grounding/earthing done for a dockside supply, say your house is along a canal and you have a boat?

  • Think from watching the video, he mention using a TN-S system, which has ground separate from neutral. Don't know, maybe they do use ground rods for those uses?
    – crip659
    Sep 17, 2022 at 14:28
  • @crip659 both TN-S and TN-C-S system support local ground rod installation in theory ( codes may prohibit, require, or allow them). After the US system is quite literally TN-C-S where the local ground rod is required. Sep 27, 2022 at 15:11
  • For TN-C-S a local rod has pros and cons. PRO: both N and PE are at the local earth voltage (rather than earth voltage wherever the utility's rod is). PRO: better safety if the neutral breaks upstream of the building. CON: earth actually becomes a current return path. The exact resistance is rarely known. (We can measure the resistance to the earth for each rod, and have standards for that, but seldom measure the resistance of the earth between local and utility rods.) Sep 27, 2022 at 15:12
  • TN-S local rod pros and cons. PRO: bonded metal components are at local earth voltage, not utility's voltage. CON: can make PE and no current N voltages further apart. A general CON of a TN-S system is that there is more distance and thus resistance for PE, which means Live to PE shorts can take longer to trip (or in extremes might fail to trip). Sep 27, 2022 at 15:13

2 Answers 2


In the UK TN-C-S is forbidden for supplies to caravans or boats, so such a supply would have to be either TN-S or TT.

Unfortunately, electricity suppliers in the UK have a nasty habbit of repairing former TN-S systems with sections of TN-C-S. So unless you have a dedicated transformer it can be difficult to be sure whether the supply is truely TN-S or not.

So often, especially for smaller scale supplies TT is the only reasonable option.


I don't know about a 'pure' dockside connection (where there is no main dwelling), but for any outbuilding, or outside supply in the UK, the earth (ground) or CPC (official term continuous protective conductor, emphasis on continuous) must be connected on a separate conductor. This would make the supply to that essentially a TN-S system. This makes a breakage of any wire or combination of wires in the external supply safe.

Exterior cabling in the UK is generally either T+E (which has an earth) in conduit, SWA armoured cable, where the armour is also the earth (and there may be an earth conductor as well), or (rarely) MICC, which carries the earth as the outer sheath

The only place in UK regs that allows combining earth and neutral in a single conductor is in the distribution network supply, which is beyond the scope of a DIY forum. As far as on-property wiring is concerned earth and neutral are completely separate concepts and have been for a very long time (I don't have access to old versions of the regs, but I guess 50s or 60s)

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