CPC always required for circuits ?

No - may circuit's don't require c.p.c.s - SELV for example, or separated circuits feeding a single item.


Circuits (and indeed entire installations) that are entirely protected by double or reinforced insulation may have their c.p.c.s omitted  - but only where the installation is under adequate supervision such that replacement of items with Class I equivalents cannot happen (so not domestics for instance) - in other situations a c.p.c. must be run to and terminated at each 'point' and 'accessory' (412.2.3.2).


A TT DB is an interesting one - I guess it could be argued that the TT earth is run to and terminated at the DB, so the requirement is satisfied - so as long as the supply cable itself need no Earthing (e.g. insulated & sheathed or singled in insulating conduit). I bet it'll confuse someone doing an EICR looking for a Zs value for the submain though....


   - Andy.

To the second point first, from Andy, Anyone confused by that shouldn't be doing the EICR Andy, but there we go.


All circuits in a TN mains installation would normally have a CPC run with them, but "with them" is not a requirement, just that one is present at every accessory and appliance etc. This could be structural steel connections, or SWA armour, or containment as long as the Zs is low enough at the point of load. This is of course what everyone measures during an EICR (or should...).


ELV cables don't necessarily have to have an Earth, in many cases it wouldn't do much good as any kind of Zs might well not be achievable, but it doesn't matter as they are not considered dangerous. There would be a value Andy, it might just be rather higher at the destination board than expected, and an R1+R2 would need a N/A and a note on the form.

The great majority of mains voltage circuits do need a CPC, but yes there are exceptions.

And of course the great majority of installation cables include a CPC.


I have previously installed two very short circuits without a CPC. The application was a transportable toilet and shower unit. Three phase 32 amp supply via a ceeform inlet. 30ma RCD.

I removed the mains voltage lights as these often tripped the RCD. Instead I fitted two all insulated fused connection units, each of these supplied a 12 volt DC 60 watt all insulated SMPSU. Two circuits each supplying all insulated bulkhead lights with 12 volt CFLs.

It "felt wrong" not to run a CPC to the fused connection units, but was in my view acceptable in the circumstances.

just a quick note to say thank you for the comments.