You have a situation where two circuits go to the same fitting. Like say a two gang switch in the hall for upstairs and downstairs lights, where each is on a different circuit. Do you simply link all the earths together?
Basically curious if this might increase RFI radiation - with so many electronics having a controlled 'bleed' to earth.
IANA electrician but I don't see why not. I can't think of a fault condition where having an extra earth path would be a problem.
Different 'earths'. In electronics you are careful about your ground return path, because that carries your return current. There's no such thing as 'ground', there's simple the positive current path from your power supply and the negative current path, forming a loop.
Current flows around that loop, causing a magnetic field (varying if the current is not DC). That varying magnetic field can induce currents in other metal things nearby, which you might not want (for example audio signal paths). (This is how hearing aid loops work)
That loop is also susceptible to induction from outside magnetic fields. That induction causes additional current to flow in the loop, which can generate voltages in your circuit (this is how AM radio antennas work). If those voltages come in places you didn't expect them, that's EMI.
So that's the reason for careful layout of your return current path, to prevent these effects.
In electrics, however, the earth wire is to carry your *fault current*. And so you need a fairly beefy path capable of taking hundreds of amps for a short period should you get a line-earth connection (before any MCB or RCD is tripped). Lower impedance is better.
A low impedance path for 50Hz doesn't necessarily mean a low impedance path for RF (due to inductance), but I would have expected the inductance of a house earthing system to be relatively low.
So, if your equipment ties its signal ground to earth, then that would null out any noise on the earthing system. I suspect the reason for the 'bleed' is that, in the event of a line->earth fault, they don't want the equipment to suddenly receive line voltage on its casing/etc - it's just an RF current path, not a 50Hz path.
If you're running Homeplugs, that's using your mains wiring system as an antenna and then the RF response and capacitance between line and earth matters rather more. But generally a bigger problem for those systems is all your switchgear - MCBs, RCDs, SPDs, etc that affect the frequency response of your wiring system. But Homeplugs are all about trying to make something out of the worst antenna system you could possibly think of, so it's hard to predict how things will turn out.
Theo
Suppose at your accessory (A) you connect the CPC for circuit 1 but not for circuit 2.
Then someone comes along and disconnects circuit 1 - e.g. to connect a new fitting before A.
Circuit 2 is still live - with no CPC connected at A.
I think 411.3.1.1 applies:
"A circuit protective conductor shall be run to and terminated at each point in wiring and at each accessory except a lampholder having no exposed-conductive-parts and suspended from such a point."
You might argue it only requires "a CPC" but I think that would be more than Jesuitical given "circuit" is defined as "[stuff] supplied from the same origin and protected against overcurrent by the same protective device(s)."
Pass.
Yes.
The situation is no different than if you were running singles in plastic conduit - you wouldn't run a separate cpc for every circuit.
It does help with some tests if earths aren't joined together, but it's unavoidable in practice.
I intentionally linked my cooker and socket cpcs together in the kitchen because I know I have undersized cpc on the old imperial socket wiring. Brings the Zs down nicely.
Owain
That might affect testing of Ze on each circuit. Though if a note is left at the meter I don't see the issue.
Given the area of a loop infers inductance, where suppression is reliant on the CPC then I would say yes.
There is no issue. It will not affect the Zs to any appreciable level, and any effect is beneficial. Parallel earth paths exist in most houses, anything connected to the copper water pipes give a (better usually) parallel path to earth. It is not a problem.
You could use an RCD/RCBO for additional protection or do an adiabatic equation calculation to determine the minimum CSA of the cpc and use an MCB.
MCB and RCD fitted :)
Can't be bothered doing anything adiabatic. Electrician said whatever it tested as was pretty good.
Owain
Indeed, it's called fortuitous earthing for a reason :-)
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