Earthing neutral

AJJewsbury:

Where is that current coming from though? Are circulating currents really that bad in magnitude? Keep in mind they are not regulated nor restricted in TN-C-S which in theory would produce parellel current in orders of magnitude more.

I don't have the experience to know what sort of magnitude circulating currents might be considered normal in a parallel source situation - maybe some of the 'big boys' here might be able help?


I might hazard a very wild guess that they're likely to be small percentage of the normal load - so maybe 1-10A for a 1000A system? Certainly something inconsequential thermally, but even a few amps can be significant if a break in a c.p.c. means you're passing it through a couple of separate electrodes to complete the circuit. Say both electrodes were 20 Ohms (the traditional BS 7430 requirement for LV electrodes) and 2A flowing between them would raise each of them by 40V and so could have 80V across the break between the two parts of the system - which probably isn't comfortable from a shock protection point of view.


With an uninsulated conductor you've also got the issue of the N current flowing through exposed- and extraneous-conductive-parts in parallel to the intended conductor - which again even if its only a few amps can be unwelcome from an EMI point of view - and just the sort of situation that specifying a TN-S system over a TN-C-S one is meant to avoid.


  - Andy.

 

AJJewsbury:

Where is that current coming from though? Are circulating currents really that bad in magnitude? Keep in mind they are not regulated nor restricted in TN-C-S which in theory would produce parellel current in orders of magnitude more.

I don't have the experience to know what sort of magnitude circulating currents might be considered normal in a parallel source situation - maybe some of the 'big boys' here might be able help?


I might hazard a very wild guess that they're likely to be small percentage of the normal load - so maybe 1-10A for a 1000A system? Certainly something inconsequential thermally, but even a few amps can be significant if a break in a c.p.c. means you're passing it through a couple of separate electrodes to complete the circuit. Say both electrodes were 20 Ohms (the traditional BS 7430 requirement for LV electrodes) and 2A flowing between them would raise each of them by 40V and so could have 80V across the break between the two parts of the system - which probably isn't comfortable from a shock protection point of view.


With an uninsulated conductor you've also got the issue of the N current flowing through exposed- and extraneous-conductive-parts in parallel to the intended conductor - which again even if its only a few amps can be unwelcome from an EMI point of view - and just the sort of situation that specifying a TN-S system over a TN-C-S one is meant to avoid.


  - Andy.