Earthing neutral

AJJewsbury:

Again, I see no harm in multiple earthing of a PE.

Because it's not just a PE in that diagram - it's also the system N - it ties the neutral points of both sources togther - and multiple earthing of a N can be a problem.


   - Andy.

 

mapj1:

It might - consider this rather simplified illustration - you will have to imagine the blackboard and the chalk...


Imagine you are holding the bare PE, but your feet are in good contact with terra-firm earth, but outside the near field of the electrodes .

Consider what happens when the TX faults HV phase to frame. The HV phase voltage goes down, and at the same time the HV and LV earth voltages at the fault

both come up to meet it part way.

The area of a circle a rod length or two around the electrode goes up a bit of course, but only the earth really near the electrode(s) rises to full fault voltage, then the volts slope off to almost nothing by 2 rod lengths radius. It is better with a large area mesh electrode of course.

Let us assume the bit of ground you are standing on remains at or near 0V

Ouch.


Now repeat that thought experiment, but now with your hand on a similar PE with a mm or 2 of green and yellow stripy plastic between you and it. Not so much Ouch.

The surge is still bad for kit further along the LV network of course, but the PE = CPC on the LV side is usually multiply earthed on the load side of things as it is bonded to water pipes and so on, so while the end near the transformer may rise to a few hundred V, the bit near the kitchen taps might only rise by a fraction of that.


There is also a cultural/ historical aspect, as around here bare earth wires more generally are pre1966 , and is seen by many rather like cotton or rubber insulation, asbestos wadding, leadwork and various other things as indicating a "state of the ark"  installation, folk just do not expect to see bare wires for any function.

Not a PE in the diagram, but a PE in real life. I can not find or see any problems if earthing it multiple times.

AJJewsbury:

Not a PE in the diagram, but a PE in real life. I can not find or see any problems if earthing it multiple times.

But it not just a PE, even in real life it's also a N - it's liable to be carrying current in normal service - even if in this case of not distributing N to the loads it'll still be carrying circulating currents. Remember that comment that it should be thought of as 'similar to a PEN'?


   - Andy.

 

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.

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.

 

 

From my view point I don't see multiple parelled sets of 4/0 PE breaking at once. Considering all the PME supplies...