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The US electrical system

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GlennSprigg:

--- Quote from: bdunham7 on June 27, 2020, 04:47:27 am ---
--- Quote from: cliffyk on June 27, 2020, 03:14:22 am --- If you draw 200 A from 120 V phase "A", and 100 A from phase "B", the "neutral" wire will be carrying 100 A--enough to be a BIG surprise for anyone believing it to be "neutral", and mucking about with it while the full phase load is unbalanced.

--- End quote ---

That is exactly what a neutral is supposed to do.  What big surprise would the neutral have?  3 volts?

--- End quote ---

Nope!!  "Funny" things used to happen, (here in AussieLand) with say 3-Phase 'Star' (neutral) connected
supply authority transformers, (like the/our  11-kv ones). The Public never saw this, but 'Un-Loaded', you
would see very strange voltages, and often MANY hundreds of volts on the Neutral Star-Point!!   :phew:
Was lethal, until loaded/grounded.  Thinking back, it was never explained to us WHY, though...   :-//

cliffyk:

--- Quote from: AlbertL on June 29, 2020, 01:34:06 am ---
--- Quote from: bdunham7 on June 27, 2020, 04:47:27 am ---
--- Quote from: cliffyk on June 27, 2020, 03:14:22 am --- If you draw 200 A from 120 V phase "A", and 100 A from phase "B", the "neutral" wire will be carrying 100 A--enough to be a BIG surprise for anyone believing it to be "neutral", and mucking about with it while the full phase load is unbalanced.

--- End quote ---

That is exactly what a neutral is supposed to do.  What big surprise would the neutral have?  3 volts?

--- End quote ---

It depends on the nature of the mucking.  In a properly installed, intact system, touching the neutral and ground (earth) shouldn't yield any great surprise.  But getting in series with a neutral, or between an open neutral and ground, could expose one to essentially the full line voltage.  And opening a current-carrying neutral could produce a significant arc.

This leads into another interesting aspect of US distribution: the multi-grounded neutral.  The utility substation typically supplies three-phase distribution feeders from the wye-connected secondary of a step-down transformer.  The neutral of the wye is grounded at the substation, travels along with phase conductors, and is grounded at intervals along the line.  Transformers along the feeder make the final step down to customer delivery voltage, which is typically 120/240V single-phase (i.e. a center-tapped 240 volt secondary) or three-phase 120/208V or 277/480V wye.  The secondary neutrals of these transformers are connected to the feeder neutral, and grounded at the transformer location.  Finally, the neutral is carried to the customer's main service equipment, where it is again grounded, often at more than one location on the premises.

The two key points are (1) the neutral is common to both sides of the distribution transformers, and (2) the neutral is grounded (earthed) at multiple locations, some of them widely separated.

--- End quote ---

Yes, that was my point--disconnecting it, thinking it's harmless would cause a surprise...

tom66:
A very dangerous condition can occur on TN-C-S electrical systems, which are very common in the UK.  A PEN connection, combined earth/neutral, is passed along with live in the supply cable.  PEN is split into neutral and earth at the main entry fuse.  Most UK homes do not have additional earthing (PME)

The risk is that if PEN fails due to cable damage, corrosion, poor maintenance etc then, although power to the building will be lost, the PEN can effectively float towards Live as any appliance that conducts current in the "off state" (think fridge compressors, heating elements, even some SMPSes) will pull that PEN node up via a low impedance node.  This will then mean if someone touches one of their now-floating appliances and an earthed device (such as water piping, or walks on wet ground outside while touching their PEN-earthed electric car) then they could receive a fatal electric shock.

This is a particular pain for EV charging installations, as far as I am aware there are only two ways to solve the problem:
- Earth rods until the impedance is below some nominal figure, which can often require 5+ earth rods to be drilled in to the driveway and fitted (expensive, time consuming)
- An EV charging station that isolates PE as well as L/N when power fails and only connects PE when the vehicle has been detected

It's for this reason PME is now standard at newer builds ... but again, one of those cost saving decisions made 50 years ago that bites us in the ass now.

Monkeh:

--- Quote from: tom66 on June 29, 2020, 12:50:52 pm ---A very dangerous condition can occur on TN-C-S electrical systems, which are very common in the UK.  A PEN connection, combined earth/neutral, is passed along with live in the supply cable.  PEN is split into neutral and earth at the main entry fuse.  Most UK homes do not have additional earthing (PME)
--- End quote ---

The majority of TN-C-S systems are PME. PME is distribution side, not customer.


--- Quote ---This will then mean if someone touches one of their now-floating appliances and an earthed device (such as water piping
--- End quote ---

This is what bonding is for, see: equipotential zone.


--- Quote ---or walks on wet ground outside while touching their PEN-earthed electric car
--- End quote ---

And this is a rather overblown concern IMO, but it is in fact proving to be a pain.

richard.cs:

--- Quote from: Monkeh on June 29, 2020, 01:37:25 pm ---And this is a rather overblown concern IMO, but it is in fact proving to be a pain.

--- End quote ---

Well yes, it's not introducing a genuinely new risk, the general public was already hopelessly unaware of the risk of outdoor class I appliances with TNC-S earthing and happily bought all manner of earthed metalwork outdoors (outside of the equipotential zone), in addition to the various light fittings, outdoor taps and gas pipes (which now seem to default to running along outside walls) that form part of the installation more generally. The only difference with electric cars is that it's a new kind of class I appliance intended for use outdoors (rather than incidentally used as such), and lots of people have got in a bit of a panic about it. Class II cars would be possible, but it's now rather too late for that.

Generally the safety case for TNC-S in the UK has been looking progressively poorer for decades as the metallic water and gas pipes that used to provide free sub-Ohm backup earthing have disappeared, and the promised reliability of double-crimped neutral connections has not really been achieved.

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