As a matter of interest, what's the typical voltage of the three wires connected to porcelain/glass insulators, mounted on wooden poles, that feed the transformers (often pole-mounted) that step down to 240V for overhead wiring to houses? I can't find anything on Google, using phrases such as "overhead power line 240V" or "low voltage overhead power line".
I presume the 240V overhead wiring is 4-wire (ie a star with a wire at each tip and one at the centre) to allow one of the wires to be designated as neutral and thus able to be earthed at each house just before the "electricity board fuse". Does this neutral wire have to be thicker than the other three because it is carrying the return current for all three phases, or do the three return currents usually cancel each other out (at least for the ideal case where all three phase currents are the same magnitude and exactly 120 degrees apart)?
11 kV is the normal distribution voltage to end users from a Primary substation (pole transformers are classed as "sub-stations").
The Primary substation will probably be fed at 33 kV or higher depending on how big it is. It will almost certainly have at least two feeds, main and reserve. The reserve to the Primary substation here is 11 kV, the main 33 kV.
What haapens on the 240 V side varies greatly, it can be 4 wire (3 phases, Neutral and Earth) any single phase supplied property taking a phase N and E. Down to two wire, phase and combined Neutral/Earth, this is what our supply is. The Neutral is bonded to physical ground at the pole.
Given that supply voltages are multiples of 11 (11 kV, 33 kV, 132 kV), I wonder why OHLE railway electrification standardised on 25 kV rather than 22 kV?
Diverging off the topic slightly I know of a couple who having decided on an "Escape to the Country " inspired move avoided looking at some d properties due to the descriptions mentioning a "Sub-Station" nearby. It was only later in the search process that they discovered not all things described as a sub station looked like a large brick and concrete box with a numerous conductors and insulators like they often saw from the train to work and the "Sub -Stations" concerned were the pole transformers for the property and sometimes some others nearby.
Good grief what do you two do to have 11 kV in the house? The supply here is 1 phase 240 V to the house from the 11 kV sub-station up a pole 10 yards from the house...
It's most likely to be 11kv. It could be 6.6kv 11kv or even very rarely 33v in the UK. It will be delta connected on the primary side. Some rural prop erties only get two phases on the primary side. Not in the UK, but in some countries abroad the primary side can be a single phase and the return is v ia the earth.
On the secondary there are multiple configurations. It will be star connect ed. Earth may or may not be combined with neutral on a single conductor or not supplied at all. There may be a single phase, two phases or three phase s.
Some remote locations have a split phase arrangement on the secondary where the supply is a single phase centre grounded. This allows a supply a littl e bit more remote from a single phase supplied transformer.
The neutral will be earthed at the substation. A combined earth and neutral conductor should be earthed at multiple points along the route.
If the load were balanced completely there will be no current on the neutra l conductor. In practice there will be current on the neutral conductor but it should not exceed the current on a single phase - it would take a very odd inductive load to do that.
Don't use this info for anything other than entertainment. I may be talking out of my arse.
If the load were balanced completely there will be no current on the neutral conductor. In practice there will be current on the neutral conductor but it should not exceed the current on a single phase - it would take a very odd inductive load to do that.
Like pikeys pinching a bit of copper that they think is an earth when it is a neutral on a three phase supply.
Without bothering to look at all the other replies, I can tell you that a 3 phase with neutral LV distribution cabling system between the 240v substation and domestic properties only subjects the neutral to a maximum current equal to that of any of the phases in a worst case scenario (either only one or two of the phases supplying their maximum rated currents with either the other two or one disconnected). When all three phases are supplying a balanced 3 phase loading, no current flows in the common neutral back to the substation transformer.
However, for each connected property, the neutral will carry the return current to the street joint onto the 3 phase supply distribution cabling where it will be cancelled out to a greater or lesser degree by the neutral return currents of adjacent properties fed from the other two phases. There will be some current flowing between the tapping off points along the street distribution 3 phase plus neutral cable but it will never exceed, even in the worst case, the currents carried by any one phase.
Compared to a simple single phase distribution using only two wires in the street distribution cabling where each conductor has to be able to safely carry 3 times the current of each phase wire in the 3 phase system, there is a significant saving in copper investment (a 33% reduction in copper costs) plus, as a bonus, elimination of volt drop in the common neutral return under maximum balanced loading conditions.
As far as the rural distribution system with pole transformers that you've described, I believe the voltage level on these lines is typically
6 or 11 KV (I'm not an expert on such rustic arrangements - 11KV is a common voltage level and I have a vague recollection of 6KV being mentioned).
The common neutral on an 11 or 6KV line cannot be used directly by the connected properties. Each property's neutral simply terminates on the neutral connection of the pole transformer's LV secondary (where it may meet neutral connections from two other adjacent properties if within reaching distance of the LV overhead cabling).
It is my understanding that the neutral connection on the LV secondary will be independently connected to a local earthing spike or spikes provided by the utility company responsible for the supply. It may not be as good a low earth resistance connection as that provided in urban substations but it'll be a damn sight safer than relying on an 11 or 6 KV distribution neutral connection (assuming such a connection is provided or even needed in the higher voltage levels of the distribution network).
I'm now going to hit send and take a look at what the 'experts' have said and see how big a fool I may have made of myself. :-)
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