Three phase earth

Hi all,
What is the reason for the yellow phase being earthed or used as the star point in a three phase system? I have seen this several times in the North
of the uk. I am told that it is something to do with impedances?
Many thanks.....Barry
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I'm not aware of any arrangements in the UK; which have this configuration from a public utility. and most certainly not in Scotland which is "north of the UK". It's not a standard colour code either for the UK.The earth cable here, is coded Green/Yellow, (was Green self colour to BS 7671). Is this perhaps a single phase, construction transformer output, you are referring to; which is rated for 110/55/110 Volts, by means of a centre tapped transformer?
Jaymack
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hi John,
Thanks for your reply. I have seen this a couple of times when I've been out troubleshooting and told that its something to do with earth fault impedances? L2 (yellow as it was) is the star point. By the way the colours I am reffering to are not the wire colours but the "old" phase rotation colours ie (R) L1 (Y) L2 and (B) L3.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Barry wrote:

This doesn't seem to make sense. If there is a star point then obviously we're talking about windings connected in star. By definition, the star point is the point where the three windings are connected together; how can L2 (or L1 or L2 dor that matter) be 'the star point'? This s the point where one end of each winding are joined together. If the star point is not earthed, then any other point could be, but I've never seen any normal supply in the UK where one of the phases is earthed. Some countries, Italy for example, had three phase railways, wher one phase was connected to the earthed running rails, and the other two to twin overhead wires, with current collection by means of a split pantograph, and cheap single to three phase converters often produce output in this form, but I can't think of anywhere else that I've seen it.
In what situation did you see it? Was it part of the public supply, or something coming out of a private generator or isolating transformer? If the latter, what was it driving?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Barry wrote:

This doesn't seem to make sense. If there is a star point then obviously we're talking about windings connected in star. By definition, the star point is the point where the three windings are connected together; how can L2 (or L1 or L2 dor that matter) be 'the star point'? This s the point where one end of each winding are joined together. If the star point is not earthed, then any other point could be, but I've never seen any normal supply in the UK where one of the phases is earthed. Some countries, Italy for example, had three phase railways, wher one phase was connected to the earthed running rails, and the other two to twin overhead wires, with current collection by means of a split pantograph, and cheap single to three phase converters often produce output in this form, but I can't think of anywhere else that I've seen it.
In what situation did you see it? Was it part of the public supply, or something coming out of a private generator or isolating transformer? If the latter, what was it driving?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Barry wrote:

I've been thinking more about this on the bus going to work. There seem to be three possibilities:
1) You are mistaken, and you weren't seeing what you think you were seeing.
2) You were seeing some strange long obsolete system.
3) You were seing something unusual, which was done for some specific reason.
Taking these in order, I don't know how experienced you are, you could be an electrical engineer with many years experience, or a total novice - or anywhere in between. This would of course greatly affect the possibility that you were mistaken. You say that you've seen this when you've been 'out troubleshooting'. You also say that you were told that 'its something to do with earth fault impedances' The fact that you were somebody who would be doing troubleshooting, and that you asked the reason, and were given one, albeit not a very clear one, rather than being told that you were mistaken, and that it was not connected that way does seem to make this possibility less likely.
Until after the War there were still various odd systems in use in the UK; there were a few places on odd frequencies like 40Hz., I think a few were even still on d.c. Odd Voltages lasted even longer, I just remember being converted from 200 to 240V, in about 1961. Things have however been standardised for a long time. When did you see this, if it was less than about 50 years ago then I think we can rule out the second possibility.
The third possibility would be the most interesting, and if this is the case, then I would like to know the reason too. I doubt that the fact that you saw it in the North of the UK is technically significant. There aren't really many technical differences from the rest of the UK; there are more hydro-electric stations in Scotland than there in the South, but I doubt that has anything to do with it. As for 'political' differences, things really have been standardised for a long time. Do you simply spend more time looking at installations in the North than elsewhere, and therefore if odd systems exist then you are more likely to see them there.
Where did you see this; was it a low Voltage supply system, or a medium Voltage distribution one? Was it in an industrial situation; if so, what sort of industry? Was there some sort of specialised equipment installed nearby?
I think it's very unlikely that any normal customer would be supplied with this system; in this country people expect neutral to be close to earth potential, which it obviously wouldn't be with this system. This suggests that either the costomer produces this themselves via their own isolation transformer, or that it is a distribution system, with the final low Voltage supply having conventional earthing arrangements, and the customer knowing nothing about the odd distribution system.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hi Guys,
Thanx for your responses. A little about me then!! I am 55 years old OUCH , and have always been an electronics engineer. I have been in the "electrical" industry for a little over fourteen years now. I am Technical Manager for my company and part of my job is sorting out customers problems when everything else fails, the buck stops here.
This goes back a few years now and I have seen the yellow phase earthed a couple of times. I remember that on one occasion it was down in a valley at a sub station in Scotland serving a distillery (no samples were given unfortunately). The problem was with a power factor transducer which was connected between the red and yellow phases for reference, it didn't work !! It transpired that the yellow phase was pulled to ground. Connecting between the blue and red phases restored normal operation.
It was then I was told of the earth fault impedence but I'm still not sure of the logic of this? Its not really important I suppose but its just one of those things that come to mind from time to time as unsolved.
Cheers,
Barry

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Are you sure it wasn't a delta winding, where there is no star point to earth?
--
Andrew Gabriel

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Are you talking about the secondary winding of a VT? It is usual to earth the yellow phase in the UK. although I don't know why. I have copied this to sci.engr.electrical.sys-protection, maybe somebody there will be able to answer your question.
Barry wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hi,
I don't think that there was a VT involved although it was a long time ago and a lot has happened since then. I would be interested to know if you get an answer.
Cheers,
Barry

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

novice
when
the
UK;
'political'
medium
This
arrangements,
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

star
North
of
The other possibility; is that the transformer is an earthing transformer to another; and for a bulk electrical supply. Without seeing the circuitry, I'm at a loss.
Jaymack
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

All the coils are either connected end to end as in delta
OR
one end of each coil are connected together as in star
If the star point is subsequently earthed then the system Neutral and Earth are at the same potential
For a three phase supply that hasnt got an equal impedance on each phase there will be a neutral current and a resulting shock risk as the neutral voltage is different than the assumed zero at the star point
Is this what you are talkin about
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.