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hivoltage (Electrical) (OP)
12 Nov 03 7:07
Good Morning to everybody,

I noticed that in the US the only Faulted Circuit indicators available on the market are Single-Phase type that are installed on each of the Medium Voltage overhead cables using a hot stick.
On the other hand common practice in Europe is to use three-phase pole-mounted fault indicators, which are sensitive to the magnetic field created by the residual current and are safely installed some metres below conductors.

Can anybody explain to me the reason for such a different practice?

Does it have to do with Standards and Regulations or just with different grounding systems?

Are there specific reasons why it would not be possible to use  three-phase residual current detectors in the US?

Thank you for your valuable help!
Helpful Member!  Bung (Electrical)
13 Nov 03 21:17
The pole mounted types don't work too well when there are multiple circuits on the pole.  They also seem to be more prone to false operation - they are trickier to set up properly, and have to be much more sensitive (being so far from the line).  We have used both , and have recently changed from pole mounted to conductor mounted devices (as much for cost and other reasons as anything else).

Bung
Life is non-linear...

hivoltage (Electrical) (OP)
17 Nov 03 4:51
Thank you Bung,
it seems that reasons are based more on customers experience, rather than on regulations or standards.
But now I wonder why in Europe you can find a totally different situation:
Conductor mounted devices are very seldom used, while pole-mounted devices are much more common.
I can hardly believe that a continent-wide preferred practice is for an overall worse solution.
There must be something more:
Is it possible that in Europe, maybe, there are vendors of better performing pole-mounted equipments?
Is it possible that differences in network arrangement between US and EU make pole-mounted devices more suitable or effective?

Thank you again

Hivoltage
jbartos (Electrical)
23 Nov 03 14:14
Suggestion: There appears to be a wider variety of transmission poles or towers in USA. Plus Bung mentioned inherent shortcoming of pole mounted fault indicators adds to the application problem.
stevenal (Electrical)
24 Nov 03 11:33
hivolt

The answer to your question is in it. "On the other hand common practice in Europe is to use three-phase pole-mounted fault indicators, which are sensitive to the magnetic field created by the residual current and are safely installed some metres below conductors."

US distribution systems are usually 4-wire multi-grounded neutral systems feeding unbalanced loads. Residual current is present during normal conditions.
polyphase (Electrical)
24 Nov 03 14:32
Here is another guess at a possible reason.  The phrase "safely installed some metres below conductors" implies a difference in the work practices necessary to install the two types.  One requiring hot line work and the other being less hazardeous.  Also there is the quanity issue of one device versus three.  I can not compare the purchase prices of the two types, but it could also be a factor.

hivoltage (Electrical) (OP)
1 Dec 03 5:04
Thank you Stevenal.
Just a remark: if distribution systems in the US are 4-wire multi-grounded, this means that residual current resulting from unbalanced loads is shared between neutral conductor and earth, isn' t it?
I suppose that the neutral conductor impedance is normally much smaller than earth impedance, so that residual current should be carried by the neutral conductor and not by earth.
This would make it possible to use three-phase fault indicators, with the additional advantage of detecting possible broken neutral wire.

What do you think about this?
Helpful Member!  jghrist (Electrical)
1 Dec 03 11:12
The current will split between the neutral and the earth.  More will flow in the neutral close to the fault, with some current flowing from the neutral into the earth at each pole ground.  After a few spans, the current in the neutral will become more or less constant.  Close to the source substation, some of the ground current will flow from the earth back into the neutral through the pole grounds.  

Using the SES FDIST program, I calculate the following for a 1000 A line-end Ø-grd fault on a 5-mile distribution line with 40-ohm pole grounds every 200 feet, a 5-ohm substation ground grid resistance, a 3/0 ACSR neutral, and 100 ohm-m soil resistivity:

Current in neutral in first span (at station) - 750 A
Current in 10th span - 500 A
Current in 30th thru 90th spans - 350 A
Current in 120th span - 600 A
Current in last span - 900 A
hivoltage (Electrical) (OP)
2 Dec 03 4:18
I see.
It seems that you are using the sky wire as a neutral conductor, isn't it?
And in addition, as Jghrist has shown (thank you for your contribution!), somewhere along the line, earth current may be even higher than neutral current.
This makes 3Ø fault indicators difficult to use because of the variable share between neutral current (balanced by the sensor) and earth current (not balanced by the sensor).
This seems to be a major drawback of this kind of indicators, at least for this application.
But ... is this medium voltage network configuration the only one used in the US?
Does anybody know where I could find on the web a survey of medium voltage network arrangements in North America, including grounding practice, tower configurations, etc.?
Thank you in advance!

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