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high leg

English translation: high-voltage phase of a three-phase delta configuration (NA electrical installations)

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GLOSSARY ENTRY (DERIVED FROM QUESTION BELOW)
English term or phrase:high leg
English translation:high-voltage phase of a three-phase delta configuration (NA electrical installations)
Entered by: Ken Cox
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12:50 Oct 12, 2005
English to English translations [PRO]
Tech/Engineering - Electronics / Elect Eng / printing
English term or phrase: high leg
about installation of a printing press

"All power supplies should be verified before powering up any equipment.
When using 3-phase power, the high leg, if it exists, should be in the center tap position on the main disconnect."

...what's the high leg in this case? a higher circuit? a phase with the higher voltage??

thanks!
Elena Ghetti
Italy
Local time: 18:57
for info
Explanation:
'High leg' refers to one phase of a three-phase delta configuration in which a center tap (and not the same kind of 'center tap' in the asker's text) is connected to ground. It's the phase 'opposite' the center tap, and thus has the highest voltage relative to the center tap.

In a wye configuration, there is no high leg (hence the 'if any' in the text).

As for the 'center tap' in the asker's text, IMO that is the terminal in the middle.

refs:

[PDF] Installation Instruction for Three Phase High-Leg Delta, 120/240 ...
Installation Instruction for Three Phase High-Leg Delta, 120/240 volt, ...
High Leg (Phase B). Low Legs (phases a & C). Nominal. system. Voltage ...
www.trapthezap.com/PDF/3DxInsta.pdf

Subject : High legs, three phase checking
However, when you use only two of the legs of a 3-phase system, it is single phase
... whether on the path through a, or through C ), therefor a 'high leg'. ...
www.elitesoft.com/sci.hvac/ithighlg.html
Selected response from:

Ken Cox
Local time: 18:57
Grading comment
Many thanks! thanks also to Cristina and Dusty
4 KudoZ points were awarded for this answer



Summary of answers provided
5 +1the phase in the middle - also known as phase B
Cristina Hritcu
4 +2for infoKen Cox
4 -1See explanation below...
Tony M


Discussion entries: 1





  

Answers


50 mins   confidence: Answerer confidence 5/5 peer agreement (net): +1
the phase in the middle - also known as phase B


Explanation:
This high leg is called as such to give one an idea about it's position by comparison with the other 2 legs from the 3 phase power systhem. It is also known as phase B
see diagram here: http://www.itvss.com/pdf/3D-Inst.pdf

Also you should know that the incoming power through the 3 legs varies:
see below: http://www.eng-tips.com/viewthread.cfm?qid=3423
"On a incoming service 3phase 240V phase to Phase. 2 legs
are 120 against neutral one leg is 208 V.
The incoming service is 600 A. The high leg has only a 15
HP compressor connected"


    Reference: http://www.eng-tips.com/viewthread.cfm?qid=3423
    Reference: http://www.itvss.com/pdf/3D-Inst.pdf
Cristina Hritcu
Local time: 17:57
Native speaker of: Native in RomanianRomanian
PRO pts in category: 4

Peer comments on this answer (and responses from the answerer)
agree  Ken Cox: probably correct (calling it phase B is probably conventional) -- see further explanation
5 hrs
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6 hrs   confidence: Answerer confidence 4/5Answerer confidence 4/5 peer agreement (net): +2
for info


Explanation:
'High leg' refers to one phase of a three-phase delta configuration in which a center tap (and not the same kind of 'center tap' in the asker's text) is connected to ground. It's the phase 'opposite' the center tap, and thus has the highest voltage relative to the center tap.

In a wye configuration, there is no high leg (hence the 'if any' in the text).

As for the 'center tap' in the asker's text, IMO that is the terminal in the middle.

refs:

[PDF] Installation Instruction for Three Phase High-Leg Delta, 120/240 ...
Installation Instruction for Three Phase High-Leg Delta, 120/240 volt, ...
High Leg (Phase B). Low Legs (phases a & C). Nominal. system. Voltage ...
www.trapthezap.com/PDF/3DxInsta.pdf

Subject : High legs, three phase checking
However, when you use only two of the legs of a 3-phase system, it is single phase
... whether on the path through a, or through C ), therefor a 'high leg'. ...
www.elitesoft.com/sci.hvac/ithighlg.html

Ken Cox
Local time: 18:57
Specializes in field
Native speaker of: Native in EnglishEnglish
PRO pts in category: 128
Grading comment
Many thanks! thanks also to Cristina and Dusty

Peer comments on this answer (and responses from the answerer)
agree  Tony M: Yes, upon reflection, and considering the differences between US and UK, I think you've hit the nail on the head there.
2 mins
  -> Thanks -- that's always better than hitting your thumb.

agree  Can Altinbay
1 hr
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53 mins   confidence: Answerer confidence 4/5Answerer confidence 4/5 peer agreement (net): -1
See explanation below...


Explanation:
First of all, Elena: no, 'center tap' is NOT some kind of intermediate socket; I'll come back to that later.

The English is not entirely explicit or clear to me, but I think it is safe to make certain assumptions based on likelihood.

I think you are right to interpret 'high leg' as one of the phases having a higher voltage than the rest; this is quite common, if the loads on the various phases are not perfectly balanced.

By 'center tap', I think they are referring to the voltage taps on a transformer of some kind. Basically, you will usually have at least 3 taps, one for 'High', one for 'Low' and a center one for 'medium' or 'Normal' etc. That way, for example, the voltage can be adjusted around its nominal value (say: 220 V) by boosting it via a tap that will give (say) +20V or -20V.

So what they're saying is:

set the phase with the highest voltage to the center 'medium' position, and then use the taps available on the other 2 phases to boost the 2 lower voltages to match.

This is clearly one way of ensuring maximum margin for adjustment.

Please contact me privately if you want further explanation...


--------------------------------------------------
Note added at 6 hrs 57 mins (2005-10-12 19:48:01 GMT)
--------------------------------------------------

In order to try and clarify some of the confusion, I'd like to just comment a bit on Kenneth's answer.

By definition, in a 'Y' (or star) connected 3-phase system, the centre neutral point MUST be equi-voltage from the others --- so no one phase OUGHT to be higher than the others (though in practice, due to load imbalance etc., it may become so) I don't see how it is possible to have a 'phase opposite the centre tap', since by nature they are all balanced about it (otherwise, neutral current would flow!)

However, I think the point that KC is making is probably valid for the US system (about which I know much less); as I understand it, the 'earthed' [grounded] neutral is in this case NOT connected to the 'true' centre tap, but to a mid-point between only TWO of the phases; in this case, I quite agree that the 'high leg' would be the one; this would explain how 2 of the phases would be at (say) 110V either side of earth, whilst the third phase would be at (I guess) around (cos 60° * 220)V = approx. 209V.

If this is a US context, then I think KC's answer certainly makes most sense.


--------------------------------------------------
Note added at 7 hrs 1 min (2005-10-12 19:51:44 GMT)
--------------------------------------------------

Sorry for ommission above:

"...I quite agree that the 'high leg' would be the one 'opposite' the neutral mid-point;..."

Tony M
France
Local time: 18:57
Specializes in field
Native speaker of: Native in EnglishEnglish
PRO pts in category: 284

Peer comments on this answer (and responses from the answerer)
disagree  Ken Cox: a good story, but I think you're off base this time (a rarity...) // center-tapped 120/240 is standard in the US, and if you have a delta config there is no 'true' neutral, so - bingo - you get a high-leg delta system.
5 hrs
  -> Thanks, Kenneth! Indeed, I think you're right on this one... // Thanks, KC, for the explanation; yes, silly me, I was forgetting, 'cos it didn't actually SAY this was US... I always forget the 'colonies'... ;-))
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