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which addresses a technical error in
Draft 3.1. I've reviewed the comment with Hank Hinrichs
and have attached his
comments below. I'm expediting this comment due to the work-in-
progress at
TIA. --------------------------------------------------------------------------------------------------------------------------------------
Comment: subclause 33.4.4
Resistance Balance, page 79
The method of measurement for
resistance balance specified in Subclause 33.4.4
does not accurately characterize the
resistance unbalance between the pair(s) and
therefore the requirement will not
suffice to control the transformer current unbalance.
Summary of technical
Issue:
The intent of Draft 3.1 subclause
33.4.4 is to provide a requirement for the resistance
unbalance between two conductors.
The requirement is specified to limit the transformer
current unbalance. Although the
intent is reasonably stated in the draft, the method of measurement
provided does not accurately
characterize the resistance unbalance between the pair(s)
and therefore the requirement will
not suffice to control the transformer current unbalance.
Proposal:
The ASTM resistance unbalance
calculation is proportional to the calculation
used to derive the transformer current unbalance. When
the ASTM method is used the
percentage of resistance unbalance
required to control the transformer current unbalance
is approximately
5%.
ASTM D 4566 states that, “the
absolute difference in resistance unbalance is calculated
by subtracting the lesser resistance
from the greater resistance”, and that, “a more useful
and generally used expression for
resistance unbalance is percent resistance unbalance, where
%Resistance Unbalance = ((max
resistance-min resistance)/(min resistance)) x 100”.
For international reference
purposes, The ASTM resistance
unbalance % can be translated into an
ISO equivalent as specified in IEC
61156-1 1994* where resistance unbalance is specified as:
The resistance unbalance between
conductors of a pair or in the same side of a quad is defined as: deltaR(%) =
(Rmax-Rmin)/(Rmax+Rmin)x100(%)
where
deltaR is the resistance unbalance;
Rmax is the resistance, in ohms, for
the conductor with the higher resistance value;
Rmin is the resistance, in ohms, for
the conductor with the lower resistance value.
The percentage unbalance of 5% based
on the ASTM method translates into ~2.5% based
on the IEC 61156-1
method.
----- Original Message -----
From: "Hinrichs, Hank" <HANKHINRICHS@xxxxxxxxxxxx>
To: "'Chris DiMinico'" <cd@xxxxxxxxxxxxxx>
Sent: Friday, August 16, 2002 12:10 PM
Subject: RE: Technical error in 802.3af Draft 3.1. > > I agree with your analysis and proposed solution of using ASTM D 4566 as the > formula for calculating resistance imbalance. The complete relationship is: > > (Rmax-Rmin)/Rmin * 179 <= 8 > > Where: Rmax = Conductor having the higher resistance > Rmin = Conductor having the lower resistance > 8 = Maximum current tolerated without degradation in > performance > 179 = Maximum current permitted in either of the two > conductors. > > This value comes from the summation of 175 mA (the maximum average current > in any one conductor) + 4 mA (1/2 the current the transformer can tolerate > without degradation in its performance. > > The ISO definition can be used as an approximation, however it will always > yield a value slightly smaller. > > As I said in our telephone discussion, it's possible we can adjust the > current imbalance value to where the formula equates to 5%. However I want > to discuss this with Steve Ellsworth first. > > The 3.5 figure was derived from an anticipated 6.125 mA of current > imbalance. This yields (6.125mA/175mA) * 100 = 3.5%. I can't remember where > 6.125 mA came from. > > Regards, > > Hank Hinrichs, Principal engineer > 12220 World Trade Dr. > San Diego, CA 92128-3765 > Tel: (858) 674-8208 > Fax: (858) 385-8000 > e-mail: henryhinrichs@xxxxxxxxxxxx <mailto:henryhinrichs@xxxxxxxxxxxx> > |