linda1'ppt

1
Impacts on Clearances Resulting from Proposed
Changes to Strength Loading in 2002 NESC
IEEE
O.C. Amrhyn, CSP Outside Plant Engineering
Consultants (OPEC) February 6, 2000
2
Revised Ice Map

• How (tentatively) clearances will be evaluated
• Has ice load increased in the area
• Is plant MCOT or ice-load limited
• Calculate vertical clearances without wind
• Ignore tension

3
Revised Ice Map (contd.)

• First, vertical sag calculated for clearance
• Related tension not used for strength calc
• Second, actual line tension calculated
• Account for wind speed changes
• Does it meet NESC conductor requirements

4
How (tentatively) clearances are evaluated
(contd.)

• Calculate tension at appropriate wind velocity
• From existing loading districts (current Code)
  use most appropriate safety factors and
  temperatures
• Or later information
• Evaluation of vertical sag and total
  cond./messenger load will determine suitability
  of proposed plant, i.e, span and conductor size

5

• If Revised Ice Map Increases Sag
• Most power lines probably remain MCOT limited
• Communication line clearance to conflict below
  line will decrease
• New lines will have to be raised
• Clearance between lines may become a problem
• Lower conductor measured without ice

6
Possible Effects on Clearances by Changes to
Strength and Loading

• Major Impact
• Changes are only proposals
• Would be significant on SC4lesser on others
• Only ice enters into calcs for clearances
• Wind no direct effect
• Importantconductors must meet strength
  requirements as well as clearances (includes wind)

7
Review of Fundamentals Affecting Sag

• Definitions
• Young Modulus
• Elastic and non-elastic components
• Creep
• Initial, final, and final unloaded sag
• Basic Span, Ruling Span

8
Review of Fundamentals Affecting Sag (contd.)

• Basic Principles
• Stress strain relationship
• Effect on S/S by non-elastic component
• Stress/Strain values
• Relationship applies only to a given curve
• Values may be different (though linear among
  themselves) for different curves of same material
• Relationship proves need to resag lines major
  storm

9
Typical Stress/Strain DiagramCable
10
Possible Effects on Clearances by Changes to
Strength and Loading

• Apparent Changes
• Some areas will change loading districts (as
  presently defined)
• Some conductors and messengers will increase in
  size
• Use of loading districts will have to be changed
• Present form would tend to complicate strength
  calcs

11
Why Should Clearances be Impacted by Changes in
S/L?

• Basis for changes to S/L requirements are
  improved data concerning ice and wind
• Sag changes must inevitably follow changes in ice
  thickness on conductor
• Totally unacceptable to accommodate in S/L, but
  ignore increases in sag
• Compromises good engineering principles
• Opens the Code to severe criticism in litigation

12
Why Should Clearances be Impacted by Changes in
S/L? (contd.)

• Ignore in S/L let overgenerous O/L factors
  continue to provide necessary strength
• Not good engineeringwasteful
• Obscures true nature of the situation
• True clearances should be known, not assumed

13
How the Code May Cope with Excessive Sag
Clearances?

• Sags must be based upon best information
  available
• Not acceptable to work with different values in
  different parts of the Code
• Physical laws and recorded data cannot be ignored

14
How the Code may cope with Excessive Sag
Clearances? (contd.)

• Engineering not a pure science
• Relies heavily on the pure sciences, but also
  upon time proven concepts and measurements
• Fluid mechanics and heat transfer rich in derived
  constants
• Both the effects of wind and force (ice) can be
  ameliorated by application of published sound
  engineering principles

15
How the Code may cope with Excessive Sag
Clearances? (contd.)

• An eyeball comparison of light and medium size
  cables
• Supply cables
• Tern and swan
• Both probably MCOT limited
• Communications cables
• Medium to heavy cables either temperature or ice
  limited
• Light cable probably weight limited
• Various combinations must be checked out

16
Cable-TernNA - Not ApplicableIntl. Tens - 5700
LB.Brkng. Str. - 23 KIP
17
Cable-SwanNA -Not ApplicableIntl. Tens. - 450
LB.Brkng. Str. - 1830 LB.
18
Cable - Telephone Alpeth 1.6 LB/FTNA - Not
ApplicableMessenger - 10MInitial Tens - 2100
LB.Brkng. Str. - 11 KIP
19
Cable - Coaxial (CATV)1/2 OD 0.098 LB/FTNA -
Not ApplicableMessenger - 6.6 MInitial Tens -
600 LB.Brkng. Str. - 6600 LB.