C-5 Insulation Resistance Calculations.pptx

8/10/2019 C-5 Insulation Resistance Calculations.pptx

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Insulation Resistance Calculations of

Airfield Lighting Circuits

35TH

ANNUAL HERSHEY CONFERENCE

Joseph Vigilante, PE


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Presentation Objective

To develop a formula to calculate insulation resistance

for an airfield lighting circuit and provide theoretical

and real-life examples.


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Presentation Agenda

Cable insulation resistance (IR) background

FAA IR guideline recommendations

Formula development

Airfield lighting circuit calculations Summary

Open Q&A


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Anatomy of insulation current flow

Capacitance charging currents, C

Absorption current, RA

Conduction current, RL

Circuit model

DC VOLTAGESOURCE

RA

C

RL

S


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CURRENT-MICROAM

PERES

100

90

80

70

60

50

40

30

25

20

15

10

9

8

7

6

5

4

3

2.5

2

1.5

1

0.1 0.15 0.2 0.25 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.5 2 2.5 3 4 5 6 7 8 9 10

SECONDS

CAPACITANCE CHARGING CURRENT


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CURRENT-MICROAM

PERES

100

90

80

70

60

50

40

30

25

20

15

10

9

8

7

6

5

4

3

2.5

2

1.5

1

0.1 0.15 0.2 0.25 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.5 2 2.5 3 4 5 6 7 8 9 10

SECONDS


ABSORPTION

CURRENT


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CURRENT-MICROAM

PERES

100

90

80

70

60

50

40

30

25

20

15

10

9

8

7

6

5

4

3

2.5

2

1.5

1

0.1 0.15 0.2 0.25 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.5 2 2.5 3 4 5 6 7 8 9 10

SECONDS


ABSORPTION

CURRENT

CONDUCTION

CURRENT


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CURRENT-MICROAM

PERES

100

90

80

70

60

50

40

30

25

20

15

10

9

8

7

6

5

4

3

2.5

2

1.5

1

0.1 0.15 0.2 0.25 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.5 2 2.5 3 4 5 6 7 8 9 10

SECONDS


TOTAL

CURRENT

ABSORPTION

CURRENT

CONDUCTION

CURRENT


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Types of insulation resistance testing

Short-time/spot reading

0 TIME 60 sec

MEGOHM

S

VALUE READ AND

RECORDED

TIME (MONTHS)

MEGOHMS

VALUES CHARTED


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Types of insulation resistance testing

Time-resistance method

0 TIME 10 Min

MEGOHM

S

INSULATION

SUSPECT

INSULATION

PROBABLY OK


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Testing airfield lighting circuits

AC 150/5340-30F, Design & Installation Details for

Airport Visual Aids - Chapter 12, Equipment &

Material, Section 13, Testing

50MNon-grounded series circuits

FAA-C-1391, Installation & Splicing of Underground

Cable


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Resistance values for maintenance

AC 150/5340-26B, Maintenance of Airport Visual Aid

Facilities

Suggested minimum values

10,000 ft. or less - 50M

10,000 ft. 20,000 ft. - 40M

20,000+ ft. - 30M


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FAA-C-1391 Installation & Splicing of Underground Cables

Spot Test

Take readings no less than 1 minute after readings

stabilized

Cable IR values = 50M, 40M& 30M

Loop IR reduced due to parallel summation

Cable IR never less than above values


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Insulation resistance formula

Constant current series lighting circuit Provides for parallel summation of circuit components

3 components

L-824 cable

L-823 cable connectors

L-830 isolation transformers

RT RN RC

I

V


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FAA minimum IR values

L-824 cable AC 150/5345-7E, Specification

for L-824 Underground Electrical

Cable for Airport Lighting Circuits

Table 1, Test #9 > ICEA S-96-659, Section 7.11.2

Corresponding to IR Constant

50,000 M - k-ft. at 15.6C


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L-823 cable connectors AC 150/5345-26D, FAA

Specification for L-823

Plug and Receptacle, Cable

Connectors Section 5.1Type I Connectors

75,000 M


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L-830 isolation transformers AC 150/5345-47C, Specification for

Series to Series Isolation Transformers

for Airport Lighting Systems

Table 3 Insulation Resistance 7,500 M


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Base formula

1/IR = 1/RC + 1/RN + 1/RT

RT RN RC

I

VIR


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Cable equation

The insulation resistance of cable for a certain length canbe calculated by the following formula:

Where: RC = Insulation resistance in Megohms of cable

K = Specific IR in Megohms - k ft at 60 F of insulation

D = Outer diameter of insulation

d = Outer diameter of bare copper wire

L = Length of airfield cable in feet

Value of K for EPR insulation = 50,000 Megohms

=

1000


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Cable equation

JACKET

INSULATION

CONDUCTOR

OD

D d

D= 9.18 mm

d= 4.58 mm


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Cable equation

=

1000

RC = 50,000 M-k ft * Log ( 9.18/4.58) * (1,000/L)

RC = 15,098,860 M/ L


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Connector equation

The insulation resistance of L-823 connector splicescan be calculated by the following formula:

RN = Rc/Nc

Where: RN = Insulation resistance in Megohms of all connectors

Rc = Insulation resistance of L-823 connector splice

Nc = Quantity of L-823 connector splices

RN = 75,000M/Nc


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Isolation transformer equation

The insulation resistance of L-830 isolationtransformers can be calculated by the following

formula:

RT = Rt/NtWhere:

RT = Insulation resistance in Megohms of all transformers

Rt = Insulation resistance of L-830 isolation transformers

Nt = Quantity of L-830 isolation transformers

RT = 7,500M/Nt


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Base formula

RT RN RC

I

V

1

=

15,098,860 +

75,000 +

7,500 M

IR

1/IR = 1/RC + 1/RN + 1/RT


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Developing IR calculation formula

L-823 Connector L-830 Isolation

Transformer

L-824 Series Lighting Cable

Supply

Return

Section 1 Section 2

Section 3


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Section 1 Section 2

Section 3


L-823 Connector L-830 Isolation

Transformer


Insulation

Resistance to

Earth

Earth Ground


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L-823 Connector

Insulation

Resistance to

Earth

Earth Ground

Supply


1 1

= 15,098,860

+ 75,000

M


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L-823 ConnectorL-830 Isolation

Transformer


Insulation

Resistance to

Earth

Earth Ground

M1 2=

15,098,860+

75,000+

7,500


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Insulation

Resistance to

Earth

Earth Ground

L-823 Connector

Return

1

3=

15,098,860+

75,000 M


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M

1

= 1 1

+ 1 2

+ 13

=1

1 1 +

1 2 +

1 3

IRsection 3IR total IRsection 2IRsection 1


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Calculation example

Vault

CCR

SECTION 1

SECTION 2

SECTION 3

Handhole

Handhole


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Calculation example

Section 1:

Cable = 5,000 feet

Connectors = 2

Isolation XFMRs = 0

Section 2:Cable = 20,500 feet

Connectors = 224

Isolation XFMRs = 112

Section 3:

Cable = 5,000 feet

Connectors = 2

Isolation XFMRs = 0


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Calculation Example

Vault

CCR

Handhole

Handhole


Connectors = 2

Isolation XFMRs = 0

=

,,

+

,

= .0003578


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Calculation Example

=

,

,,+

,+

,= .0192777


Connectors = 224



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Calculation Example

Vault

CCR

Handhole

Handhole

=

,,

+

,

= .0003578


Connectors = 2

Isolation XFMRs = 0


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Calculation Example

IR total= 50 M

Circuit length is 30,500 ft, so the circuit IR is well over the

recommended minimum value.

1

2=

20,500

15,098,860+

224

75,000+

112

7,500= .0192777

1 1 = 500015,098,860 + 275,000 = .0003578

1

3=

5000

15,098,860+

2

75,000= .0003578


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Modifying a circuit

Vault

CCR

SECTION 1

SECTION 2

SECTION 3

Handhole

Handhole


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Modifying a circuit

Assume:

Segment 1:

Cable = 5,000 feet

Connectors = 2

Isolation XFMRs = 0

Segment 2:Cable = 20,500 feet

Connectors = 430


Segment 3:

Cable = 5,000 feet

Connectors = 2Isolation XFMRs = 0

Total circuit:

IRsection1 = 2,795 M

IRsection3 = 2,795 M

IRsection2:

RC = 50,000 * Log ( 9.18/4.58) * (1,000/L) =15,098,860/20,500 =755 M

RN = 75,000/Nc = 75,000/430= 174 M

RT = 7,500/Nt = 7,500/215 = 35 M

IRsection2 = 1/(1/755 + 1/35 + 1/174)IRsection2 = 28 M

IR = 1/ ( 1/2,795 + 1/28+ 1/2,795 )

IR = 27.4 M

Each circuit modification warrants a reevaluation of the IR for that circuit


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Modifying a circuit

Total circuit IR = 27.4 M

Circuit Length = 30,500 feet

Recommended value for +20k feet circuit = 30 M

If you remove the transformer component,

the circuit IR = 128 M

C t d l M i IR f TDZ Ci it


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Case study example: Measuring IR of a TDZ Circuit

Total circuit IR = 33.2 MCircuit Length = 16,430 feet

10k-ft20k-ft = 40 M

W/O Transformers; IR = 164.3

Measured value = 58.10 M


Connectors = 5

Isolation XFMRs = 0

Section 2:

Cable = 13,200 feet

Connectors = 365


Section 3:

Cable = 1,615 feet

Connectors = 5

Isolation XFMRs = 0

C t d mpl M ri IR f REL Cir it


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Case study example: Measuring IR of a REL Circuit

Total circuit IR = 61.6 MCircuit Length = 30,857 feet

+ 20k-ft = 30 M

Measured value = 998 M


Connectors = 5

Isolation XFMRs = 0

Section 2:

Cable = 27,777 feet

Connectors = 180


Section 3:

Cable = 1,540 feet

Connectors = 5

Isolation XFMRs = 0


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Summary

Good engineering practice to perform circuit load andinsulation resistance calculations

Best practiceestablish field test baseline and trackresults

Standards provide recommended values - reductions areallowed

Check & verify transformers, connectors and cable typesand sizes

Minimum allowable component valueshigher factory

test values High initial field value does not necessarily indicate a

good circuit


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Questions

C-5 Insulation Resistance Calculations.pptx

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