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Insulation Resistance Calculations of Airfield Lighting Circuits 35 TH ANNUAL HERSHEY CONFERENCE Joseph Vigilante, PE.

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Presentation on theme: "Insulation Resistance Calculations of Airfield Lighting Circuits 35 TH ANNUAL HERSHEY CONFERENCE Joseph Vigilante, PE."— Presentation transcript:

1 Insulation Resistance Calculations of Airfield Lighting Circuits 35 TH ANNUAL HERSHEY CONFERENCE Joseph Vigilante, PE

2 Presentation Objective To develop a formula to calculate insulation resistance for an airfield lighting circuit and provide theoretical and real-life examples.

3 Presentation Agenda Cable insulation resistance (IR) background FAA IR guideline recommendations Formula development Airfield lighting circuit calculations Summary Open Q&A

4 Anatomy of insulation current flow Capacitance charging currents, C Absorption current, RA Conduction current, RL Circuit model DC VOLTAGE SOURCE RA C RL S

5 Anatomy of insulation current flow CURRENT - MICROAMPERES SECONDS CAPACITANCE CHARGING CURRENT

6 Anatomy of insulation current flow CURRENT - MICROAMPERES SECONDS CAPACITANCE CHARGING CURRENT ABSORPTION CURRENT

7 Anatomy of insulation current flow CURRENT - MICROAMPERES SECONDS CAPACITANCE CHARGING CURRENT ABSORPTION CURRENT CONDUCTION CURRENT

8 Anatomy of insulation current flow CURRENT - MICROAMPERES SECONDS CAPACITANCE CHARGING CURRENT TOTAL CURRENT ABSORPTION CURRENT CONDUCTION CURRENT

9 Types of insulation resistance testing Short-time/spot reading 0 TIME 60 sec MEGOHMS VALUE READ AND RECORDED TIME (MONTHS) MEGOHMS VALUES CHARTED

10 Types of insulation resistance testing Time-resistance method 0 TIME 10 Min MEGOHMS INSULATION SUSPECT INSULATION PROBABLY OK

11 Testing airfield lighting circuits AC 150/ F, Design & Installation Details for Airport Visual Aids - Chapter 12, Equipment & Material, Section 13, Testing –50M Non-grounded series circuits –FAA-C-1391, Installation & Splicing of Underground Cable

12 Resistance values for maintenance AC 150/ B, 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

13 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

14 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 RTRT RTRT RNRN RNRN RCRC RCRC I V

15 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 , Section –Corresponding to IR Constant 50,000 M - k-ft. at 15.6°C

16 FAA minimum IR values L-823 cable connectors –AC 150/ D, FAA Specification for L-823 Plug and Receptacle, Cable Connectors –Section 5.1 – Type I Connectors 75,000 M

17 FAA minimum IR values L-830 isolation transformers –AC 150/ C, Specification for Series to Series Isolation Transformers for Airport Lighting Systems –Table 3 Insulation Resistance 7,500 M

18 Base formula 1/IR = 1/RC + 1/RN + 1/RT RTRT RTRT RNRN RNRN RCRC RCRC I V IR

19 Cable equation The insulation resistance of cable for a certain length can be 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

20 Cable equation JACKET INSULATION CONDUCTOR OD D d D= 9.18 mm d= 4.58 mm

21 Cable equation

22 Connector equation The insulation resistance of L-823 connector splices can 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

23 Isolation transformer equation The insulation resistance of L-830 isolation transformers can be calculated by the following formula: RT = Rt/Nt Where: 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

24 Base formula RTRT RTRT RNRN RNRN RCRC RCRC I V M¹M¹ IR 1/IR = 1/RC + 1/RN + 1/RT

25 Developing IR calculation formula L-823 Connector L-830 Isolation Transformer L-824 Series Lighting Cable Supply Return Section 1Section 2 Section 3

26 Section 1Section 2 Section 3 Developing IR calculation formula L-823 Connector L-830 Isolation Transformer L-824 Series Lighting Cable Insulation Resistance to Earth Earth Ground

27 Developing IR calculation formula L-823 Connector Insulation Resistance to Earth Earth Ground Supply L-824 Series Lighting Cable M¹M¹

28 Developing IR calculation formula L-823 Connector L-830 Isolation Transformer L-824 Series Lighting Cable Insulation Resistance to Earth Earth Ground M¹M¹

29 Developing IR calculation formula L-824 Series Lighting Cable Insulation Resistance to Earth Earth Ground L-823 Connector Return M¹M¹

30 Developing IR calculation formula M IRsection 3 IR total IRsection 2 IRsection 1

31 Calculation example Vault CCR SECTION 1 SECTION 2 SECTION 3 Handhole

32 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

33 Calculation Example Vault CCR Handhole Section 1: Cable = 5,000 feet Connectors = 2 Isolation XFMRs = 0

34 Calculation Example Section 2: Cable = 20,500 feet Connectors = 224 Isolation XFMRs = 112

35 Calculation Example Vault CCR Handhole Section 3: Cable = 5,000 feet Connectors = 2 Isolation XFMRs = 0

36 Calculation Example IR total = 50 M Circuit length is 30,500 ft, so the circuit IR is well over the recommended minimum value.

37 Modifying a circuit Vault CCR SECTION 1 SECTION 2 SECTION 3 Handhole

38 Modifying a circuit Assume: Segment 1: Cable = 5,000 feet Connectors = 2 Isolation XFMRs = 0 Segment 2: Cable = 20,500 feet Connectors = 430 Isolation XFMRs = 215 Segment 3: Cable = 5,000 feet Connectors = 2 Isolation XFMRs = 0 Total circuit: IRsection1 = 2,795 MΩ IRsection3 = 2,795 MΩ IRsection2: RC = 50,000 * Log ( 9.18/4.58) * (1,000/L) = 15,098,860/20,500 = 755 MΩ RN = 75,000/Nc = 75,000/430 = 174 MΩ RT = 7,500/Nt = 7,500/215 = 35 M IRsection2 = 1/(1/ /35 + 1/174) IRsection2 = 28 MΩ IR = 1/ ( 1/2, /28 + 1/2,795 ) IR = 27.4 MΩ Each circuit modification warrants a reevaluation of the IR for that circuit

39 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

40 Case study example: Measuring IR of a TDZ Circuit Total circuit IR = 33.2 M Circuit Length = 16,430 feet 10k-ft – 20k-ft = 40 M W/O Transformers; IR = Measured value = M Section 1: Cable = 1,615 feet Connectors = 5 Isolation XFMRs = 0 Section 2: Cable = 13,200 feet Connectors = 365 Isolation XFMRs = 180 Section 3: Cable = 1,615 feet Connectors = 5 Isolation XFMRs = 0

41 Case study example: Measuring IR of a REL Circuit Total circuit IR = 61.6 M Circuit Length = 30,857 feet + 20k-ft = 30 M Measured value = 998 M Section 1: Cable = 1,540 feet Connectors = 5 Isolation XFMRs = 0 Section 2: Cable = 27,777 feet Connectors = 180 Isolation XFMRs = 90 Section 3: Cable = 1,540 feet Connectors = 5 Isolation XFMRs = 0

42 Summary Good engineering practice to perform circuit load and insulation resistance calculations Best practice – establish field test baseline and track results Standards provide recommended values - reductions are allowed Check & verify transformers, connectors and cable types and sizes Minimum allowable component values – higher factory test values High initial field value does not necessarily indicate a good circuit

43 Questions


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