Capacity of Carrying Lightning Currents in LPS Copper Cables and Connectors

Experimental verifications

Hélio E. Sueta1, Miltom Shigihara1, Luis E. Caires1 , Geraldo F. Burani¹

1 Instituto de Eletrotécnica e Energia da USP São Paulo - Brazil sueta@iee.usp.br

Jobson Modena2

2 Guismo Engenharia São Paulo -Brazil

jomodena@gmail.com

Abstract— This paper presents the results of a series of tests on patches of bare copper cable, usually used in LPS to check their ability to conduct lightning currents.

Keywords- LPS; connectors; cable splices; lightning conduction

I. INTRODUCTION

The current Brazilian standard [1] (under revision) allows the use of copper 16 mm² to 50 mm² conductors in different LPS subsystems.

A future revision, based on the IEC 62305 series [2] [3] [4] [5] provides the use of copper cables of at least 50 mm² (air-termination, down-conductor and earth-termination systems).

Conductors to be used in the LPS, in Brazil, have non-compulsory accreditation. There is a parallel market in Brazil for copper conductors of low quality that, besides having questionable composition may have a cross-sectional area smaller than the rated one.

Thus, while most countries use copper cables with an effective cross-section of 50 mm², in many cases in various parts of Brazilian installations, cross-sections don’t reach 23 mm².

This study examines the importance of the cross-section of LPS cables conducting lightning currents and presents some results of laboratory tests conducted on cables and connectors used in these systems.

II. DEVELOPMENTS Initially, a verification of the material sold in the Brazilian

market was carried out to compare the effective bare copper cross-section conductors to a rated cross-section indicated by the manufacturer.

This research has shown that the difference between the cross-sections is about 60%, i.e., the product called "generic" has cross-sectional area lower than that declared.

Another studied point regards the ability to conduct lightning current by LPS components.

We tested 36 samples of bare copper cable splices by using connectors of the split-bolt type. The cable splices had nominal gauges of #16 mm², #35 mm² and #50 mm²; half of them had 2 connectors by splice and the other half only had one. Regarding the quality of the splices, we simulated them with good torques (2 N×m) and weak torques (< 2 N×m). The idea was to simulate weak connections that may take place in inadequately maintained LPS.

Initially we measured the electrical resistance (contact) of all samples before current applications. Table I describes the characterization of the samples and the values of the electrical resistance (1) before the current applications of continuity, (2) after the applications and (3) after the impulsive current applications (in some of the samples where the applications were made). For the measurement of electrical resistance, we used a microohmmeter with the following features: Digital Micro-ohmmeter of MEGABRAS, model MPK204 used in the range of 5 A. Figure 1 is a photograph showing the measurement of electrical resistance in a sample.

Figure 1. Measurement of electrical resistance.

Tests were conducted simulating the continuing currents with a pulse current of about 600A with 580 ms of duration (charge of around 350 C). In some samples, additional applications were made with higher current or duration values.

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2011 International Symposium on Lightning Protection (XI SIPDA), Fortaleza, Brazil, October 3-7, 2011

978-1-4577-1897-7/11/$26.00 ©2011 IEEE

We also carried out some tests with impulse currents in some of the samples. These tests were performed with the highest possible values in the current configuration of the impulsive current laboratory at the IEE-USP.

TABLE I. SAMPLE CHARACTERISTICS AND RESISTANCE VALUES MEASURED

Samples C. 1 (mm²)

C. 2 (mm²)

Number connec. Torque

Electrical Resistance ( )

Before After I cont

After I imp

#1 50 50 2 Strong 251 237 - #2 50 50 2 Strong 237 236 - #3 50 50 2 Strong 243 235 - #4 50 50 2 Weak 243 245 - #5 50 50 2 Weak 276 265 - #6 50 50 2 Weak 240 251 - #7 50 35 2 Strong 273 275 - #8 50 35 2 Strong 286 280 - #9 50 35 2 Strong 271 274 - #10 50 35 2 Weak 271 273 - #11 50 35 2 Weak 281 282 - #12 50 35 2 Weak 277 275 - #13 50 16 2 Strong 413 415 - #14 50 16 2 Strong 435 421 - #15 50 16 2 Strong 411 428 - #16 50 16 2 Weak 422 475 - #17 50 16 2 Weak 440 450 - #18 50 16 2 Weak 427 428 - #19 50 50 1 Strong 300 292 - #20 50 50 1 Strong 270 274 - #21 50 50 1 Strong 266 287 - #22 50 50 1 Weak 252 336 - #23 50 50 1 Weak 270 298 - #24 50 50 1 Weak 291 327 290 #25 50 35 1 Strong 304 310 - #26 50 35 1 Strong 293 300 - #27 50 35 1 Strong 300 294 - #28 50 35 1 Weak 208 663 - #29 50 35 1 Weak 303 295 - #30 50 35 1 Weak 281 274 288 #31 50 16 1 Strong 500 417 - #32 50 16 1 Strong 488 502 - #33 50 16 1 Strong 438 450 - #34 50 16 1 Weak 470 - - #35 50 16 1 Weak 456 444 - #36 50 16 1 Weak 488 607 542

NB: Columns “C.1” and “C.2” correspond to the gauge of cable portions of each splice. Column “Nº conec.” refers to the number of connector for each splice (half the samples used 2 connectors and the other half only made use of one).

A. Comparison between effective cross-section of the conductors with the rated cross-section A first batch of samples were analyzed in relation to cables

effective cross-section, where it was found: a rated cross-section of 35 mm² having effective cross-section of 23 mm²; a 50 mm² rated cross-section having effective cross-section of 31 mm²; 70 mm² rated cross-section having effective cross-section of 43 mm² and a 95 mm² rated cross-section having effective cross-section of 62 mm².

This first batch showed an approximate ratio of 60% between the rated and effective cross-sections.

A new batch, used for testing, was also examined and include the rated 16 mm² cross-section that is still used in Brazil to LPS down-conductors.

There is also a movement in Brazil to have a compulsory certification for this product.

Thus, it is hoped that with this accreditation, the products used in LPS have better quality and ensure that at least meet the specifications stated by the manufacturers.

The samples used in the trials were measured using a measuring microscope Mod MF - A1010B together with the data processing unit 2D - QM-DATA 200. Figure 2 shows a cross section cable #50 mm², seen with the aid of the microscope used for measuring the cable section. Each cable wire was analyzed separately (three measures) and the sum of the averages of each wire area average was calculated to make it possible to compare the actual copper cross section of the cable with the nominal section informed by the manufacturer.

Figure 2. Cable section of #50 mm².

The results were as follows: the nominal section cable of #16 mm² has a cooper real section of # 17.68 mm²; the nominal section # 35 mm² presents a real section of # 34.27 mm² and the one of #50 mm² has a real section of # 48.05 mm².

Regarding the tested cables, their sections are very close to the nominal values presented by the manufacturer.

B. Tests on samples of cables and connectors Samples for testing consist of small patches of bare copper

cables with varying cross-sections, some with cable splices of the same cross-section and some with splices in sections of cables with different cross-sections. Table I shows the characteristics of each tested sample.

Samples considered to have a strong torque are the ones adjusted with a torquimeter with a value of torque of 2 N×m. The ones with a weak torque were tightened in a way cables 1 and 2 were put close to each other. Thus, it was not easy to move the cables manually.

C. Continuing current tests

All samples were submitted to continuous current pulse application with the following features for continuing current simulation:

• Median current value: 595A

• Duration of application: 585 ms

• Approximate charge value: 350 C

Figure 3 shows an oscillogram of one of the applications.

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CURRENT

100 ms/div

A 0.0

692.3

-692.3

Figure 3. Oscillogram of continuing current application.

After each application, the samples were visually inspected and none of the 36 samples presented any kind of damage whether electric arc, welding or melting.

Based on these results some samples were selected for further verification.

Samples #22, #28 and #34, each with one connector and a very weak torque were more loosened so as to manually move the cables. We applied the same current pulse and the result of visual inspection was identical to previous tests.

In these same samples, it was made an application that lasted longer: Current: 590 A, duration: 2000 ms corresponding to a load of approximately 1184 C. The result was similar to the previous ones.

Still in these samples, we made an application with higher current values and different duration from the initial one, i.e., current: 5780 A and duration: 630 ms, corresponding to a charge of approximately 3640 C. In sample #34, there was the melt of cable #16 mm². Figure 4 shows the sample before the test and Figure 5 shows the sample after the test. Samples #22 and #28 showed a similar result to previous ones.

Figure 4. Sample before the tes.

Figure 5. Sample after the test.

It is very important to stress that in the case of the cable #16 mm² application, test values were way much higher than the standardized ones. Still, through Figure 5 we can see that both the cables and the connector weren’t damaged in the connection region. Fusion took place at the #16 mm² cable splice.

After applying continuing currents, electrical resistance measurements were repeated and results are shown in Table I.

Analyzing the results from both continuing current tests and electrical resistance measurements after and before current applications, we verified that the tested samples conducted continuing currents with values which are close to and much higher than the standardized ones. Regarding the electrical resistance values before and after current applications, we noticed that these values were very close, except for samples #22 and #28 which were loosened for additional verification. In these cases, the electrical resistance value increased.

D. Impulse current tests Samples #24, #30 and #36 were chosen to have impulse

current injected.

The current impulse generator (see Figure 6) was set up to obtain impulse currents with an average peak of 55 kA and waveform of 5.0/10.2 s as shown in Figure 7. Three current impulses were injected in each sample, all with the same characteristics.

Figure 6. Assembly of current impulse test.

-40

-30

-20

-10

0

10

20

30

40

50

60

-1,0 -0,5 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0

Time (us)

Cur

rent

(kA

)

Figure 7. Impulse current waveform 5.0/10.2 μs.

We chose the most fragile samples once they have only one connector and a very weak torque. Electrical resistance measurements, as seen in Table I, were repeated. Electrical resistance value was very close to the initial one in these samples.

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After tests and measurements, samples were diassembled and visually inspected. Except for a few scorched spots (see Figure 8), all samples, when tested, behaved properly.

Figure 8. Detail of sample #36 submitted to all tests.

III. CONCLUSIONS Currently many LPS in Brazil have splices between bare

copper cables of different cross-sections, e.g. 35 mm² cables assembled in the air-termination system with 16 mm² cables assembled in the down-conductors system, or 16 mm² down-conductors with 50 mm² cables in the earth-termination system.

Moreover, these cross-sections are rated values and these sections may actually be even lower due to the parallel market in the country.

The IEC Standard [4] indicates, for these protection subsystems, 50 mm² copper cables.

In previous work [6], it was presented a comparison between projects developed according to current Brazilian standard series and the IEC.

This study showed that projects implemented with the same philosophy, e.g., using external down-conductors and buried grounding system (copper cables), have very different costs for the two standards.

In some cases, the cost of a IEC Standard LPS model may be more than 2 times the cost of LPS in the same building made according to the current Brazilian Standard.

The contribution of copper conductors (cross-section, length and amount) on this cost increase is very large.

In this study, we verified that, under the values and conditions used, the behavior of samples with 1 or 2 connectors type split-bolt, different gauges (#50 mm² with #50 mm², #50 mm² with #35 mm² and #50 mm² with #16 mm²) and different torques (weak or strong) is very similar. With that in mind, all sample behaved properly during the tests. Overall, other tests using other types of connectors and other current levels such as 100 kA currents with waveform of 10/350 s are recommended for the verification of other effects, such as mechanical stress on conductors and splices.

ACKNOWLEDGMENTS

Authors would like to express their thanks to QUALIFIO for the first measurements and analysis in materials traded in Brazil.

We thank the technical staff at IEE-USP, especially the Laboratory for High Currents (Laboratório de Altas Correntes - Ivan Raposo e Eduardo Chinen), The Laboratory of High Voltage (Laboratório de Alta Tensão - Clóvis Kodaira) and the Low Voltage Section (Seção de Baixa Tensão - Tadeu Osano e Jorge Rufca) for performing the tests.

REFERENCES

[1] ABNT – Associação Brasileira de Normas Técnicas, “ABNT NBR 5419 – Proteção de estruturas contra descargas atmosféricas” , 2005, in portuguese.

[2] IEC – INTERNATIONAL ELECTROTECHNICAL COMMISSION – IEC 62305 -1, First edition, 2006-01 – Protection against lightning – Part 1: General principles.

[3] IEC – INTERNATIONAL ELECTROTECHNICAL COMMISSION – IEC 62305 -2, First edition, 2006-01 – Protection against lightning – Part 2: Risk Management.

[4] IEC – INTERNATIONAL ELECTROTECHNICAL COMMISSION – IEC 62305 -3, First edition, 2006-01 – Protection against lightning – Part 3: Physical damage to structures and life hazard.

[5] IEC – INTERNATIONAL ELECTROTECHNICAL COMMISSION – IEC 62305 -4, First edition, 2006-01 – Protection against lightning – Part 4: Electrical and Electronic Systems within Structures.

[6] Sueta, H. E.; Grimoni, J.A.B.; Modena, Jobson; Oliveira, J. Barbosa; Alves, Normando V.B, “Comparative analysis of LPS designs developed in accordance with both ABNT 5419/2005 and IEC 62305/2006”, Proceedings of the GROUND 2010 & 4th LPE -I International Conference on Grounding and Earthing & 4th International Conference on Lightning Physics and Effects, Salvador, Brazil, Nov. 2010.

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