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A comparison of NFPA 780 and IEC 62305White Paper

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Contents

Rigorous Calculations

Strike Level and Design Parameters

Risk Analysis

Separation Distance

Air Terminal Selection

Down Conductor Design

Earthing Design

Test and Maintenance

Surge Protection Devices

Actual Lightning Discharge Testing

2


WP045/USA/1016 © Copyright 2016 DEHN, Inc.

IntroductionThe objective of this article is to present why the IEC 62305 Certified Master Label Compliant Lightning Protection Systems (LPS) are comparable and useful as an alternative to NFPA 780 designs for North American markets. This article promotes the acceptance and utilization of IEC 62305 certification as a more thorough approach, produc-ing a superior LPS design, than NFPA 780 for UL96A master label for LPS installations by showing that IEC accomplishes all safety and best practice requirements for North American installations.Per UL announcement, they will grant their Master Label to a LPS that is compliant with UL 96A. This means that UL will pro-vide their Master Label for a LPS designed to meet IEC 62305 if installed by a UL certified installer and passing a UL inspection by a UL LPS inspector.To show that the IEC 62305 is a more thorough approach pro-ducing a superior LPS design this article will first provide some background. After this background, the following sections will illustrate how the IEC standard produces a more detailed and superior LPS implimentation:

¨ Design philosophy & calculations methods

¨ Rigorous calculation

¨ Strike Levels and Design Parameters

¨ Risk analysis required vs optional

¨ Separation distance calculations methods

¨ Air terminals selection

¨ Down conductor design

¨ Earthing design

¨ Test and maintenance

¨ Surge Protection Devices

¨ Actual lightning discharge testing

BackgroundBoth the IEC and NFPA documents are peer reviewed on a regular basis by subject matter experts.As of the date of this report, the IEC 62305 standard is pub-lished as Edition 2:2010 and is updated by committee TC81 under the regular maintenance process with new versions ex-pected in 2017 and 2018.The IEC 62305 suite of standards is ilustrated as a systematic approach in Figure 1.The NFPA 780 standard is currently published under the 2014 edition, with public input for consideration of the Technical Committee for the next 2017 edition (Figure 2).

Design Theory and PhilosophyThe basic requirements of IEC 62305 cover a detailed risk fac-tor assessment which then guides the air terminal design, down conductors, bondiong and application of SPD‘s to reduce the risk below a tolerable threshold. This risk assessment re-quires a fairly detailed knowledge of the structure and end use, and software is almost always used to perform the calculation.

Protectionagainst lightning

IEC 62305

Part 1General principles

Part 2Risk management

Part 3Physical damage to

structures and life hazard

Part 4Electrical and electronic

systems within structures

Figure 1 The IEC 62305 suite of standards is divided into four use-ful segments.

Figure 2 The NFPA 780 standard is presently under 2014 revision.

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The calculation includes, Human Life, Public Services, Cultural Heritage, Economic risk, and penalties for denial of service. This risk assessment is both good and bad in that pole barns will have fewer requirements than would be found in NFPA 780, but high value assets will have greater requirements.The basic approach to lightning protection is to work within the accepted step leader behavior of lightning strikes. Imagine the final lightning strike distance of a bolt seeking any path to ground as a radius of a sphere that could touch anywhere. The more stringent the class of lightning protection, the smaller the radius and more risk the bolt can find the building.The IEC risk mitigation offered by virtue LPS installation of-fers tiered criteria structure with more sensitive applications requireing more stringent LPS protection coverege. LPS IEC Class III (45 m radius sphere) meets the demand of most typi-cal commercial buildings and is most similar to the standard NFPA (150 ft sphere) . This IEC LPL class III implies a protection system capable of intercepting lightning strikes with currents as low as 10 kA and as high as 100 kA 10/350 μs (see lightning strike figure). Based on these parameters, the rolling sphere method can been employed to develop a safety canopy con-structed with a geometry of a 45 m (146.25 ft) radius using air terminals, down conductors tested and earthing system capa-ble of handling 100 kA direct lightning currents.

NFPA 780 Design Theory and PhilosophyThe NFPA LPS design does not include a mandatory risk review but instead is based on the standard NFPA Class I lightning protection system for lower height ordinary building not more than 75 ft (23 m) tall. For this class of structure there is no requirement for a rolling sphere, so the basic air termination placement, down conductor cross section design and earth-ing rod matching system are employed per relevant NFPA 780 clauses. To meet the standard, each rod height can be selected at 24 in (61 cm) tall, with interconnection along the roof and down conductors terminating into a dedicated ground elec-trode earthing rod. NFPA 780 provides guidelines for how often to place air termi-nals, spacing’s for cross and down conductors, ground rod and

loop requirements, surge-protection requirements, and how to install protection for trees, towers and similar structures. The standard however has two primary short falls in that it does not require a detailed analytic calculation of the integrity of the installed systems ability to handle direct lightning strike events, nor does it take into consideration what asset the LPS is protecting. In other words, NFPA 780 has the same require-ments for a pole barn as it does for a high value asset.Table 1 defines the basic NFPA classes of lightning protec-tion and the mechanical difference in materials for structures above 75 ft (23 m) in height.

Rigorous CalculationsThe IEC 62305:2006 standard requires an actual assessment of the lightning protection system to insure that it is capable of handling a lightning strike. The lightning strike calculations are far more significant for both the time domain parameter (10/350 μs vs 8/20 μs) and the actual strike amperages (100 kA to 200 kA) than the US industry standard (often only 20 kA for UL 1449 and UL 467). IEC 62305:2006 calculations that are re-quired include:

¨ The peak lightning strike current to be carried on individual conductors in DC amps to ensure that current carrying ca-pacity is not exceeded.

¨ Time-domain analysis of the lightning strike on the specific structure. This is critical to understanding the amperage carrying capacity of the conductors.

¨ The application of the rolling-ball theory of lightning pro-tection tested against 3D computer models of the structure and surrounding area.

¨ Spark gap and arc-flash calculations to allow for flashes from the lightning protection system to adjacent conduc-tive utilities.

¨ Separation distance to avoid flashover from down conduc-tors to electrical aparatus.

05 50

LPL

I 3 kA(99 %)

200 kA(99 %)

5 kA(97 %)

150 kA(98 %)

10 kA(91 %)

100 kA(97 %)

16 kA(84 %)

100 kA(97 %)

II

III

IV

200150100

Ipeak/kA

e.g. for LPL II:97 % of all light-ning currents > 5 kA and 98 % < 150 kA

NFPA 780-2011

Item Less than 75-ft More than 75-ft

Name Class I Class II

Air terminal min 3/8-in dia (9.5 mm)

min 1/2-in dia (12.7 mm)

Main / downcomer

57 kcmil (between #2 and #3 AWG)

115 kcmil (between 1/0 and 2/0)

Earthing Rods Rods and loop

Figure 3 The IEC 62305 standard divides applications by likely hood of capturing both the high and low severity lightning events.

Table 1 The NFPA 780 standard divides applications into two broad categories by height.

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Strike Levels and Design ParametersAdditional advantages of the IEC methods show up in strike levels and design parameters.In the IEC 62305 standards, the frequency spectrum of the lightning strike on the specific structure leads to a higher charge transfer than seen in NFPA strike levels. This forms the basis for both surge-protection and for timing of circuit break-ers to prevent power outages.Figure 3 shows the Lightning strike levels suggested under IEC to select the LPL measures leading to suggested rolling sphere diameter (lower strike likelyhood implies a smaller diameter).

Risk AnalysisThe IEC requirements imply a normative risk analysis. It is re-quired, and through this excerise the mitigation measures are selected and possibly drive the Lightning Protection Level from a default LPL III to a higher more sever level with smaller roll-ing sphere diameter and higher density of lightning rod air ter-minals. The analysis takes into account the physical structure, hazard location level and occupency to build the un –protected

risk level and then allows selection of features to reduce the risk. The final rolling sphere coverage result will confirm the structure is protected.NFPA 780 annex L describes the informative (suggested but not required) risk review procedure. It does include many of the features of the IEC review, but does not actually help deter-mine the LPL severity as in the IEC method. The designer does not actually select a more rigorous approach as a result of the NFPA risk review.It is important to note the NEC does require critical operations providers to conduct the annex L risk assesment. The final LPS design may incorporate these findings but the system does not drive improvement.The review from NFPA annex L is shown in Figure 4 and lends itself to a spreadhseet calculation.The IEC risk review walks through the structure to develop a fully exposed base line and prompts the designer to increase mitigation levels until a tolerable risk level has been estab-lished. These features and lightning strike levels then guide the designer to a suitable LPS for the specific structure and use.

DETAILED RISK ASSESSMENT WORKSHEET

Equivalent Collective Area

(for rectangular structure)

See Table L.4.2.

See Table L.4.2.

See Table L.4.2.

See Table L.6.7.1

Direct Strikes to Structure

Strikes Near Structure

Strikes to an Incoming Service

Ae = LW+6H(L+W)+9�H2

Nd = (Ng)(Ae)(Cl)(10-6)

NM = (Ng)(Am - Ae)(Cl)(10-6)

NL = (Ng)(Al )(Cl)(Cl)(10-6)

L = Ae =

Ae = Nd =

Ng =

Cl =

W =

H =(substitute formula forother structures)

Probability of Damage

Annual Threat of Occurence

Ng = Nm =

Am =

Ae =

Cl =

Ng = NL =

Al =

Cl =

Injury Due to a Direct Strike – PA

Physical Damage Due to a Direct Strike – Pn

Failure of Internal Systems Due to a Direct Strike – PC

Failure of Internal Systems Due to a Direct Strike – PM

See Table L.6.7.3

See Table L.6.7.4

See Table L.6.7.5

Without coordinated surgeprotective devices – PM = 1.0

UW is the lowest withstand voltage of protected equipment

See Table L.6.7.7

See Table L.6.7.6

PA =

Pn =

PC =

PM =

KS =KS = (KS1)(KS2)(KS3)(KS4)

KS1 = KS2 = 0.12ω

KS4 = 1.5/UW

KS1 =

KS2 =

KS3 =KS4 =

Figure 4 An excerpt of Annex L risk assessment shows it is well suited for the user to create their own work sheet.

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NFPA has only a loose discussion on allowing sufficient sep-eration between the LPS conductors and unbonded electrical systems. A sample calculation from NFPA 780 is shown in Figure 8.But the IEC involes a detailed review of the separation of down conductors from electrical aparatus and wiring

Figure 5 The DEHN Support Toolbox helps calculate the direct and indirect strike collection area.

Figure 6 The DEHN software helps the user through the whole process to derive the protection measures needed to reduce risk below the tolerable thresholds and drive continuous improvement.

Figure 7 Water cooling towers and electrical distribution centers are illustrated in 3D with the overlay of the theoretical blanket of protection.

The direct (solid lines) and indirect (dashed lines) expo-sure area calculations for IEC risk analysis are shown in Figure 5.The calculations for intolerable risk, and then mitigated and tolerable risk levels can be illustrated like in Figure 6. The final protected state using the LPL rolling sphere diameter determined through risk mitigation selection will then be il-lustrated to show coverage like in Figure 7.

Seperation DistanceAny electrical component or wire will be influenced by the EMP from the down conductor as it diverts lightning to ground. The

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(Figure 9). Software modling allows the designer to see how many cm spacing would be required around the LPS based on the suggested maximum surge likely to be introduced per the strike level chart. This allows a designer to take flashover into account systematically through the structure and not neglect these effects accidentally.Isolation spacing and insulation materials are suggested under the IEC methods to prevent touch and step voltages next to the down conductors and earthing electrodes.Figure 10 illustrates the 3 m (9.75 ft) limit for touch and step insulation as recommened by IEC. This is a key issue in select-ing LPS components with intrinsic safety insulation features. This allows the designer to minimize spacing and cost to achieve the safest effective control of risk.

Air Terminal SelectionNFPA 780 for Class I designs allows the spacing of the air ter-minals to be placed at intervals of 25 feet (7.6 m) instead of the 20 foot (6 m) spacing required for air terminals of less than 24-inches (61 cm) in height.According to the IEC 62305 standard, longer rod designs are allowed that have been tested to show the integrity to han-dle real lightning strike energies. These longer rods can also reduces the total number of rods needed to avoid any strikes to the building or any roof top mounted electrical systems. Be-cause there can be unknown features on the roof, these rods assure lightning will not seek weaker metal structures and still provide a minimal impact to the aesthetic view of the building.

Down Conductor DesignThe IEC 62305 indicates the down conductors vary from 10-meter to 20-meter (32.5 ft to 65 ft) spacing, where as in the NFPA 780 a one-size-fits-all 30-meter (76 ft) spacing is em-ployed. The material for higher Class II roofs does double in diameter over 75 ft (23 m) from 57 to 115 kcmil to account for higher weight and a more extreme environment.

Earthing System DesignIf there are adjacent structures between which electrical pow-er supply lines and measuring/control lines are installed, the earth-termination systems have to connect between reinforce-ment, down conductor and earthing systems for equipotential bonding. It is advantageous to reduce the currents in the lines via many parallel paths. This aim is fulfilled by means of an intermeshed earth termination system. Concrete columns that are used for down conductors must be tested a 0.2 ohms or less continuity, and rebar must be welded with 20 x diameter overlaps. These must be bonded to the floor slab according to IEC 62305.Ground rings are required for all non-conductive buildings, buildings housing electronic systems, and certain risk factors

Figure 10 Both IEC and NFPA indicate where insulation must be installed on the down conductors near public access areas.

Figure 8 Separation distance is calculated in NFPA 780.

Figure 9 The DEHN support tool helps show the flashover distance (cm) to guide designers in spacing and separation dis-tance.

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in the IEC 62305 guidelines. Individual rod installations (before attachement into the ground ring) must be tested so that each electrode is at the same resistance to ground as in the NFPA.

Test and MaintenanceThe IEC specifies that a LPS will inlcude test points for future measurements of the LPS connections and earthing integrety (Figure 11).The implementation of a long term preventative maintenance program is required to uphold the integrity of the LPS system components. The IEC standards identifies this to be impera-tive in order to sustain safe AC electrical power reference, the diversion of lightning and the return on investment of both the LPS and customer equipment.The key inspections include:

¨ Inspection at the design stage

¨ Inspections during the construction phase

¨ Acceptance test

¨ Visual inspections

¨ On-site inspections

¨ Measurements

¨ Documentation

¨ Maintenance

¨ Final Inspection and UL Master Label Certification

In Figure 12 we see the short term degeneration of the down conductor portion of the LPS due to local wear and tear effects.

Surge Protection DevicesThe application of spark gaps between lightning conductors and other metallic objects must be considered. In addition, the IEC 62305 standards suggests that incoming utility services (such as overhead power lines) and adjoining public spaces

may also be required to have protection systems installed, based on the risk assessment.Both internal SPD’s and external lightning protection systems are mandatory in the IEC 62305 suite of standards.The IEC 62305:2006 has stringent requirements for annual test-ing and inspection of the lightning protection systems. This of course, goes along with mandatory maintenance requirements.The NEC does require SPD’s to be installed at emergency panels, critical operation providers and where wind generation facilties connect back into the grid, but this requirement does not extend to the prevention of loss at industrial or commercial facilities.Figure 13 shows a diagram depicting all of these IEC best practice concepts to achieve a system level solution with high-est withstand and safe, noise free operation.

DNO

Ex i

MEB

heater

gas

Telecontrol /telecommunication

Measuring and control equipment

PROFIBUS

external LPSfoundation earth electrode

Figure 11 Typical test joints and inspection boxes.

Figure 12 Here is an example of extreme wear and tear of the down conductor. (Photo courtesy MHD)

Figure 13 Here, the equipotential bonding of incoming metal pipes and the application of SPD’s are illustrated with connec-tions to the Master Earthing Bar (MEB).

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Actual lightning discharge testingThe IEC standards dictate that all lightning protection rods, down conductors and clamps must meet the extreme direct lightning currents.All SPD’s are tested to assure they can withstand both the indirect strike 8/20 μs testing seen in UL 1449 standards, but also the extreme 100 kA 10/350 μs withstand associated direct lightning strikes. Both the NFPA 780 and UL standards advise testing to 20 kA 8/20 μs.The IEC establishes a higher energy transfer mechanism, shows the mathematical relationships for charge transfer. This assures mechanical integrity of the components through labo-ratory proof testing.In Figure 14 we see the IEC accredited, DEHN laboratory fa-cilities in Germany which accommodate the leading surge test equipment and safety practices.

SummaryIn summary, the IEC 62305 certification for achievement of UL 96A master label requirements meet best practice installa-

tion and protection of the value of the building asset. UL will now offer a UL Master Label for IEC 62305 compliant lightning protection systems. An important limitation of NFPA 780 is that each building that is part of a complex LPS system must be endowed with all of its own LPS lightning rods, down conductors and earth-ing, with no consideration of the benefit of the rolling sphere coverage offered by adjacent, taller structures. IEC 62305 risk assessment is normative while the NFPA less detailed risk as-sessment is informative. IEC 62305 risk assessment drives im-provement and results in four LPS classes, with air termination, down conductor, and earth termination systems dependent on LPS class. NFPA 780 risk assessment results in a go / no go one size fits all LPS, with a rolling sphere equivalent to IEC LPS Class III. NEC requires risk assessment for critical operations power systems (COPS).The IEC suite of standards provides a more comprehensive and thorough set of design tools for the creation of effective light-ning protection systems.

Acknowledgements: DEHN protects. For the past 105 years we’ve led the way in Lightning and Surge protection solutions for people, industry and electrical / electronic systems against the effects of light-ning and surges.

About the author: Mark Hendricks – Mark has contributed to various IEC and IEEE standards groups and has served the power quality in-dustry for over 19 years.

References: NFPA 780:2014, IEC 62305:2010, UL 1449 4th edition, UL 467

Figure 14 All DEHN products are 100% tested to show they with-stand lightning events.

(Photo courtesy DEHN)


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