~ National Lightning Safety Institute ~

Section 5.5.7

1. Summary

Lightning incidents to heavy industry bring serious consequences to people safety and to productivity. Lightning strikes upon high value assets are not well recorded – statistics are elusive. This paper presents a case study of lightning threats to a multi-million dollar mining investment in Peru. Efforts to mitigate the consequences of lightning are described in two broad categories: detection and protection. The presentation is illustrated with many examples of lightning vulnerabilities and solutions.

2. Background

Client company operates a large mine that employs over 500 people. Lightning arrives over the mountains with little advanced notice. In November 2004, several employees were injured while working outdoors. This raised labor questions and management concerns for possible future lightning events. NLSI was retained to produce a site assessment to improve safety and increase productivity.

3. Lightning Protection Sub-systems

A matrix of lightning protection sub-systems is shown below and can be applied to specific sites to reduce the risk.

Caveat: Because of its arbitrary, capricious, random stochastic, and unpredictable nature, absolute protection from lightning's effects is impossible.

4. Lightning Detection

Past procedures employed small, hand-held detectors. False positives and arbitrary distance determination of these equipments caused faulty safety estimates and excessive downtime. In one case, a hand-held detector went into alarm during nearby electric (arc) welding. In another example, it was shown the detectors were sensitive to nearby RBG computer monitors and microwave ovens. Confidence in existing detection equipment was low.

Detection equipment upgrades were studied. Client sent a team of four technical engineers to the U.S. to assess more reliable equipment. Visits to three vendors included product demonstrations and discussions with engineering and technical persons. Improved equipment accuracy and user confidence resulted in client replacing existing detectors. Factory technicians assisted in the installation of new detectors at the client mine site.

A refined lightning safety shutdown program was instituted. Both policy and procedural plans were adopted. In brief, at the initial stage these are:

• Yellow Alert when lightning enters zone between 60-32 km
• Orange Alert when lightning enters zone between 32-16 km; deactivate this alert if 15 minutes has passed without lightning entering the 16 km zone
• Red Alert when lightning enters zone between 16-0 km
• Red Alert if electric field within 4-km zone exceeds 2000 V/m

The above defaults may be changed to more restrictive thresholds as empirical studies merit.

5. Lightning Protection

Client's lightning protection for such structures as refueling depots, crushers, ANFO storage, refining laboratory, administration buildings, maintenance buildings, and truck shop were inadequate. Active mining operations, such as blasting, drilling, loading, and hauling, sometimes used inefficient lightning safety criteria. Each site and each activity was studied from the client mantra of "Every Worker Goes Home Safe Every Day." Appropriate modifications were suggested, studied and implemented.

• By example, personal safety for outdoor workers was addressed by using large metal shipping containers (quasi-Faraday Cages) as safe refuges. They are skid-mounted and moved to needed locations as required.
• By example, close-in lightning called for suspending mine face Reed Drilling and loading/hauling. It was determined that operators inside operating equipment robust cabins were safe from direct lightning effects. This work could continue when lightning threatened.

Air terminal designs of the Early Streamer Emission (ESE - Thor Aerodynamico Ionizante Pararayos) type were widespread. Vendors to client asserted that the ESEs provided safe zones for personnel and that ESEs assured a large protective radius. Neither of these statements are correct. It is not the place of this paper to re-introduce discussions about ESEs, already well-described elsewhere. It should be sufficient here to state that these designs are not approved by most code-making authorities nor by mainstream science (Uman & Rakov, 2002). Corrective measures here also were suggested, studied, and implemented.

6. Conclusion

NLSI recommendations have been implemented. NLSI may re-visit the client mine in 2006 to inspect modifications. This case study can serve as a template for open-pit mining operations elsewhere.

7. References

• "Accidente Rayo en Plataforma Chancado Primario," client document dated Nov. 2004.
• "Politica de Alertas Por Tormentas Electricas," client document dated Jan. 2006.
• M. Uman and V. Rakov, "A Critical Review of NonConventional Approaches to Lightning Protection," American Meteorological Society, Nov. 2002.