It is disappointing to note the amount of myths associated with static electricity or lightning that have been accepted on faith as truth for a phenomenon that has been recorded as early as 600BC. We are often confronted with these so called "truths" presented as facts. At Static Electricity Control (SEC) we only focus on facts related to static electricity and lightning as supported by testing and empirical or analytical data. 

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TRUTH SUPPORTED BY FACTS

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Static electricity that results in product missbehaviour costs

companies millions of dollars in production downtime and

associated costs. In 2016 SEC was involved in an industry

study that looked at production downtime and associated

costs of 4 manufacturing companies. In a case study related

to an Australian Food Manufacturer it was calculated

that the company lost 1,729 hours in downtime and

$2,297,000+ in associated costs over a 12 month period due

to static electricity and its effect on production until a solution

was introduced.

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The following comment is a common one in relation to static

electricity in a hazardous area: "We haven't had a fire in 50 years".  

However it may need to be adjusted to "We are lucky we haven't

had a fire in 50 years", once you consider all the facts.

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• In the USA alone, static electricity causes on average 280 industrial incidents each year resulting in injuries and fatalities, tens of millions of dollars of direct property damage, lost production or plant downtime, and environmental release issues. (Source: Understanding the Shocking Truth – Graham Tyers, Newson Gale Inc, 2010)

 

• Static electricity is the prime culprit for at least two serious fires or explosions in industry worldwide every day of the year, according to the National Fire Protection Association (NFPA) and the U.K.'s Institution of Chemical Engineers (Source: Understanding the Shocking Truth – Graham Tyers, Newson Gale Inc., 2010)

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• A review of 310 accidents by the Japanese chemical industry found that improper grounding caused 70% of all accidents involving static electricity. (Source: Understanding the Shocking Truth – Graham Tyers, Newson Gale Inc., 2010)

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• In the USA damage to infrastructure and buildings from lightning costs companies millions of dollars in production downtime and associated costs.

 

​STATIC ELECTRICITY

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​The energy released in a static electricity discharge can vary. The energy in joules can be calculated from the capacitance (C) of the object and the static potential V in volts (V) by the formula E = ½CV2.

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One experimenter estimates the capacitance of the human body as high as 400 picofarads, and a charge of 50,000 volts, discharged e.g. during touching a charged car, creating a spark with energy of 500 millijoules (mJ).

 

Another estimate is 100–300 pF and 20,000 volts, producing a maximum energy of 60 mJ.

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According to IChemE - Institution of Chemical Engineers the capacitance of the human body  is as high as 200 picofarads, and a

charge of 20,000 volts, discharged e.g. during touching a charged car, creating a spark with energy of 90 millijoules (mJ).

 

The human body is capable of accumulating enough static electricity that when discharged under certain conditions can ignite a significant number of solvents, fuels and combustible dusts that exist in general industry application and processes today.

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IEC 479-2:1987 states that a discharge with energy greater than 5000 mJ is a direct serious risk to human health. IEC 60065 states that consumer products cannot discharge more than 350 mJ into a person.

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The maximum potential is limited to about 35–40 kV, due to corona discharge dissipating the charge at higher potentials. Potentials below 3000 volts are not typically detectable by humans. Maximum potential commonly achieved on human body range between 1 and

10 kV, though in optimal conditions as high as 20–25 kV can be reached. Low relative humidity increases the charge buildup; walking 20 feet (6.1 m) on vinyl floor at 15% relative humidity causes buildup of voltage up to 12 kilovolts, while at 80% humidity the voltage is

only 1.5 kV.

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As little as 0.2 millijoules may present an ignition hazard; such low spark energy is often below the threshold of human visual and auditory perception.

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Typical ignition energies are:

• 0.011 mJ for hydrogen

• 0.2-2 mJ for hydrocarbon vapors

• 1–50 mJ for fine flammable dust

• 40–1000 mJ for coarse flammable dust.

 

The energy needed to damage most electronic devices is between

2 and 1000 nanojoul. A relatively small energy, often as little as 0.2–2

millijoules, is needed to ignite a flammable mixture of a fuel and air.

For the common industrial hydrocarbon gases and solvents, the 

minimum ignition energy required for ignition of vapor-air mixture

is lowest for the vapor concentration roughly in the middle between

the lower explosive limit and the upper explosive limit, and rapidly

increases as the concentration deviates from this optimum to either

side. Aerosols of flammable liquids may be ignited well below their 

flash point. Generally, liquid aerosols with particle sizes below 10

micrometers behave like vapors, particle sizes above 40 micrometers

behave more like flammable dusts. Typical minimum flammable

concentrations of aerosols lay between 15 and 50 g/m3. Similarly,

presence of foam on the surface of a flammable liquid significantly

increases ignitability. Aerosol of flammable dust can be ignited as

well, resulting in a dust explosion; the lower explosive limit usually

lies between 50 and 1000 g/m3; finer dusts tend to be more explosive

and requiring less spark energy to set off. Simultaneous presence of

flammable vapors and flammable dust can significantly decrease

the ignition energy; a mere 1 vol.% of propane in air can reduce the

required ignition energy of dust by 100 times. Higher than normal

oxygen content in atmosphere also significantly lowers the ignition energy.

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There are five types of electrical discharges:

• Spark discharge, responsible for the majority of industrial fires and explosions where static electricity is involved. Sparks occur between objects at different electric potentials. Good grounding of all parts of the equipment and precautions against charge buildups on equipment and personnel are used as prevention measures.

• Brush discharge occurs from a nonconductive charged surface or highly charged nonconductive liquids. The energy is limited to roughly 4 millijoules. To be hazardous, the voltage involved must be above about 20 kilovolts, the surface polarity must be negative, a flammable atmosphere must be present at the point of discharge, and the discharge energy must be sufficient for ignition. Further, because surfaces have a maximum charge density, an area of at least 100 cm2 has to be involved. This is not considered to be a hazard for dust clouds.

• Propagating brush discharge is high in energy and dangerous. Occurs when an insulating surface of up to 8 mm thick (e.g. a teflon or glass lining of a grounded metal pipe or a reactor) is subjected to a large charge buildup between the opposite surfaces, acting as a large-area capacitor.

• Cone discharge, also called bulking brush discharge, occurs over surfaces of charged powders with resistivity above 1010 ohms, or also deep through the powder mass. Cone discharges aren't usually observed in dust volumes below 1 m3. The energy involved depends on the grain size of the powder and the charge magnitude, and can reach up to 20 mJ. Larger dust volumes produce higher energies.

• Corona discharge, considered non-hazardous.

 

 

LIGHTNING & LIGHTNING PROTECTION SYSTEMS

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It is somewhat suprising that industry continues to refer to static electricity and specifically lightning protection as being part of the "black arts". That the techniques and or solutions are considered mysterious and or not based in science.

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Standards that apply to Lightning Protection systems such as  Australian and New Zealand Standard for Lightning Protection AS1768, IEC International Standard 62305 series – Protection against Lightning and US NFPA 780 – Standard for Installation of Lightning Protection Systems provide clear and detailed information on lightning protection and how to apply a Lightning Protection system.

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If a lightning protection solution is specified and designed correctly than it is no different to a standard electrical system installation. If a standard electrical system was not maintained there will be some element of risk associated with the electrical systems ongoing performance and safety therein. Explaining how a lightning protection system works for a client and the risks associated is critical.

 

The basic premise behind a lightning protection system is to safely redirect lightning away from infrastructure, services, processes, personnel etc. to an independent earth. It is important to understand how each piece of equipment in a lightning protection system works and to what degree each piece of equipment assists in redirecting that lightning to earth. Sometimes it requires a combination of equipment in order to provide a solution that minimises the associated risk.

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A traditional lightning rod system placed at all four corners of a building does not necessarily safeguard that building from a lightning strike.

 

There is no lightning protection system that can completely safeguard a site from lightning. Just like there is no electrical system that can completely safeguard an electrical grid/terminal station from a blackout. The irony is that most electrical blackouts that affect a terminal station occur during a weather event and are sometimes the result of a lightning strike. 

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To reduce the risk associated with lightning, understanding the risk is essential. Some of the questions to ask during pre qual so as to assist in gathering the correct information with a view to specification and design...What is the risk to human life, services provided to the public, cultural heritage, loss of income or revenue? What is the source(s) of damage? Direct strike to the structure, Strike near the structure, Strike near conductive paths such as metal pipes...Will a direct strike and subsequent lightning current create a mechanical or thermal effect causing a fire or explosion...and so on...

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Understand your risk and apply the appropriate lightning protection system.

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References

 

M.A. Kelly, G.E. Servais and T.V. Pfaffenbach  An Investigation of Human Body Electrostatic Discharge, ISTFA ’93: The 19th International Symposium for Testing & Failure Analysis, Los Angeles, California, USA/15–19 November 1993

ESD Terms". eed.gsfc.nasa.gov. 

Static Electricity Guidance for Plant Engineers Graham Hearn – Wolfson Electrostatics, University of Southampton