The Third Age of 15 kV and 38 KV Medium Voltage Switchgear puts Arc Flash Safety First

Arc Flash Accidents are on the Rise All Across America

The Occupational Safety and Health Administration (OSHA) and the National Fire Protection Agency (NFPA) investigate a growing number of serious arc flash accidents every year.  In the United States alone, there are 30,000 arc flash incidents per year, about 5 to 10 per day.  7,000 of these result in burn injuries with 2,000 requiring serious hospitalization.  400 die from the accident with 80% of the deaths due to the burns – and not the electrical shock.  Medical costs for an accident survivor averages up to $1.5 million, and lost production, insurance and legal costs can add up to $10 million per accident.  The statistics are telling – just one arc flash incident is not only costly to human life, but injurious to the social and economic fabric of the workplace and community.

No electrical operator or maintenance personnel, nor any manager or business owner, nor any corporation, seeks to conduct their business with such callous that there is a thriving environment for arc flash hazard conditions everywhere.  Everyone tends to treat electrical energy with enough respect that we all look out for conditions that are unsafe.  But if you have ever worked on your home’s 120 VAC circuits enough times in your life, you may have come across a live wire you were sure was turned off at the circuit breaker panel, or a tool slipped and touched a live wire to ground, creating an arc flash.  The more you have worked on your house’s electrical circuits, the more likely the chance of an accident, because over time the probability of an accident increased.  Similarly, the probability of an accident decreases the more careful we are regarding the electrical work.

It's Not About the Probability of Occurence - It's About the Damage by the Occurrence Itself

But there is a big difference between a 120 VAC arc flash and a 38 kVAC arc flash – since the potential voltage is magnitudes larger, so is the arc fault current and its ability to fuel an explosive arc flash.  To better understand the explosiveness of an arc flash, let’s break it down into its components.  There is thermal energy in the form of outward expanding searing heat.  There is acoustic energy in the form of a deafening sound wave followed by a high pressure energy wave that can bring about a concussion.  And the thermal, acoustic and pressure waves can tear through steel enclosures and accelerate the debris as projectiles that penetrate nearby objects.  Any of these elements can cause serious injury to a person – all together they can be fatal to anyone within a 5 to 10 foot range of the explosion.  The probability of someone being near the explosion is very low - the damage to the 7,000 people statisically per year affected by such an explosion - very catastrophic. 

During a 38 kVAC arc flash, the temperature at the flash point can reach 35,000 °F.  To put that into perspective, the sun’s surface is 6,000 °F to 23,000 °F depending on where it is measured.  The acoustic and pressure wave generated by the arc flash explosion often destroys hearing and causes nearby personnel to be blown off their feet. And for gear that is not arc resistant, sheet metal is torn into pieces and thrown as projectiles at nearby personnel at high velocities.  Unlike the size of the damage done by a 120 VAC arc flash, the damage done by a 38 kVAC arc flash is not just linearly larger, it’s significantly larger due to the longer time it takes for the arc fault current to be extinguished.

Once an Arc Flash Explosion Gains Momentum, It's Destructive Power Rapidly Explodes Outward

Once a medium voltage arc flash erupts and begins its rapid expansion, if the arc fault current is not extinguished within 200 milliseconds, cables nearby the arc flash are vaporized.  By 300 milliseconds, nearby copper bus bars are vaporized.  And once solid copper is vaporized, its vapor expands to 67,000 times the volume of solid copper and propels the arc flash explosion outward even faster.  If there is still arc fault current driving the arc flash, by 400 milliseconds untreated sheet steel of the equipment is torn to pieces by the expanding arc flash blast – and now that sheet steel is turned into deadly shrapnel and launched as projectiles into the surrounding environment. 

Once started, it does not matter what caused the arc flash – by the time 500 milliseconds is reached, a 10 by 10 by 10 foot volume around the accident has been hit with searing heat, a deafening concussion and lethal shrapnel.  Even if using arc resistant switchgear to protect human life, after 500 milliseconds the gear itself is internally destroyed and needs to be completely replaced.  Any arc flash explosion is always a costly accident.

As More Medium Voltage Switchgear is Made, the Probability for a Deadly Arc Flash Occurence Rises

Arc flash issues were not as problematic before the last decade, as most factories relied upon low voltage based power control which required low voltage power distribution systems.  Most of the medium voltage gear was utilized in power generation facilities or fenced off in cages along one wall of the factory.  But the push for greater production drove the need for greater power control, and so medium voltage automation and control systems became more prevalent.  At the same time, increasing government incentives for renewable energy production enabled these industries to become economical, which drove rapid growth in wind and solar power generation farms – which all use medium voltage power distribution systems for their power collection systems.

Medium voltage power distribution continues to rapidly grow in use among many industries.  But unlike low voltage with its well-known UL and NEC certifications and codes, medium voltage has been guided by utility-led ANSI and IEEE standards and practices that are not as well known, especially in the renewable energies markets that rely upon IEC certified products.  So for many years, care and handling of medium voltage power distribution was practiced by very knowledgeable and experienced personnel who were well trained in the handling medium voltage power distribution systems, so the probability of arc flash incidents was constrained and remained low for decades.

But as more personnel entered the growing medium voltage power distribution systems market, many with only low voltage power distribution systems experience, the number of arc flash incidents began to rise.  As many experienced medium voltage power distribution engineers retired, medium voltage power distribution systems knowledge gradually slipped away just as medium voltage power distribution system emerged in the renewable energies markets.  For several years in a row, OSHA and NFPA have been reporting increases in arc flash accidents, because the experience in handling medium voltage power distribution system has eroded to some extent.

Arc Resistant Switchgear is Not Ideal Protection from an Arc Flash Occurence and Its Explosive Aftermath

So in response, armed with the only solution available – heat and blast resistant sheet metal armor – the electrical industry addressed arc flash explosions by ducting them away to areas where the explosion was thought to be safe to vent – there was no other technology available to do anything better at the time.  Electrical enclosures were made thicker to handle the heat and blast pressure of an arc flash and then vent it somewhere else, instead of allowing the pressure to build up and cause the enclosure to explode.  For the past two decades, arc resistant switchgear utilized this design approach to address the explosion – but not its cause, the unconstrained rise in fault energy fueling the arc.

There are significant unresolved issues with this design approach.  If someone is walking near the vented area and there is an arc flash explosion, they will be subjected to the full force of the explosion’s heat and blast.  It may seem odd that a person would even be near such a vent for any reason, but even though arc flash resistant switchgear has been installed in its own protective room – its ducting has been vented into a hallway in one instance.  The contractor did not fully understand the purpose of the venting, and left to their own accord, implemented the venting incorrectly.  Deployment of arc resistant switchgear is not foolproof.

Even though this is a unique example, it still illustrates arc flash explosion venting is inherently dangerous.  Engineers will insist if arc resistant switchgear is implemented correctly, there is no danger – until the door of that switchgear is opened for the gear to be worked on, a condition in which arc flash protection is lost.  65% of all arc flash incidents occur with the door open during maintenance.  Once an arc flash starts, arc resistant gear does not extinguish the explosion, it allows it to expand so much that the gear is damaged and usually has to be replaced, leading to production downtime ay minimum.  The design approach of arc resistant switchgear is not only inherently dangerous – it also has an unfavorable ROI if in the process of protecting someone it destroys itself.  What business manager would approve of such equipment if another alternative was known to be available?

Now that many industrial, marine and renewable energy markets are demanding more power managed within less volume, arc flash resistant gear vented within these confined spaces combined with the possibility of flammable elements nearby, increases the probability of a significant arc flash accident.  There are indications these hazardous conditions are happening more often. One such possible condition is a wind turbine that uses hydraulic systems for blade pitch control and has a nacelle made out of composite fiber plastic.  If exposed hydraulic fluid is within range of an arc flash explosion – the resultant fire can be catastrophic.  There have been pictures on the internet of a turbine’s nacelle burning down to the ground.  One wonders if everyone got out safely without injury – fatalities within turbines have been traced back to arc flash incidents. 

Another hazardous condition is an oil drilling or production ship or rig managing through an oil & gas blowout accident.  The quick cutting of medium voltage power using arc flash resistant switchgear increases the probability that an arc flash could cause an explosion with fatal results.  How does a drilling platform in the middle of the ocean end up with a rig wide fire? – when an arc flash meets venting oil & gas.  We need to examine the arc flash hazard consequences from the perspective of the damage caused by one single arc flash – it’s not about how often they happen – it’s about what happens when they happen.

Mitigation of an Arc Flash before it Erupts is Always the Best Method of Eliminating the Explosion

It is always better to contain and mitigate the arc flash, than allow it to erupt into an explosive ball of disruptive energy.  So, rather than allow the explosion to proceed out of control using the current design approach of “pointing the gun the other way”, what if there was a way to “stop the bullet in the barrel before it left the gun”?

In this design approach, the arc flash needs to be detected at the speed of light, with a correspondingly quick cut-off of the arc fault current to starve the explosion of its energy.  This design approach – detecting the arc flash light along with the consequent rapid rise in pressure, and then using that information to quickly trip a circuit breaker – was invented a decade ago, but it took years of testing to make it reliable and safe.  Sensors had to become both precise and reliable as well as cost effective, electronic relays had to quickly and reliably trip without nuisance tripping, and circuit breaker contract opening response time had to speed up and reliably work time and time again. 

Today, all three technologies have evolved together to provide robust and reliable arc flash detection and fault current interruption within 60 milliseconds or better for 38 kV “arc flash mitigation switchgear”.  An arc flash incident extinguished by 60 milliseconds will still leave damage about the size of a softball for 38 kV gear – but operators and maintenance personnel need to wear only PPE 1 protective clothing.  When used with 15 kV switchgear, arc fault current interruption is accomplished within 6 milliseconds when using an active grounding switch, resulting in almost no damage to the switchgear.

Since the explosive energy of an arc flash is significantly contained, arc flash mitigation switchgear enclosures can remain sealed with no ducting or venting, and so the probability of an arc flash explosion outside the switchgear is eliminated when using arc flash protective relays.  By using today’s advanced techniques of “electronic watchdogging”, the relay continuously performs a self-check on its own circuitry, thereby assuring the operator and maintenance personnel of the condition of the protection system at all times.  As a result, are flash mitigation switchgear is more compact yet handles just as much power as its larger arc resistant switchgear cousin with greater safety.  And since arc flash incidents do not erupt into the kind of explosions that destroy arc resistance switchgear, not only is arc flash mitigation switchgear safer – it has a very favorable ROI for business managers looking for safer and less destructive alternatives.

Arc Flash Mitigation is Only the First Step to Significantly Improving Medium Voltage Switchgear Safety

To further enhance the safety of arc flash mitigation switchgear, manufacturers will include remote rack in & out of the circuit breaker with the enclosure door sealed shut as a standard feature of the gear.  The next safety feature switchgear manufacturers should include standard is an internal grounding switch, to ensure the safety of maintenance workers from accidental up or downstream electrical backfeed while working inside the switchgear.  And of course, insulated busbars and insulating boots around all busbar connections is a demonstration of prevention is the best medicine, by eliminating arc flash incidents due to accidental tool drops.  By implementing these additional safety counter-measures along with arc flash mitigation protection, medium voltage power distribution systems can be taken to a new level of safety and performance regardless of operator or maintenance personnel knowledge and experience regarding medium voltage power distribution systems, without compromising cost per watt or ROI.

In Summary

Let’s work together to drive the 30,000 annual arc flash incidents down dramatically, as well as drive the 2,000 annual hospitalizations and 400 annual fatalities from arc flash accidents to zero, now that medium voltage arc flash mitigation switchgear is available.  And let’s set our sights on using similar technologies to address these same issues that have been plaguing high voltage power transmission and distributions systems for years. There are always newer and better ways to improve safety and performance without compromising cost, whenever one is open to evaluating and assessing new ideas.