Arc-flash, also known as flashover, refers to a large discharge of energy that occurs due to unexpected arcing in electrical systems. It can release heat up to 35,000 degrees Fahrenheit and air pressure blasts that are enough to kill. While circuit breakers are meant to prevent arc-flash, the current during an event may remain low enough that breakers do not trip. Arc-flash incidents are common in the U.S., injuring many workers, and data center outages have been linked to arc-flash failures. Some experts argue accidents will always occur, but others like Chris Crosby believe more can be done to prevent unnecessary risks by not working on live equipment and ensuring proper redundancy as required by

1. Source: US MSHA
Arc-flash: discharging your moral duty
17 April 2015 By Michael Kassner
Working on live power systems can kill. Is 100 percent uptime worth it?
Chris Crosby is passionate about this industry. His company Compass Data Centers will
write a check for $100,000 (URL=http://www.datacenterdynamics.com/news-
analysis/compass-offers-100k-refund-for-delayed-builds/89207.fullarticle) to any client that
doesn’t get a fully operational data center within six months. So when Crosby tells us that
preventing arc-flash accidents is a moral imperative
(URL=http://www.compassdatacenters.com/the-moral-imperative/) , we should listen. Arc-
flash, also known as flashover, can cause injury and death.
What is an arc-flash event?
Summer lightning is a dramatic arc-flash event,
but Crosby is talking about discharge in the
data center.
“Arc-flash events occur when electrical systems
do not do what you want them to,” he explains.
“A lightning bolt, basically, comes out. It comes
out with heat approaching 35,000 F and often
creates a pressure blast of up to 2,100 pounds
per square inch. That’s enough to kill someone
without getting electrocuted.”
An arc-flash will release energy rapidly, due to
unexpected arcing between two phase busbars, or a phase busbar and a neutral or
ground. Mike Holt (URL=https://www.mikeholt.com/mojonewsarchive/NEC-
HTML/HTML/What-is-Arc-Flash~20040512.php) , NEC expert and noted author, says it is
a self-sustaining process like that used in electric-arc welding.
“The fault has to be started by something creating the path of conduction or a failure such
as a breakdown in insulation,” says Holt. But it continues after the physical fault is
removed.
“The cause of the short normally burns away during the initial flash and the arc fault is
then sustained by the establishment of a highly-conductive plasma. The plasma will
conduct as much energy as is available and is only limited by the impedance of the arc.
This massive energy discharge burns the bus bars, vaporizing the copper, and thus
causing an explosive volumetric increase; the arc blast, conservatively estimated, has an
expansion of 40,000 to 1.”
Shouldn’t the circuit breaker trip?
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2. One would think that circuit protection would prevent this from happening. I asked Paul
Estilow (URL=http://www.dlbassociates.com/about/) , principal engineer at DLB
Associates and a thirty-year veteran in the electrical power industry, why circuit breakers
do not always prevent arc-flashes. “The problem is that the fault current during an arc-
flash event may be less than the rating of the circuit breaker,” Estilow told us in a phone
conversation. “Because there is high resistance in the arc, the current level remains
relatively low, while the amount of energy builds, leading to an explosion.”
Estilow added there are circuit-protection devices designed specifically to mitigate arc-
flash, but they are expensive and due to the vagaries of arc-flash the specialized devices
may not always work as expected.
How serious is this?
Arc-flash events or more correctly accidents are not new, they have been around as long
as man-made electricity. What may surprise most people is the prevalence of arc-flash
events and the resultant injuries. Richard B. Campbell and David A. Dini, authors of this
Fire Protection Research Foundation (associated with the NFPA) report
(URL=http://www.nfpa.org/~/media/files/research/research-foundation/research-
foundation-reports/electrical/rfarcflashoccdata.pdf?la=en) write, “A common estimate of
arc flash occurrence is that there are 5 to 10 arc flash explosions in electrical equipment
every day in the US, but the origins of this estimate are unclear.”
Campbell and Dini found the estimate seems to be borne out by official documentation.
“Literature on electrical injury has tended to focus on shock and electrocution, while
devoting comparatively little attention to injuries resulting from arc flash or arc blast,”
conclude the authors. “Research on electrical burns nevertheless shows that burns from
electric flash are responsible for many of the work-related burns treated at burn centers.”
• A Michigan burn center found that 34 percent of patients injured on the job received
flash injuries.
• Arc flash injuries represented 55 percent of the electrical work-related burn injuries
in the Ontario research.
• A study of electrical injuries over a 20-year period at a Texas burn center found that
40 percent of burns were electrical arc injuries
Why is this happening?
As to why this is happening, some experts say it is the nature of the beast: accidents will
happen. Estilow provided a “for instance.” A technician could innocently switch a circuit
breaker on and trigger an arc-flash event; simply because a nest built by a field mouse in
the electrical box controlled by circuit breaker shorted out.
Accidentally initiating an arc-flash event is possible anywhere electricity is used. That is
until scientists and engineers figure out a preventative solution. However, Crosby’s
concern is more focused. Being in the data-center business, he is alarmed by what he
considers to be an inordinate amount of arc-flash events in data centers.
A well-published example would be Siobhan Gorman’s 2013 Wall Street Journal article
Meltdowns Hobble NSA Data Center
(URL=http://www.wsj.com/articles/SB10001424052702304441404579119490744478398) .
Gorman says electrical problems caused ten meltdowns in a 13 month period, primarily
due to arc-fault failures in the back-up generators. She adds that an official described one
occurrence as a flash of lightning inside a two-foot box that led to a fiery explosion.
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3. Crosby feels that an excessive focus on availability adds to the problem. When data-
center operators guarantee 100 percent uptime, staff end up working on energized
equipment, in order to to keep the customer-facing equipment up and running.
According to Crosby and Estilow there are only three reasons to work on energized
equipment — a major reason arc-flash events happen in data centers:
• Interrupting the electricity would endanger human life
• Shutting down power would make the life-threatening situation worse
• Certain electrical tests that can only be performed wih the equipment energized
Crosby contends that there are many instances where work is being done on energized
equipment in data centers for reasons other than those stated above. Crosby also
contends that those reasons should not exist. If a data center is offering 100 percent
uptime, the redundancy required to meet that guarantee needs be in place, and that same
redundancy should allow any work on electrical systems to be accomplished with the
systems in a de-energized state.
The Uptime Institute has provisions for that in the organization’s tier-rating system
according to this Green Server Room paper (URL=http://www.greenserverroom.org/Tier%
20Classifications%20Define%20Site%20Infrastructure.pdf) that defines site infrastructure
with regards to the tier-rating system:
• Tier III Concurrently Maintainable Site Infrastructure: A concurrently maintainable
data center has redundant capacity components and multiple independent
distribution paths serving the computer equipment. Typically, only one distribution
path serves the computer equipment at any time.
• Tier IV Fault Tolerant Site Infrastructure: A Fault Tolerant data center has multiple,
independent, physically isolated systems that each have redundant capacity
components and multiple, independent, diverse, active distribution paths
simultaneously serving the computer equipment.
DLB Associates’ Estilow explained the difference. Think of Tier III as have two
independent power systems A and B. If system A is down for repair or maintenance,
system B is powering the data center. However, if system A is down for maintenance, and
then an outage occurs with system B, a Tier III data center could go down.
Tier IV provides two redundant power systems and is fault tolerant, meaning that upon
failure of any individual equipment the system will switch to the redundant system without
affecting the IT load.
In any case, both Tier III and Tier IV allow technicians to work on electrical equipment
when it is powered off.
Crosby’s reasoning
This is a complicated issue, with data center management having to weigh risks versus
consequences. To that end, Crosby shared a personal experience:
“Spreading the word about the danger of arc flash is important to me personally because I
have seen how devastating it is when there is a fatality on a project. At a company I
worked at previously, a worker tragically was killed during construction of a data center
when a pressurized safety system exploded.
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4. “I don’t want anyone in our industry to be exposed to unnecessary risks, and the risk of
arc flash is dramatically minimized when the right procedures are in place. Data center
employees deserve to be safe and their families deserve to know that safety is a top
priority in our industry.
“Arc flash is deadly, and companies in our industry should be doing much more to create
safer working conditions that minimize the chance of it happening. It’s the law, but it’s also
our responsibility as employers who have the lives of our workers in our hands.”
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