Maintenance Technology July 2017 - 14

PLANT PROFILE

The M2 switchgear (left) is the safest medium-voltage equipment in any Mercedes-Benz facility, worldwide. Power-system functions
are conducted at the HMI (human-machine interface) module (right), located safely away from the switchgear.

instead of having to make a mad dash to
get it all done over a weekend."

Technology's safety benefits
Safety was a major emphasis for
Sagan when designing the new unit
substations for the plant expansion.
Power-system studies revealed that
the most dangerous arc-flash hazards
in the plant were found at the 480-V
main breakers at the unit substation
transformers. A typical unit substation
steps down from 13.8 kV to 480 V for
distribution in the shop areas. In a
conventional transformer protection
scheme, the primary disconnect switch
has fuses that protect the transformer.
"The problem is that if there is an
arc-flash event when someone is opening
or racking out a 480-volt main circuit
breaker, you are relying on the fuses
on the high side of the transformer to
blow to turn that arc flash off," Sagan
explained. "Because the fault current
has to go through the windings of the
transformer, the fuses are operating in
their time-delay range. The amount of
current available at the transformer, and
the longer clearing time of the fuses,
result in extremely high levels of arc-flash
energy. There is no personal protective
equipment available that can protect a
person in that situation."
To mitigate such dangerous levels of

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MAINTENANCE TECHNOLOGY

arc-flash energy, Sagan used a transformer differential protection scheme on
the new unit substations. The fuses in the
primary disconnect switch are replaced
with a medium-voltage vacuum circuit
breaker. A differential relay measures
the current going in and coming out of
the transformer. If there is a difference
between the two, then the relay assumes
there is a short circuit inside and issues a
high-speed trip command. The relay also
has the ability to detect fault current on
the 480-V main breaker.
"Because it can trip the primary circuit
breaker so much faster than fuses would
operate, this scheme reduces the arc-flash
hazard to much safer levels," Sagan
explained. "All of the new substations
utilize that scheme. And, with the addition of remote-control panels for all of the
circuit breakers, the whole unit substation
is much safer to operate."
A substation such as this may have
between 15,000 and 20,000 A of fault
current available. The arc flash that would
be created could be between 30,000 and
35,000 F-four times hotter than the
surface of the sun.
"We are definitely dealing with
extremely high levels of energy," Sagan
said. "It can be extremely dangerous to
workers who have to interact with this
gear. By utilizing protection schemes
that are designed to reduce the arc-flash

energy levels, we are also in effect making
the system more reliable. An arc flash
with that much energy could kill somebody and is also doing a tremendous
amount of damage to the equipment. So,
if I can reduce the amount of damage
to the equipment I may also be able to
reduce the cost to repair or replace it."
Sagan achieved his goals to make the
power system safer and more reliable.
With almost four decades of experience,
he said he considers this the greatest
engineering achievement of his career.
And it all started with a blank page
and unlimited opportunity. MT
Visit maintenancetechnology.
com/1707profile to watch videos in
which electrical engineer Randall
Sagan provides more detail about
the Mercedes-Benz power system
he designed.

Michelle Segrest is president of Navigate
Content Inc., and has been a professional
journalist for 28 years. She specializes
in creating content for the industrial
processing industries and has toured
manufacturing facilities in 46 cities in
six countries on three continents. If your
facility has an interesting maintenance
and/or reliability story to tell, contact her
at michelle@navigatecontent.com.
JULY 2017

http://www.maintenancetechnology.com/1707profile

Table of Contents for the Digital Edition of Maintenance Technology July 2017