The Development of Short
Circuit Ratings for Industrial Control Panels
Custom industrial control
panel
Until
the late 1990s Industrial Control Panels (enclosed electric devices not
intended for residential use) did not have standardized short circuit ratings.
These panels were built to accommodate diverse industrial applications and
could not be easily standardized.
A
short circuit occurs when the electrical dielectric (insulation or air gap) fail. When
this happens the “flow” or current of electricity increases to levels that could cause a
fire, explode, or cause a person to be shocked.
Enclosed devices damaged by
short circuit
Short circuits are usually prevented by means of protective devices, such as a
circuit breaker or a fuse. Protective devices are designed to stop short
circuit currents by opening (creating an air gap that the electricity cannot jump
across). Some protective devices are faster to open an air gap than other
protective devices.
This
means that some devices lets through more short circuit current than others. It was an engineering problem whether the protective devices could
actually protect the other devices in the panel.
When I was clean shaven
with dark hair
When
I worked for Moeller Electric (no longer exists) it had very good protective
devices. It limited the let through current (the overcurrent that gets through
the protective device before it opens) we engineers were able to easily
determine what devices could be protected without fire, explosion, or shock. The devices were mainly International Electrical Commission (IEC) designs.
Although
we could determine short ratings for industrial control panels, these ratings were not
always accepted by the regulating authorities or by the customers.
So
we set out to create a national standard that included short circuit ratings
for industrial control panels.
The first step was to create a new standard at
the National Electrical Manufacturers Association (NEMA). I was assigned this
task and a subcommittee of NEMA SC2 was formed. Because Moeller Electric
initiated the project, I was made chair. We met mostly in Clearwater Beach
Florida which my wife and I enjoyed. We still miss the area.
One of our articles on
short circuit ratings for panels
The
subcommittee did not last long as Underwriters Laboratories Inc. took over the
effort by drafting its own standard UL 508A. This was actually a better
situation for the development of short ratings as the editors were not
competitors.
Once
written and approved UL 508A became the basis for the National Electrical Code
(NEC) published by the National Fire Protection Association. An increased degree of
safety was added to the electrical construction industry. I am still happy
today for being involved with the creation of the then new safety standard.
Some of the competitor manufacturers had old
style equipment and resisted the inclusion of IEC types of electrical devices. Their resistance to the new
technology lasted until all of the big manufactures could develop or buy IEC
devices to compete with devices made in the European Union.
Unfortunately
the resistance to IEC devices continues today to the disadvantage of American
panel makers and consumers. For example the science of insulation dielectric is
prohibitive on the feeder side of a branch circuit device in the USA. There is no scientific or safety reason for this
prohibition.
There
were many heated arguments during our advocacy of short circuit ratings
for industrial control panels. Despite them we understood that we were representing the
interests of a companies for which we worked. Although heated and loud the arguments
were never personal. In 2004 I went to work at the College of the Holy, but
have kept in touch with our former adversaries.
