Electrical Hazard -- Power Press
KP had worked for many years at a plant that manufactured prefabricated
structural components for buildings. One assembly line made use
of a power press that fastened the wood sections together. Power
for the press was delivered by way of several electrical conduits
that ran underneath the assembly table. The electrical conduits
were solid bars surrounded with insulation except for a groove where
a brush made contact with the conductor.
Periodically, a brush assembly would come detached from the conduit
and the press would come to a halt. When this happened, the main
disconnect switch, which was located in a nearby shed, was turned
off ; and the switch on the power press was turned off.
On the day of the accident, the above procedure was followed. The
power light on the power press was off when KP's supervisor told
him to go underneath the table and check the brushes. A few moments
later, KP indicated that, indeed, there was a problem with one of
the brushes. A scream was then heard. KP had been electrocuted .
Subsequently, it was determined that the main power switch, which
was a rotary knob, had failed to break the circuit when it was rotated
to the "off" position. Considerable effort was allocated to determining
the mechanical causes of the switch's failure.
HF Issues: This graphic shows the method used to identify
some of the issues in this case.
HF Investigation and Analysis: Material reviewed included
accident reports, video of the accident site; equipment manuals;
and depositions of all the parties involved.
There were several human factors issues. The employee who rotated
the main disconnect knob to the left towards the "off" position
did not rotate it far enough to break the circuit, though he believed
that he had. Human factor considerations included the location of
the control's legend, which was obscured by the operator's hand;
changes in control resistance with knob rotation; and other sources
of feedback as to the position of the knob.
| In addition, consideration was given to
whether the electrical conduits were provided adequate guarding in
terms of both proximity to the operators (guarding through distance)
and the dimensions of the gap in the insulation..
Another human factor's issue concerned the status of the indicator
light on the power press's control panel. KP's supervisor maintained
that he had turned off the power press, and that the indicator light
was dark. An engineer examined diagrams of the circuitry of the
power press and opined that power to the indicator light would be
interrupted if the brush assembly for one, and only one, of the
conduits was not properly aligned. In other words, the conduits
could still be "hot" even if the indicator light on the power press
indicated that no power was being delivered to the machine.
To illustrate the interaction between the main power disconnect
switch, the alignment of the electrical conduit brushes, and the
status of the indicator light on the power press, I constructed
an interactive computer model in Working Model 2D (Knowledge Revolution).
The model of the main power disconnect switch could be rotated and
end up in one of three positions. The far right position resulted
in the power being disconnected, while the circuit remained intact
in the remaining two positions. The model permitted manipulation
of brush contact with the conductor for three conduits. As shown
in the following two graphics, if, for example, the middle brush
was not making contact with the conductor, the power indicator light
on the power press would correctly indicate that power was still
being delivered (provided the main disconnect switch was not rotated
all the way to the right).
However, if under the same conditions, the brush
of the conduit on the far right did not make contact with the conductor,
the indicator light on the power press failed to remain on. (See
figure below.) In my opinion as a human factors specialist, this
accident could not be solely attributed to defects in the main power
disconnect switch. Further, it illustrates the importance of adopting
a systems approach to identifying what can go wrong in equipment
Steamboat Island NW
material may be used freely provided you
reference this source:
G. David Sloan, Inc., 1999, www.gdsloan.com, Olympia, WA, USA
design by Alex Gheorghiu, Chuck Mathison & Pam Johnson