August 2015 13
contained and not propagate beyond the box. Obviously,
with the joules potentially available in a vehicle fuel tank,
the design is not intrinsically safe; nor are the electrical
circuits in the fuel tank housed in an explosion proof box.
Please explain the concept employed to prevent
explosions in vehicle fuel tanks.
— Dennis Quinn
AIf you are not a licensed electrician, you are mighty insightful into a deep, dark secret of the electrical
trade. The National Fire Protection
Association’s National Electric Code
(NEC) Article 500 classifies areas
where gasoline vapors are (Division
1) or may be present (Division 2) as
a Class I/Group D hazard location
(there are also temperature and zone
classifications, but I am trying to
keep it simple). So, fuel tanks would
be classified as Class I/Group D/
Division 1 locations when specifying
the necessary safety precautions
for electrical/electronic equipment.
[The NEC is written to cover every
conceivable situation, and is thus a
very complicated volume which is
for those who really like to read and
follow various circuitous paths to find
a simple answer.]
I worked in a Class I/Group D/
Division 1 chemical plant and our
electrical installations were both
expensive and exacting: A general-purpose enclosure was
made of stamped sheet metal with a simple latch, whereas
an “explosion proof” enclosure was cast aluminum with 20
or 30 bolts to close the gasketed cover, and the threaded
hubs were designed for a tight seal to prevent infiltration
QUESTIONS and ANSWERS
Post comments on this article at
Re: Burn Marks on LCD TV Screen
Thanks for answering my question in the March issue
of Nuts & Volts. I turned down the backlight and the
brightness, and that resolved most of the problem. I used
a snow pattern and a pixel protector, but neither worked.
Probably because the image is too persistent. Thanks
Mark Erickson, El Paso, TX
Mark, I am glad I could help you. Technology today is
moving so rapidly it is sometimes hard to keep up with. So,
the questions of our readers are helping me to stay on top
of the rapidly changing field of electronics.
Re: Op-Amp Accuracy Question
Thank you for the answer to my question on op-amp accuracy in the May 2015 issue. It was exactly the
answer I was looking for. I will order the Analog Devices
part and test it for future use. In the past, I have found
that any trimpots in the circuit are subject to temperature
variations that add to the inaccuracy.
Also, I need to look for an app that gives me lunar-planetary alignment. Thanks.
Milton, that is what I love about doing the Q&A —
helping readers solve their electronic problems. A couple
of websites for astronomy apps for the iPhone are: www.