conversation during a spring NAB Show workshop turned to tightening
fuses and disconnect box connections.
Fig. 1: The results
of a hard fault on a three-phase disconnect.
are always a good idea, but should nevertheless be performed only by
Wayne Eckert of
Channel 1 Images sent in this picture of a three-phase electrical
service, fed underground via the PVC conduit at the bottom of the
panel. Cabling runs up to the knife switches at the top of the panel,
then down to the fuses and finally into the building through the
lower conduit exiting the rear of the panel.
There was a hard
fault on the leftmost phase (phase A); its fuse failed to extinguish
or contain the arc and exploded. The plasma from that explosion
caused a phase-to-phase arc between phases A, B and C. This vaporized
the fuse holders and burned up the B and C phase fuses, the shells of
which can be seen lying in the bottom of the panel (Fig. 2). The
current finally rose to a point that the power company’s
distribution transformer primary fuses dropped and killed the
Think about the risk
of personal injury here. Plasma pressures inside the panel were
sufficient to bend the cover. Heat vaporized the rear of the panel,
exposing the wall behind it. This all happened as a flash, not a
long-term arc, lasting only 30 to 60 cycles or a second or less.
Imagine what this
level of energy would do to flesh, muscle and bone had the panel been
opened when an engineer was standing before it and not wearing proper
Leave electrical panel work to licensed electricians. You certainly
may know how to maintain electric panels; but operating as an
unlicensed electrician can still leave you open to liability, should
* * *
DeSpain hails from Fort Worth, Texas, and is a registered
professional engineer. He writes to warn engineers about using even
very fine sandpaper to burnish relay, contactor or switch contacts.
Fig 2: There’s not
much left after this fire.
Yes, the sandpaper
may clean off the dirt, but disastrous results will follow. The
abrasive used in the sandpaper will embed itself in the relatively
soft contact metal, preventing complete contact closure. This can
result in intermittent operation or accelerated failure. Sandpaper
also can remove too much of the contact metal and change the contour
of the contact.
Use a diamond
contact cleaner or a piece of typing paper instead. David also
recommends a strip of crocus cloth, which is similar to emery paper
but softer and sometimes hard to find locally. It’s not cheap but
can be found online at a source like Olsen Industrial
A 3-inch by 50-yard
roll is about $60, but a little goes a long way. Maybe buy a roll and
divide the cost among your market engineers.
* * *
the last day of the NAB Show, the Telos Alliance celebrated the
wedding of Janis and Inga (Jahimcika) Timma. Janis is a software
engineer at the company’s Riga office.
It reminds me that
25 years ago, Delta Electronics’ AM stereo technician Chris Wilk
married Delta sales secretary Michelle Howe at the NAB Show.
Judith Gross, then
Radio World’s editor, and I witnessed that wedding at Circus
Circus. It’s hard to believe that many years have passed. Chris and
Michelle are still happily married, have three grown girls, and Chris
is now the chief engineer for Delmarva Broadcasting’s Delaware and
Salisbury/Ocean City Maryland properties.
Good things do
happen in Vegas. We extend our congratulations to both couples!
Workbench. You’ll help your fellow engineers and qualify for SBE
recertification credit. Send Workbench tips to firstname.lastname@example.org. Fax to (603) 472-4944.
John Bisset has spent 44 years in the broadcasting industry and is
still learning. He handles West Coast sales for the Telos Alliance.
He is SBE certified and is a past recipient of the SBE’s Educator
of the Year Award.