Navigating the Hazards of Engineering
Our past few issues have
generated some interesting responses from our readers. Let’s start with an
observation from contract and projects engineer Ed Dulaney. After reading Mike
Vanhooser’s tip about using concrete to protect copper strap (Workbench July 18), Ed said he had to
shudder. He writes, “It’s a very bad idea to put a ground strap into anything
so rigid as concrete.”
1: Straw, grass, and nesting materials don’t mix with a 50 kW ATU.
Ed explains that when lightning hits an electrical conductor, it induces
a massive electromagnetic field. When this EMF travels along the conductor, it
literally “vibrates” with the energy that is induced into it. Ed has seen
photos of concrete pads that had copper rods in them, which were then struck by
lightning. The damage is minimal and collateral damage is limited, as most of
the pieces of concrete fall less than 10 feet from the point of entry. But the
forces are evident.
However, imagine if the same thing
happens to the base pier of a tower, where the strap is covered in concrete. A
few fractures in the pier are all it would take for the tower to come tumbling
Ed says he’s all for covering copper straps in
tar, paint or other pliable material. But he’d never put anything that has the
possibility of carrying a million Joules of energy into a completely rigid
Ed raises a valid point; but for the trenches in
Mike’s case, the strap was laying in the trench, covered with dirt, and the
concrete was used as a cap to discourage digging. Amazing the schemes we have
to come up with to save our copper!
Marc Mann provides an interesting comment,
after reading Greg Richwine’s clean-up of beryllium contact shavings (Workbench
Aug. 1). To prevent the spread of the shavings and dust, Greg carefully wiped
up the residue that lay under the contactor. His caution was warranted, as
beryllium is a known carcinogen. Marc wanted to bounce Greg’s procedure off a
friend who is a retired industrial hygienist and OSHA inspector —but don’t hold
that against him, Marc adds.
More important, the OSHA inspector mentioned
something that Marc hadn’t considered: There is a reasonable probability that
during the erosion of the beryllium contacts, a micro aerosol of BeO (beryllium
oxide) may have been generated. If the device had a cooling system, i.e., a fan
or blower, and the effluent (heat and BeO aerosol) was not vented to the
exterior atmosphere, the room containing the transmitter may well be
contaminated and an air test would be prudent.
Now in Greg’s case, this was not a
transmitter, but a phasor cabinet – which I guess could have a blower
associated with it but more than likely, does not. However, the hazard still
remains and should be considered by engineers doing maintenance.
Anyone in a similar situation should stay
aware of this and notify any employees who frequent the area of this potential
health hazard. Marc realized some readers think OSHA requirements are too
strict at times; however, beryllium is nasty stuff and should be treated with
the same respect as when transformers with PCBs need remediation.
How much is too much, according to OSHA?
The amount or length of exposure to beryllium
necessary to cause a specific individual to develop CBD is not known, but
recent information suggests that even short exposures to levels of beryllium
below OSHA’s Permissible Exposure Limit (PEL) of 2 µg/m3 averaged over an
8-hour day may lead to CBD in some workers.
Fig. 2: Check the tower base spark gap, making
sure the balls are smooth.
Our Radio World Links page (radioworld.com/links)
has a link to an OSHA safety bulletin that readers may find useful. Simply
Googling “Beryllium contact hazard” will provide even more information. Thanks
to Marc Mann for his thoughts.
No beryllium evident in Fig. 1; just a lot of
rubbish, mostly collected by rodents. This abandoned 50 kW site is soon to be
back on the air, but only after some extensive cleaning and maintenance. Be
sure to use a facemask when removing the rodent droppings and nesting materials
from inside ATUs. Tighten the connections, and ensure things like the spark gap
on the tower (Fig. 2) is properly set.
Note the gouge on the bottom ball. To guard
against sustained arcing across the gap, use crocus cloth to smooth out any
pitting caused by lightning strikes. Thanks to Sim Mangga of Malben Engineering
for the pictures.
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Author John Bisset has spent 43 years in the broadcasting industry, and
is still learning! He is SBE certified, and is a past recipient of the SBE’s
Educator of the Year Award. He recently joined Elenos USA, an FM transmitter company
based in Miami.