14 March 2018
THE HAM‘S WIRELESS WORKBENCH ; BY WARD SILVER N0AX
How High is High?
Let’s talk a bit about what the “high” in “high voltage”
means. To a designer of logic ICs, a few tens of volts might
be “high voltage.” In the AC power world, voltage might
need to exceed 1 kV to be properly classified as “high
voltage.” To the Tesla coil builder or EMC (electromagnetic
compatibility) engineer, several kilovolts (kV) might be
needed to get the high voltage juices flowing. This column
will assume you’re working with AC line voltages and
Start by referring to the sidebar “High Voltage Safety”
in the November issue. All of those things are important.
A comment in the digital Nuts & Volts issue by Larry
Reynolds K4MLA noted an old and effective rule: “When
working with high voltage equipment, keep one hand in
Why this is a safe practice is discussed in a sidebar this
month. Heed that rule and all of those in the November
issue — we want you as a reader for a long time!
We’re all familiar with the maximum current rating
for a fuse, but fuses have a voltage rating too. If you have
a glass cartridge fuse handy, on the end with the current
ratings stamped into or printed on the metal there will also
be a voltage; for example, “3A 250V.” The rating tells you
how much voltage the fuse can withstand after the element
melts and opens.
Be aware that a fuse needs to withstand the peak AC
voltage which is 1.414 times the RMS voltage. Peak voltage
in a 120V AC circuit can easily exceed 170V. In higher
voltage circuits, special HV fuses must be used.
If the voltage across the fuse is higher than its rating,
an arc may develop. The arc may continue to conduct
current, defeating the purpose of the fuse entirely! Some
fuses for higher voltages are filled with sand or another
insulating material that block current after the metal
element melts. Never use a low voltage fuse in a higher-voltage circuit.
When can you use an automotive-type blade fuse? They are inexpensive
and widely available, but the standard
ATO fuse is only rated for 32V.
These must not be used in a circuit
connected to, say, the AC line’s 120V
Another tip — when wiring a panel-mount holder for cartridge-style fuses:
Connect the “hot” or “live” circuit to
the rear contact so that when the fuse
is removed from the holder, its rear
contact disconnects from the power
source. Otherwise, the holder will be
energized as you pull the fuse out.
Working with High
he November 2017 installment of Ham’s Wireless Workbench
about vacuum tubes and tube-based equipment included a
sidebar about safety with tube-level voltages — it seemed to
me a column on some of the issues associated with those voltages
would be interesting. There are lots of circuits for generating high
voltages out there, but what about the “gotchas?”
Hand in the Pocket
One of the oldest rules in the ham’s safety list is to keep one hand in your
pocket or behind your back whenever adjusting or measuring equipment
containing a voltage high enough to be a safety hazard. Why? Most workbench
shocks come from a hand or arm touching an energized component or wire.
If the lowest resistance path for that current goes through your other hand,
that conducts current through your heart and that is the most dangerous path. So,
don’t create that path!
If you’re wearing pants and shoes with insulating soles, the current path has
a much higher resistance so less current will flow. Better yet, the path from a
hand to a foot doesn’t go through the heart. Make it a habit to work one-handed
around energized equipment and you’ll be a step ahead of electricity.