[Amps] Vacuum Relay Question

[MU 4CX250B]

I’m open to your suggestion, Rob,  but don’t understand it. What do
you mean by “low v high current setup”?  The matching network is in a
very compact weatherproof enclosure at the base of a 70 foot
self-supporting aluminum mast, with sixty 120ft radials. It is an
unusual design with a large toroid providing the needed inductance.  I
was initially apprehensive about using a toroid in this application,
because I wasn’t sure I could provide adequate HV isolation. However,
I have used the network for several years, with no toroid heating,
flashovers, or problems of any sort, until a few weeks ago, when the
entire device was destroyed by a direct lightning strike.

The strike literally exploded the enclosure and destroyed its
components, including the enclosure, several ceramic standoffs, a
length of 3/16 inch copper tubing, a sheet of 1/8in Teflon insulation,
and a Kilovac H16 vacuum relay, the latter rated at 12kV/50A. It
looked like a hand grenade had exploded inside the outdoor-rated
plastic enclosure. Interestingly, the toroid core survived, though its
12AWG winding had mostly vaporized.

The matching unit is 800ft from my station, and operated remotely. It
has been tested at 3kW on 80m and 160m and fed with buried hardline
and a two conductor control cable. A large grounded 100uH airwound
(of 16awg wire) inductor is permanently attached to the aluminum mast
to bleed off charge.

I’ve now installed a large knife switch to disconnect the entire
network during our summer monsoon season, but I’m not sure two inches
of additional isolation  will be adequate to protect against a direct
strike. The network is at DC ground and is tied to the radial field.
In hindsight it might have been better to let it float when it isn’t
in use. I can argue that both ways.
73,
Jim w8zr
Sent from my iPhone

> On Jul 26, 2021, at 4:08 AM, Rob Atkinson <ranchorobbo at gmail.com> wrote:
>
> I would not use a vacuum relay in a matching network for a 1/4 w.
> antenna.   That's a low v. high current setup calling for open frame
> contactors or at least large open relays.  Perhaps this information
> will render your question moot.
>
> 73
>
> Rob
> K5UJ
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