Date: Sat, 24 Dec 2016 20:17:43 -0800
From: Grant Saviers <grants2@pacbell.net>
To: john@kk9a.com, towertalk@contesting.com
Subject: Re: [TowerTalk] Lightning Protection for Orion Rotor
MOVs provide overvoltage protection but have a downside as they degrade
with every snubbing shunt and eventually fail. On an AC power strip, a
failed to short MOV burned up a nearby ham's shack here when the plastic
power strip case ignited. If on antenna control lines, sufficient
degradation can be experienced over time to prevent normal operation.
As one antenna manufacturer discovered before they switched to TVS
diodes. Of course any transient suppression device can be vaporized
with sufficient power surges, but some pretty hefty capacities are
available.
http://www.littelfuse.com/products/tvs-diodes.aspx
Grant KZ1W
## Which is why a lot of MOVS come with an internal fuse built into them.
These
types have 3 x leads. The 3rd lead is slightly offset from the mid point
between the
2 x main leads. Done that way so the MOV can not be inserted into the PC board
backwards.
## Fuse is in one leg of the standard MOV.. but internal to the MOV. They tap
the output junction of
the fuse.. then wire to the 3rd lead. There is 120 vac on that 3rd
lead...which activates the red led
on the better quality power bars. IF the MOV eventually shorts out, from one
too many hits, the
internal fuse will open off...and with no 120 vac on output of the fuse, the
led will also go out.
That tells the end user, he no longer has any protection available.
## Movs are great, but they should be fused. The fusing can be done
external..done all the time.
You see this done on the main 200A panel in homes. We used the 208 3 phase
version at the telco
I worked for, with 3 x huge hockey puck sized MOVS, one per phase, and each
with its own fuse.
Output of fuse was wired over to both a Neon... now a led used these days....
and also the 120 vac
coil of a 4PDT mech relay. If a mov shorts out, neon or led also goes
dark..and the normally operated relay
drops out, due to no more coil voltage. With relay dropped out, contacts
close, and that puts a ground to the
alarm gathering point. Then the alarm can be seen at a remote alarm
monitoring center.
## of course, when the commercial AC power goes off for any reason, like a
fallen tree etc, ALL the neons
-leds will go out... and all 3 x relays will drop out.... bringing in the same
alarm....... failed lightning protection.
No big deal since the incoming power to main panel is also alarmed, so it too
will go into alarm.
## Plenty of house fires started by shorted... UN fused MOVS in everything
from power bars to you name it.
Sad really. You install a device thinking it will protect the delicate
equipment, but after successive hits, it eventually
shorts out, creates a ton of heat, fire starts, meanwhile the 15A breaker in
the panel never tripped.
## any of these devices will work, but use the correct sized..and type of fuse
for them.
Jim VE7RF
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|