Working for the power company, I've seen a fair amount of carnage from MOV
failures in our poletop SCADA radio equipment. MOV's usually short or
partially short when they get overloaded unless or until they get blown or
burned apart! The larger they are, the more dramatic the failure and the
bigger the flame potential. I wouldnt want any of these directly on my AC
supply in a plastic box or unsealed metal box or without proper fuse or breaker
protection. Note that most outboard power strips now contain MOV surge
suppression but they all have breakers built in. I've had a few of these fail
in my shack as well, a couple with evidence of minor charring outside and
completely destroyed inside. However, you have to keep in mind that even if
it's hanging off a 15A breaker in your panel, there is the potential there for
it to create up to 2KW worth of heat before that breaker trips. There is
definitely potential for a fire there, but I'm not sure its any greater than if
werent there as long as they are used properly. If the MOVs hadnt been there
to burn up, something else a lot more expensive probably would. Your meter
base solution sounds like a great way to go as long as it's fused and sealed
well, and you dont have flammable siding! The potential power dissipation at
that point is huge if something goes wrong, but like you said, at least it's
Jan 3, 2011 07:11:49 AM, firstname.lastname@example.org wrote:
I've been in the fire service for 27 years now and I can't tell you the
number of surge suppressors of the type that are typically installed on
a 1/2" or 3/4' knockout hole*http://tinyurl.com/2cpxn7a *that created
the structure fire that we were responding to. On a couple of
occasions, I've seen them violently shoot green flames out of them in
every direction as they burned themselves to a cinder.
My power company installs the meter base style suppressor which places
it outside of the structure the majority of the time. I'm sure that's
probably what you're also looking at.
Jon Pearl - W4ABC
## The Joslyn brand surge protector's are used by the 10's of thousands by
telco's across NA.
They are mounted below, or close by a main panel. Conduit between main panel +
The protector's we used at work, were all in steel enclosures, and those things
shut tighter than a drum. Each of the two [ 3 movs used for 3 phase] huge movs
between hot and neutral [ ditto with 3 -phase]. Each mov had a 20A slow blow
fuse in front of it.
[600V rated... big cartridge fuses.. like the .375" to .5" x 2" long variety]
Output of each fuse feeds a neon. Output of neon wired to same neutral. A 4pdt
relay..with a 120 vac coil
is also used... one per mov. 120 vac coil wired between output of fuse and
## OK, with everything normal, all the neons are lit green. If one or more
MOV's shorts out, the mating fuse
that feeds it blows open asap, and MOV is now out of the circuit. Associated
neon also goes dark. Mating
120 vac dpdt relay also drops out..... bringing in an alarm to the alarm
center/NOC etc. All the relay's of course,
in normal operation, are always operated 24/7..... except when a mov fuse blows
## I never saw one of those joslyn protector's ever catch fire in 34 yrs I was
there. [ no equipment failures from
lightning, transients/spikes/surges either]. As long as they are fused
I don't see how the mov could burst into flames. Even if it did, none of it
would get past the steel box. Those box's
were sealed very good, with a pair of real heavy duty latchs. They were not
like a regular steel electrical box either.
These things were water proof, fire rated nema box's. The seal design was the
best yet. If they were the slightest fire hazard,
the insurance co would have been reading us the riot act. [ factory mutual in
the US is the main insurance co for most of the telco's
in the usa + canada.] In some parts of Florida, etc, home insurance co's will
insist that whole house protector's are employed.
Insurance co's were getting tired of paying out claims in lightning prone areas.
## I have seen some of the whole house protector's..sold at home depot etc..
that use way smaller mov's.... and contain
no internal fuses... but use a spare 40-60A breaker position on the main panel.
The box's they use look flaky to me. They also
use plane romex, etc, from main panel to plastic box... and no conduit.
Later... Jim VE7RF
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