490 V Clamping Volt For MOV: Seems High ?
 

Hello,

Thinking of installing an Intermatic AG series surge protector across a
single 110 V
line to help protect a circuiit board in a furnace.

Clamping voltage is given as 490 V.

Seems awfully high.

Wouldn't most solid state devices blow (well) before this level ?

Thanks,
B.

According to Robert11 :
Hello,

Thinking of installing an Intermatic AG series surge protector across a
single 110 V
line to help protect a circuiit board in a furnace.

Clamping voltage is given as 490 V.

Seems awfully high.

It does. Are you _sure_ it's not intended for a 240V circuit?

IIRC, a MOV for 120V should clamp somewhere around 180-200V.

It might be worth calling Intermatic, it may be a communication
goof between engineering and tech writers.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.

"Robert11" wrote in message
...
Hello,

Thinking of installing an Intermatic AG series surge
protector across a single 110 V
line to help protect a circuiit board in a furnace.

Clamping voltage is given as 490 V.

Seems awfully high.

Wouldn't most solid state devices blow (well) before
this level ?

Thanks,
B.


That's a decent figure, actually, considering that
there is already protection there on the ckt board,
even though you're getting storm damage or whatever.
It should help substantially, but that's not the
primary protection figure.

In my experience they were 375V, but it varies by
manufacturer and purpose, level of protection,
application, etc.. "clamping voltage" is the steady
state voltage that will occur across the protective
device during a fault condition, and the fault
conditions are specced to be able to exist for a period
of time, usually in microseconds, sometimes
milliseconds. instead of thousands of volts, this
component will only allow 490V to the secondary
protection ckts already present in a standard ckt board
design.
At that voltage, the part is about to become an open
ckt after x amount of time. If the load goes away in
time, you still have a good part. If not, the part is
no longer functional and can no longer protect
anything. Other things fried, too, but they lasted a
lot longer than they would have. This is a typical
scenario for trying to protect against lightning
surges, etc..
In a system, the rating of the intermatic has to be
higher than the protective devices already present in
the ckt board. Otherwise they cannot share the load
during a surge, and successfully direct current away
from the protected equipment.

The more important figure is the "knee" voltage, the
point at which the device begins to conduct current
away from the equipment being protected. Then there is
a time/current and energy curve, detailing what happens
as the part becomes overloaded and nears its own
destruction.
The two most important figures are the joules of
energy it can absorb (basically watts, but not exactly)
and for how long it can handle that level of energy,
which is measured in anything from micro to milli
seconds. 1,000 milliseconds - 1.0 second. If the
clamping voltage is reached across the part, that
probably would indicate, based on design, that the
actuall applied voltage is in the order or many
thousands of volts. So, say a 15,000 volt spike will
clamp at 490 volts due to the component being there,
and only 490 V is left for the remianing protective
cktry to handle. It's sort of a staging cycle in most
good designs.
The switch you have isn't as simple as an MOV, but
it operates almost like one. Try a google for MOV or
Metal Oxide Varistor and you'll get a feeling for what
the part you have can do.

Hope I haven't just made things more confusing!


Pop

Hi,

Thanks for such good info.

BTW: When an MOV fails due to being unable to dissipatge too many Joules,
or overcurrent, how does it fail ? As an open or as a short between the
lines it was trying to protect ?

Thanks,
Bob
-------------------
"Robert11" wrote in message
...
Hello,

Thinking of installing an Intermatic AG series surge protector across a
single 110 V
line to help protect a circuiit board in a furnace.

Clamping voltage is given as 490 V.

Seems awfully high.

Wouldn't most solid state devices blow (well) before this level ?

Thanks,
B.

"Robert11" wrote in message
...
Hi,

Thanks for such good info.

BTW: When an MOV fails due to being unable to dissipatge too many Joules,
or overcurrent, how does it fail ? As an open or as a short between the
lines it was trying to protect ?


These short the line so that the fuse blows out, thus effectively
disconnecting the load from the power source.

--

SVL

On Tue, 9 Aug 2005 20:02:33 -0400, "Robert11"
wrote:

Hi,

Thanks for such good info.

BTW: When an MOV fails due to being unable to dissipatge too many Joules,
or overcurrent, how does it fail ? As an open or as a short between the
lines it was trying to protect ?

PMFJI, but...

MOV's fail open, and AFAIK, that's universally true. A bit sad since,
if you have no indicator light showing they're still alive, they can
take their last hit and depart the circuit leaving you none the wiser
and unprotected to boot.

A more hard-core approach is something like a TransZorb(R), which is
designed to fail short. In this case of course there must be a fuse or
fast-acting circuit-breaker to open or you have a fire hazard. With a
TransZorb and suitable current limiting in place you have far superior
protection than a MOV, and you can get a clamping voltage much closer
to what you want to protect.

http://www.transzorb.com/diodes/prot...sd/trans-zorb/

When I was using these they were a General Semiconductor product. It's
Vishay now I guess.

-=s



Thanks,
Bob
-------------------
"Robert11" wrote in message
...
Hello,

Thinking of installing an Intermatic AG series surge protector across a
single 110 V
line to help protect a circuiit board in a furnace.

Clamping voltage is given as 490 V.

Seems awfully high.

Wouldn't most solid state devices blow (well) before this level ?

Thanks,
B.

According to PrecisionMachinisT :

"Robert11" wrote in message
...

Thanks for such good info.

BTW: When an MOV fails due to being unable to dissipatge too many Joules,
or overcurrent, how does it fail ? As an open or as a short between the
lines it was trying to protect ?

These short the line so that the fuse blows out, thus effectively
disconnecting the load from the power source.

MOVs generally fail _open_, and the fuse don't blow. That's why most
protective devices have LEDs - to tell you whether the MOV is fried
or not.

Consider the situations under which they trip - very high transient voltages
and effectively an arc. The guts of the MOV are destroyed.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.

An MOV fails open-circuit. No surge protection ckt I
ever knew of would fail open circuit as that presents a
further hazard in the wiring to the device during the
overcurrent until it pops the breaker.
The poster who mentioned tranzorbs, etc. is also
correct in his post. A "safety" device will not create
an additional hazard (as in popping a breaker) when it
operates.

I hope you'll come back and let us know how your
solution works; I've seen your past posts about this
and found them interesting.

Just a final note: You do know that the ground to the
whole system is good, right? Surge protection such as
you need requires a good ground to be reliable. You
probably already answered that, but ... can't hurt to
check.

HTH,
Pop


"Robert11" wrote in message
...
Hi,

Thanks for such good info.

BTW: When an MOV fails due to being unable to
dissipatge too many Joules, or overcurrent, how does
it fail ? As an open or as a short between the lines
it was trying to protect ?

Thanks,
Bob
-------------------
"Robert11" wrote in message
...
Hello,

Thinking of installing an Intermatic AG series
surge protector across a single 110 V
line to help protect a circuiit board in a furnace.

Clamping voltage is given as 490 V.

Seems awfully high.

Wouldn't most solid state devices blow (well) before
this level ?

Thanks,
B.