1- MOV failure mode is short circuit, not open circuit. Never put
such a device on the fridge or a freezer line !
2- Use exclusively MOVs on AC lines
3- Clamping voltage is not a big problem: Electronic devices plugged
to the grid are designed for withstanding at least 1500V peak. For
indirect shocks on AC lines, typical surge waveform is very short:
8?s rise time & 20?S overall duration and, in home distribution
circuits, it rarely exceeds this value. There are other schemes,
depending on the nature of the shock (direct or indirect) and the
nature of the upstream protection.
Of course, the earthing circuit is of a prime importance: the the
surge path must have a very low stray inductance and its resistance
be compliant to the standards.
As a matter of example: At home, located in a quite high keraunic
area, all the grid distribution cables are buried and it's a
favorable situation. I have a 40y-old 220Vac protection still alive,
with 3 varistors (phase-neutral, phase-earth and neutral-hearth)
400V clamping voltage @ 1mA, with a limitation of 700V for a 10Amp
shock, 1200V for a 2500A shock, 1400V for 5kA or 1500V for 10kA.
Response time is better than 50ns. The device has it's own earth 1m
below with a straight cable, the protection is placed at the head of
the distribution cabinet located 5m after the company's counter. my
earth ground network resistance is well below 50 Ohms when the soil
is dry. Since it's been installed, the standard voltage has been
establish to 230V and I didn't see any difference.
I never had any issue for 40 years. It's not the same in my
neighborhood !!
Then at the computer level, just because I'm paranoiac (!), i have
the same scheme but with 275 V service varistors I replaced the
original 250Vac by Chinese components because they were too close to
the peak value of the grid voltage. The earth path is shorter than 1
meter.
Read this:
and
for those who can read French Language. This book is very rich hence
highly recommended, written both in worker language ans engineer's
one ; worth to be translated with Googletrans.
On 17.05.24 23:43, BuffaloJohn via
wrote:
Yes, like I wrote earlier, that is a MOV - Metal Oxide
Varistor and they have either two or three leads. The ones
with three leads can be used to indicate when the MOV has
blown. The ones with two leads are a crapshoot if they are
still good, depends how they failed.
Most surge suppressors use MOVs. The biggest problem with
MOVs is that the clamping voltage is really high. A MOV with
an AC working voltage of 130VAC has a clamping voltage of over
350V, so you can still blow the electronics out.
BuffaloJohn