The (US) UL standard (1449) requires protection from H-N, H-G, N-G.
w_ is right that the impedance back to the panel is too high for really
effective earthing to occurr. But they work by clamping.
UL 1449 has required a thermal disconnect for leaking MOVs since 1998.
As noted in both guides the protected equipment may be connected across
the MOVs and be disconnected with them, or it can be connected to stay
live (and unprotected) if the MOVs are disconnected.
UL1449 is a safety standard; says nothing about electronics
protection. A power strip protector can fail during UL testing and
still obtain UL 1449 approval. Underwriters Laboratories does not
care if a protector stops providing protection - if its fuse
disconnects protector parts. UL 1449 only cares about things that
might threaten human life.
How can one obtain UL 1449 approval? MOVs connect to AC mains via a
tiny fuse. Wrap that fuse with the MOV so that heat causes that fuse
to blow faster. Now the grossly undersized protector will trip a fuse
long before too much heat creates explosive vaporizations. Fuse is so
tiny as to not power or disconnect appliance.
MOV disconnects from surges faster - less transistor safety but more
human safety. Since power strip protectors are not effective for
transistor safety, then disconnecting an MOV faster actually promotes
sales among the naive (see 6 paragraphs down).
Undersized protectors that meet UL 1449 simply disconnect protection
faster; leaving protection inside the appliance to protect that
appliance. MOVs can disconnect during UL 1449 testing - stop providing
any transistor protection - and still the power strip gets a UL 1449
approval sticker.
Human knows MOVs disconnected because a power strip's light says
"failure". Human then assumes that power strip provided protection.
Reality - power strip disconnected from a surge as fast as possible;
leaving the appliance to fend for itself. That same fuse is described
by Arno as "a thermal fuse on each MOX resistor". That fuse
disconnects MOVs ASAP while leaving appliance connected to surges.
That is called protection?
Sometimes that UL 1449 protector does not disconnect the MOVs fast
enough. Again the scary pictures:
http://www.hanford.gov/rl/?page=556&parent=554
http://www.westwhitelandfire.com/Articles/Surge Protectors.pdf
http://www.ddxg.net/old/surge_protectors.htm
What does that light report? This picture shows a protector saying it
is OK even when MOVs are removed:
http://www.zerosurge.com/HTML/movs.html
No MOVs inside a power strip and still that light says it is OK?
Yes, because MOVs were removed manually; did not fail by being too
undersized and blowing that fuse.
UL 1449 said nothing about electronics protection. But to promote
myths, some will intentionally confuse UL 1449 with claims of
protection. Some will even claim the protector meets C62.41 which is
only testing waveforms. Some will claim UL 1449 defines protection.
Reality: UL 1449 is about human safety. To meet UL 1449, grossly
undersized power strip protectors may provide even less transistor
safety. It may disconnect MOVs during surges too small to overwhelm
protection already inside household electronics. Disconnecting faster
- providing less transistor protection - even promotes ineffective
protectors to the naive.
In an April 1990 article, Martzloff defined two types of surges.
" ... transients with low amplitudes (less than 1000 volts) are
buffered by the computers' power supply but might still couple into
circuits and cause glitches". These surges may be sufficient to blow
that fuse; disconnect MOVs inside the grossly undersized power strip.
A naive human then assumes that "my protector has sacrificed itself to
protect my computer". But as Martzloff notes, protection already
inside appliances provided the protection.
Same surge struck appliance and power strip simultaneously. Surge
could not overwhelm 1000 volt protection inside appliances. Some
computers define that internal protection at 2000 volts. But grossly
undersized power strip also had to meet UL 1449 human safety
requirements - disconnected to protect itself - not the computer.
Effective protectors must earth direct lightning strikes and still
remain functional. Responsible manufacturers provide a 'whole house'
protector that clamps (shunts) surges to earth. A product even
available in Lowes and Home Depot for under $50. UL1449 says nothing
about transistor protection. UL1449 is about human safety because
those scary pictures - threats to human life - are most dangerous in
power strip protectors.
Those with IEEE access and a few decades of engineering design
experience also know that UL approval is about human safety - does not
define transistor safety.
