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Old 30th August 2010, 09:42 PM   #21
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what good is blowing a fuse in series with an MOV??
That permits the biggest part of the spike to get through? No?

Or are you saying put the fuse in series with the load and have the MOV blow that??

I think it is not a bad idea to put in a super heavy duty high current MOV, since they do degenerate with pulses over time... but I haven't read anything on their degeneration WRT the size of the MOV vs. the current that hits... that would be good info.

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Old 30th August 2010, 10:27 PM   #22
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Yes. Put the MOVs at the end of the fuses, across the load receptacle. Inline holders work OK. If you're going to supply loads with no ground don't even run that out to the receptacle. It could be just another arc path you don't need. Put a plastic plug down in the hole so no one relies on a connection that isn't there. Put a spark gap across the receptacle too if you want.

If you can get a fast acting type fuse rated slightly above normal running current to hold the inrush for several years, that's the one to use. Nothing gets out of the way of current faster than vapor and the element has to hit that state in a piece of a millisecond to do real well at keeping energy off the arrestors/clamps. Maybe a nice expensive breaker can put a lot of pulse energy into throwing the contacts far away in that short of a time, but I don't think the ones I've seen in a 20 dollar power strip can race a fast acting fuse. Especially not less than a 5A AGC. They might Start opening right away, but it'll hold a low voltage arc for too long.

Last edited by Andrew Eckhardt; 30th August 2010 at 10:34 PM.
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Old 31st August 2010, 12:52 AM   #23
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I cheated. The attached file is actually postscript but your PDF reader should open it.

Fusing the neutral might not be legal everywhere, but depending on where the lightning hits your neutral and ground might not be the closest thing to ground anyway, and inside of 10uS there's probably not too much help from it due to inductance. One time I had something smack the phone line and the arc jumped off the modem board (this was a while ago) and onto the bottom of the computer case. From there it put a pin hole on the side of the case to the inside of a steel desk with a shelf at the front underside that the PC was sitting on. It look like someone started a TIG welder on the computer. There was nothing around the desk but air, in a very old house with asbestos tile flooring over tongue and groove hardwood. It must have been the capacitance of and other little ionized paths off the desk that loaded the case. I was in another room and heard a sharp crack. If anything went out the ground wire in the supply it didn't matter. The PC locked up but rebooted OK. The modem was gone.
Attached Files
File Type: pdf protect.pdf (10.9 KB, 157 views)

Last edited by Andrew Eckhardt; 31st August 2010 at 12:55 AM.
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Old 31st August 2010, 03:37 AM   #24
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doesn't open in 9.0

I still am not sure what you're suggesting.

A fuse is not fast enough to stop a HV pulse from lightning, afaik...
By the time an MOV has shunted enough current to flash a fuse, a whole lot of current has passed, so IF the MOV has shunted it, then the danger is gone, if not, then the fuse won't likely help... but I'd like to see what your pdf shows, maybe I am not "getting it".

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Old 31st August 2010, 04:17 AM   #25
star882 is offline star882  United States
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What about add a large air core inductor or two to slow down the transient? Space it out enough and it would take a lot of voltage to arc over it. One problem is that it could cause transients of its own, but maybe a large capacitor across the load side can prevent it.
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Old 31st August 2010, 08:04 AM   #26
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by star882 View Post
What about add a large air core inductor or two to slow down the transient? Space it out enough and it would take a lot of voltage to arc over it. One problem is that it could cause transients of its own, but maybe a large capacitor across the load side can prevent it.
If you want dood lightening protection talk to people who operate radio transmitters in Florida. Orlando sees a lightenig storm almost daily every summer afternoon and antenna up on towers are easy targets and they all have big coax cables that lead right to some expensive gear. They can design systems that can take direct hits.

The think that seem to work is to provide a very low impedance path to ground using a conductor that can take lots of current. I use coax feed line and connect the shield to a field of three 6ft groud stakes just outside the house. These in turn are bonded to the building's ground system. The hope is that a direct strike will travel on the outside of the coax and then into the ground stakes. Well at best it can only divide the voltage and I might still get a few KV in the house. The coax then right after it passes through the wall goes into a comercially made lightening arrester. This is basically a gas discharg tube The tube and low value resistor from a voltage divider. the tube normally looks like an open circuit but when the gas ionizes is a good path to ground. There is also an LC network to shunt transsients.

Ths white paper pretty much describes current best practice. The ideas in this paper are well tested and known to work
http://www.dxengineering.com/pdf/Ins...Protection.pdf

In the past I owned a sailboat. It is not fun sitting under 65 foot aluminum mast in the thunder storm at sea. You are the tallest thing around for many miles. But it can be safe again the idea is to offer the lightening a very easy path to the water. The boat had a 6,000 pound lead keel that extended about 6 feet under the hull. Connect that to the mast with some solid copper strap and it's as safe as can be. Old timmers would wrap a chain around the mast and put the other end of the chain overboard. This gets most of the current off the boat. Same idea as the tower ground system in the above white paper.

Bottom line is that it's common for electronics to take a direct hit and survive and yu don't have to invent protection scheems
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Old 31st August 2010, 06:34 PM   #27
TechGuy is offline TechGuy  United States
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Quote:
Originally Posted by bear View Post
doesn't open in 9.0

I still am not sure what you're suggesting.

A fuse is not fast enough to stop a HV pulse from lightning, afaik...
By the time an MOV has shunted enough current to flash a fuse, a whole lot of current has passed, so IF the MOV has shunted it, then the danger is gone, if not, then the fuse won't likely help... but I'd like to see what your pdf shows, maybe I am not "getting it".

_-_-bear
Fuses have limited voltage flash over arcing. A big voltage spike will vaporize the fuse filmament and turn it into ionized gas that conducts current. Breakers are even worse, and have very slow switching times. Thermal breakers take a couple of seconds, a Magnetic breaker is around 500 ms.

Quote:
If you want dood lightening protection talk to people who operate radio transmitters in Florida. Orlando sees a lightenig storm almost daily every summer afternoon and antenna up on towers are easy targets and they all have big coax cables that lead right to some expensive gear.
They rely on the tower to take the bulk of the lighning strike. They usually protect the attenna and isolate the coax cable from the tower. Gas Discharge tubes are used to divert any surges that are conducted into the coax cable.

http://www.sylvador.com/samples/man_light.pdf

For Lightning, Gas Discharge tubes work very well. For Solar flares, EMP, Gas Discharge tubes are too slow.
Here is an article that describes various Surge protection devices and some of the common circuits for Surge protection:

Multiple Protection Devices Guard Against Transients Page of
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Old 31st August 2010, 10:31 PM   #28
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Quote:
Originally Posted by bear View Post
Grounding to a gas pipe is likely a violation of code.
Remove it asap - that is dangerous.
It is a violation of the NEC 2008

250.52 (B)
Not permitted for use as Grounding Electrodes

The following systems and materials shall not be used as grounding electrodes:
(1) Metal underground gas piping systems
(2) Aluminum
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Old 31st August 2010, 11:13 PM   #29
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Bear, Dratt! 8.0 works in linux. I'll post in pdf soon.

A "real" MOV can handle 20kA pulses with 200J energy for under 5 bucks.

Here's just an example.

Digi-Key - 495-3647-ND (Manufacturer - ETFV25K130E4)

You probably couldn't find any glass if an AGC fuse had to try pushing that into it, about the same energy as a small handgun. The fuses would more than share the energy dissipation (that's the point of blowing a fuse with an MOV, anything to keep the pulse from getting out of the clamp and into the equipment), a sacrifical part anyway, may as well go out with a bang. MOVs are sort of like lamp filaments. Several times below their max ratings they can run for a very long time, over the spec kiss it goodbye.

They don't put a good fraction of that much MOV in your typical 20 dollar power strip. If they did we probably wouldn't be having this discussion.

Last edited by Andrew Eckhardt; 31st August 2010 at 11:19 PM.
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Old 1st September 2010, 03:34 AM   #30
pjp is offline pjp  India
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Quote:
Originally Posted by Andrew Eckhardt View Post
A "real" MOV can handle 20kA pulses with 200J energy for under 5 bucks.
How realistic are those MOV ratings anyway ? The part specified in the digikey link has 20kA specified for 20uS only!

20uS doesn't seem long enough to me.

I ask because I once built a surge protector using a bank of 20mm varistors (431K20D, rated 6.5kA) and it wasn't of much use. Whenever there was a spike, the varistors would blow up with a very impressive bang even though the current through them was limited to 4A by a fuse. (Which seems quite reasonable given the total device dissipation is specified as 1W)

Last edited by pjp; 1st September 2010 at 03:35 AM. Reason: added stuff
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