Over voltage protection

On a sunny day (Wed, 22 Jun 2011 21:37:49 -0400) it happened "tm" wrote in :

I, I disconnected the ground from the mains lead of my scope. At least I can measure something that way :-)

If you're serious, that's a Very Bad Idea.

On a sunny day (Thu, 23 Jun 2011 07:44:57 -0400) it happened JW wrote in :

No it is not, often you have to measure something that floats (almost always there are ground potential differences, especially in the mV range). You just have to know what you are doing. Of course when you are an obamaist commie leftist socialist then would want to make things so safe that even an idiot could use a scope. Too bad idiots cannot interpret what it shows, so in that sort of politics matches, kitchen knives, and slippery shoes are all forbidden, and after 2 generations everybody knows nothing, and is kept in a constant state of paranoid ignorance by the politically controlled media, and if not die at early age because of Darwin awards actions, will be overrun by your enemies who dare to use a scope without ground, and a few thing more. Oh, and I forgot lawyers.

"DonMack" wrote in news:itton5$g42$ snipped-for-privacy@dont-email.me:

if the ground is not connected you still get protection from the MOVs,they act in series. Just that the voltage breakdown level is doubled.

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Jim Yanik
jyanik
at
localnet
dot com

Yes, yes, I would never _recommend_ anyone else do it.

On a sunny day (Thu, 23 Jun 2011 10:53:30 -0400) it happened Spehro Pefhany wrote in :

I recommend everyone to do it :-)

I'll do it.

In well-designed boxes, there's always a fail-open fuse or breaker in addition to the MOV. Alas, even the best 'phone line' type protectors seem to omit this (and a failed or leaky MOV takes down every telephone in the house).

"DonMack" wrote in news:itton5$g42$ snipped-for-privacy@dont-email.me:

if the ground is not connected you still get protection from the MOVs,they act in series. Just that the voltage breakdown level is doubled.

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Is that a good thing?

range).

Granted, but why not use an isolation transformer on the equipment being worked on?

Too bad idiots cannot

and slippery shoes

controlled media,

by your enemies who

Why does every discussion always degenerate into political rants around here? My view on politicians? I'd like to take almost all of them, Democrats and Republicans alike and feed 'em to the sharks.

Lawyers too.

On a sunny day (Fri, 24 Jun 2011 08:17:09 -0400) it happened JW wrote in :

range).

This is not always feasible, but yes, in some cases that works. Also if your scope has 2 channels and an invert and add option, then you can do a differential measurement in many cases.

If you are measuring signals in the mV range in your HiFi setup, and amp and the rest is grounded, and your scope too, at an other outlet and even then, it is nice to be insulated. Also if you want to measure that voltage across that high side shunt, or the trigger pulses to some thyristor that is at a high voltage to ground. Battery powered scopes and special isolated probes could be a nice solution too I think. Anyways I have the ground disconnected, never a problem with that, many problem with it connected.

Too bad idiots cannot

and slippery shoes

controlled media,

by your enemies who

Oh well, many do it, It was my turn, equal rights no?

It is a difficult job, and for sure I am not qualified for that. I have to say the GW Bush seemed like he had fun doing it. I think Obama is overstressed and trying to hard to 'do the right thing', while having no clue what is right.

We will see where it goes, if anywhere at all. I would not want that job.

Too many lawsuits for stupid things (was thinking patents), they may be useful for some things. Some time ago I was watching that movie about the end of the Roman empire. Was almost a snapshot of what is happening now in the world, with Rome replaced by Washington of course. Obama = Nero? I think not, but what will come after him? Bush as Caesar, well maybe. :-)

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Lawyers don't eat politicians. ;)

"DonMack" wrote in news:iu0jht$3ot$ snipped-for-privacy@dont-email.me:

better than nothing. people should not be disabling grounds anyways. and not to expect surge protection with a disabled ground.

--
Jim Yanik
jyanik
at
localnet
dot com

Feeding lawyers to sharks is redundant.

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It's easy to think outside the box, when you have a cutting torch.

They aren't cannibals. Professional courtesy, and all...

Not all sharks are courteous...

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It's easy to think outside the box, when you have a cutting torch.

If ground is disconnected or too far away, then hundreds of joules in an MOV must absorb destructive surges - hundreds of thousands of joules. Protection already inside every appliance makes hundreds of joules irrelevant. Protectors are so that hundreds of thousands of joules do not overwhelm that protection. Protectors without a short (ie 'less than 10 foot') connection to earth will not even discuss where energy is absorbed. Pretend that near zero protection is 100% protection as long as they do not provide and you ignore the numbers.

MOVs do not do protection. Either the protector connects short (low impedance) to what does protection. Or it magically makes hundreds of thousands of joules just disappear.

Francois Martzloff was the US-NIST surge guru with many published papers. One paper looked at the energy absorbed in a MOV on branch circuits of 10m and longer and surges on power service wires up to a very strong lightning strike (200kA) to the overhead primary wire on the nearest utility pole. The maximum was a surprisingly small 35 joules. In

13 of 15 cases it was 1 joule or less. Plug-in protectors have much higher ratings.

(There are a couple reasons if anyone is interested.)

Martzloff discusses it.

Protection is not "inside every appliance". For the equipment that has protection, it does not likely have as high ratings as plug-in protectors. What equipment has ratings of "hundreds of joules"?

It is westom's compulsion to save the world from the scourge of plug-in protectors.

Both the IEEE and NIST say plug-in protectors are effective. Some even have protected equipment warranties.

It is only magic for westom.

Everyone else (like the NIST and IEEE) say plug-in protectors are effective.

=================================== In the US, UL listed plug-in surge protectors have MOVs connected H-G, H-N, N-G. They don't need to limit in series; they limit between each power wire pair.

Any external wires (like cable) to a set of protected equipment need to go through the protector. Plug-in protectors work primarily by limiting the voltage between all wires (power and signal) and the ground at the protector. They do not primarily work by grounding (earthing) a surge.

With no ground wire they still protect the connected equipment, but the ground level at the equipment can shift - not recommended. It is somewhat safer if connected downstream from a GFCI.

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bud--