Simple AC Mains switch (power switch) question.

[Andy Peters]

  I would suggest a third fuse for the round/grounding pin

Noooooooooooooo!

That would totally defeat the purpose of the ground wire.

Think about it.

-a

 

[jwhmca]

yeah... that doesn't seem like a good idea.

It would be nice to implement our own "set of standards"

There are so many things that can be so dangerous... not to mention Audio Quality.

 

[Brian Roth]

Well, I was being somewhat sarcastic....as in "the electrician's novice assistant totally screwed up the connections of the three wires".  IOW, if we are all so paranoid about any possible miswiring, then we need to have fuses on EVERYTHING!  <g>

That brings back a somewhat-related memory of Crown power amps (and other gear as well).  Crown had an external fuse holder on the back of the amps. BUT, they also had an internal fuse holder that held a somewhat larger rated fuse, wired in series with the external fuse.  Why?  Well, in case some bonehead replaced the external fuse with a "1000 amp No-Blow"...ie, a piece of tinfoil from a cigarette or gum wrapper twisted around the blown fuse's body.

Hence, we need a few dozen fuses wired all over the place in order to cover every possible problem.  KIDDING!

Bri

 

[ricardo]

You may like to measure the voltage between earth and neutral at your own house.  If you want some fun, put an ammeter instead of voltmeter between earth & neutral.  ;D

OK.  I shouldn't have said that.  Don't try this at home.  8)

There should not be currents between them unless something is broken. Earth at least here in Finland is a quite literal earth: a +25m meter thick copper cable dug deep in the ground. Both neutral and safety earth are joined to this fat copper earth somewhere in the main electricity box. Condo's are more complex, but the main idea should still be the same.

I should clarify.

Please do see what happens with an AC voltmeter between Neutral & Earth on all your wall sockets.  If you see more than 50V in the US or 100V in Europe, get an electrician to check all your house wiring.

You can buy little plug testers at HomeBase in the UK to do this.  The POMs are allowed to do their own house wiring ..  or they did when I lived there.

But even if everything is wired right, I'll bet you find some 10s of VAC.

DON'T try the ammeter.

 

[JohnRoberts]

OK, now it's official, you guys are officially over thinking this.

For Brian re: the occasional extra internal fuse, UL sometimes requires internal fuses in medium to high high current paths to prevent the product from catching fire due to heating from random component failures like shorted rectifier diodes, etc that would not draw enough current to take out the main fuse, but would try to pump a bunch of current into the caps and make enough heat to ignite nearby flammables. The internal fuse is generally invisible to the end user but might prevent your house from catching fire and burning down some day..

UL is very thorough in their safety testing, mimicking typical common component failures. 

Think about it, when was the last time you saw a UL approved product catch on fire? Not release smoke but literally burn?

JR

 

[ricardo]

In the last millenium, I wrote an internal engineering note on how to design stuff so it could easily meet ALL international safety (and also the new EMC) regulations.  I wish I still had a copy of that.

The big difference in philosophy between the 115V standards like UL and the 230V European  standards like BS, SEMKO etc (now all harmonised under EU I think) was the 115V standards were paranoid about stuff catching fire while the 230V standards concentrated on shock protection.

There were various dodges that met the letter of various standards (if not the spirit).  But by & large, if you thought about such things from the start, you had a much easier time.  Eg. you should design stuff that met both 'earthed chassis' and 'double insulated' standards.

One dodge that sticks in my mind is UL approved mains wire which if you cut through the insulation would show 2 colours so met 'double insulated' standards.

I think all the standards are actually quite sensible and worth trying to meet from the very start.  It's expensive if you don't think about these things early on.

 

[abbey road d enfer]

There should not be currents between them unless something is broken. Earth at least here in Finland is a quite literal earth: a +25m meter thick copper cable dug deep in the ground. Both neutral and safety earth are joined to this fat copper earth somewhere in the main electricity box. Condo's are more complex, but the main idea should still be the same.

That's quite interesting! In France the earth is exactly what you say, but it is mandatory to avoid connection between earth and neutral. I believe it's a remnant of the days of what we called "di-phase", where 220V was two 110V anti-phase; others would call it "balanced power".

 

[Kingston]

There should not be currents between them unless something is broken. Earth at least here in Finland is a quite literal earth: a +25m meter thick copper cable dug deep in the ground. Both neutral and safety earth are joined to this fat copper earth somewhere in the main electricity box. Condo's are more complex, but the main idea should still be the same.

That's quite interesting! In France the earth is exactly what you say, but it is mandatory to avoid connection between earth and neutral. I believe it's a remnant of the days of what we called "di-phase", where 220V was two 110V anti-phase; others would call it "balanced power".

I actually had to check how it goes before I wrote that. I hadn't realised it before just how much differences there can be with our power systems between one country and another.

Another fun fact about the three-phase power we get from the grid as an electrician puts it: "you divide it to two and send them to the opposite sides of the house, play by the ear"... And here I was thinking our mains boxes would have fancy clean up tricks etc. balancing transformers to optimise whatever comes from the grid.

[edit]

and yes, it looks like France and Belgium have (or had?) another variant of three-phase power where neutral isn't actually neutral at all, but roughly 130V above safety earth. And hence the safety relays are disjoncteur bipolaire.

[edit2]

Looks like my electrician friends had oversimplified the situation for me. The joined earth/neutral connection is sometimes done in old houses where new three prong sockets are installed. It's a cheap and questionable practise, something that was apparently done in our last place. I didn't realise I should have demanded a proper treatment. In modern installations there is a clear distinction between neutral (that comes with the three-phase grid power) and the literal safety ground/earth. And apparently this "divide to two" mentality is a lazy electricians cheat as well.


....and verging too far off topic, sorry.

 

[abbey road d enfer]

The joined earth/neutral connection is sometimes done in old houses where new three prong sockets are installed. It's a cheap and questionable practise, something that was apparently done in our last place.

Now that you mention it, I remember having read about it; it's cheap, but it works. Although not allowed in France /Belgium, it is clearly documented in official documents as an existing practice. IMO it's not worse than allowing un-earthed sockets in domestic installations. In France/Belgium, earthed sockets are mandatory only in wet rooms (kitchen, bathrooms, laundry).

 

[ricardo]

Kingston, Abbey, could you just stick an AC voltmeter between neutral & earth of a 3 pin mains socket in your respective houses and tell use what you measure in Finland & Marcelland?

And any of yus Yanks too please?

I'm really interested.

 

[Kingston]

It's hovering between 0.1-0.3VAC. I would be curious how it looks but the scope (CAT I) manual specifically warns against this.

(As a cool modern life upgrade, they installed a top notch kW/h metering system in this place and I can now monitor hourly electrical usage from a web service accurate down to 0.01kWh, database and all. There have been revelations. Specifically how expensive coffee and laundry is, and how running tube gear, LCD monitors etc. computers is relatively inexpensive.)

 

[abbey road d enfer]

Measuring a pretty constant 0.9Vac with an rms VM. I'm too lazy to put the 'scope at work but I guess it must be pretty dirty.

 

[Twenty Log]

Have seen 20V between earth and neutral in a building (USA) with long runs of wires to the electrical panel (and a failing grounding rod into the earth)...  who knows... could have been wired by the owners (of which all were engineers)    I dunno.... they also had a multi-killowatt shaker table from the 1940s with tubes that was plugged in and could have a mild fault placing current onto the earth line....

What about a simple mechanically-tied double pole circuit breaker for hot and neutral on the back of the chassis?  Somewhat idiot-proof as long as it can still trip even if duct-taped in the O.N. position...

I like the double fuse (internal and externally user serviceable), especially when out at a gig and the external fuse blows... the show must go on... and then poor bloke and the tin foil gum wrapper is forgotten about until playing a gig at the Fire Royale Theatre...

However, the circuit breaker idea is expensive, but hand soldering multiple components/fuses (while probably still less expensive than a circuit breaker soldered once) in manufacturing can still add up...

This was in Mix magazine a while ago... FWIW
http://mixonline.com/studios/design/mix-interview-arthur-kelm//index.html

 

[sahib]

As Ricardo would say "in da auld days" we did not have such thing like a seperate safety earth  and connecting the neutral to the earth contact on the wall socket was the standard way of providing safety. It worked then and it still works now where there is no safety ground.

This was of course in Turkey but we generally followed European standards. My dad is no longer with us otherwise I would have asked him how they did in '40s but in '70s when I was wiring houses it was an accepted practice.

If there is mains leak to the equipment chassis this simply short circuits at the wall socket and blows the fuse. The problem of course is when the neutral connection fails and that is where the safety ground protects you.

 

[JohnRoberts]

To continue this veer, coincidentally a week or more ago I sent an email to a magazine article author who told the story about a studio installation where they drove a separate ground rod, just for the studio outlet grounds. My question was about bonding the neutral to ground, for safety handling of fault currents. The neutral is bonded to ground at the service drop, but who knows what the soil resistance is between the two spaced apart, ground rods. So the question I had is will a fault current into a chassis ground connected to this second earth ground rod, draw enough current to take out the fuse and protect the meat puppets. (The author didn't answer me).

My understanding when you drop in a local ground rod, you need a local service panel with neutral bonded to ground at that local panel, but I am not a electrician.

JR

 

[Speedskater]

JR, this is the correct way.  Notice that in the remote building the Neutral is isolated from the Ground.

 

[JohnRoberts]

In that image the two grounds are tied together, so yes, ground is bonded to neutral but at the main service panel (where I guess it should be).

I need to go back and check the article i read (no drawings), but the sense I got from the wording was that the new local ground was not telescoped back to the service panel, just tied to the new local ground rod. In my judgement that could allow dangerous voltage rise from a mains fault to chassis ground, and perhaps not draw enough current to take out the fuse and protect the humans. It is possible the author was not clear about the actual wiring, or I was not clear in my comprehension skills.

Note: that drawing literally shows a "ground" loop (pun intended), while I believe ground loops are over-stated as signal integrity problems

JR

 

[Speedskater]

In the US style power system, there are many ground loops thru Mother Earth!  The power company transformer Neutral is connected to M.E.  Each home may have ground rods, water pipes, telephone, cable TV & internet and satellite ground connections, so do all your neighbors.  Ground currents will use all the available paths back to their sources.  Ground rods being rather high resistance won't be the favorite path of most electrons.

Typical residential ground rods when measured (not cheap or easy) often measure 25 to 100 Ohms.

 

[Kingston]

Typical residential ground rods when measured (not cheap or easy) often measure 25 to 100 Ohms.

How exactly is this measured by the way? Stick an alligator clip to the nearest bedrock? 

 

[Twenty Log]

I thought I heard about the 25 ohms once...  If so, then 120VAC (USA) which is ~170Vpeak, would draw ~7 Amps(peak) into ground on the earth line during a fault, which is not enough to trip the usual 15A breaker on 14 AWG...  (never mind what can be paralleled into the meat puppets)....  I guess the good news is that it is 7 Amps (peak) less the 15A tripping point into the meat puppets is "better" than 15A straight....  GFCI anyone?  100 Ohms would be "worse" in this case if the meat puppets were in a salt water bath...

hmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm.....

I am thankful I live in the country where I am on my own power utility transformer, cable TV drop, copper Bell line... and very far away from neighbors that I can electrocute induced with an EMP event... er... I mean.....  Nothing to see here...