Grounding for Dummies

[Guest]

Hi folks,

I was finally excited to get an electricity analogy I could understand...basically an "expanded" water explanation, with the addition of "sea level" and "below sea level" to account for ground and negative charge, respecively.

Sorry, I do have questions. I'm wondering:

What's the purpose of multiple grounds? I understand having a chassis ground, and the safety in having such, but why multiple grounds? If they all equal zero volts, what's the point? (I've built the Seventh Circle stuff and wondered why there's a common ground, a pwr ground, and an A ground).

I assume that the purpose of a grounding (not chassis) is the use of 0v is to achieve proper voltages at certain points in a circuit. Is this correct?

Thanks. Please see signature.

Kelly

 

[tmbg]

'ground' is a reference voltage by which your signal voltages are compared. A 10V signal is only 10V as referenced to ground.

Chassis ground, or earth, is a safety mechanism. It allows unsafe voltages a way to get back to the earth, where they are harmless. You want the mains ground of your AC in to be connected to chassis ground, and you want the ground of your circuit to connect to chassis ground. Also, if the whole chassis is metal, and grounded, it acts as an RF/EMI shield.

Because ground is a relative concept, you ideally want one path to ground. This is called 'star grounding'. You pick one point, and ground everything to there. The reason for this is that any conductor has resistance, and any time there is current flowing through that resistance, there will be a voltage drop. Sometimes the voltage difference between two ground points in a circuit can be quite significant, which means that a voltage that's referenced to one ground can be different than that referenced to another ground.

I may be explaining this badly, but there you go

 

[Steffen]

a good reading:

http://www.rane.com/note151.html

steff

 

[Guest]

I just found plenty after spelling "grounding" correctly in the search box. Sorry I bothered!!


kb

 

[PRR]

> why multiple grounds? If they all equal zero volts, what's the point?

Ground is NEVER zero volts!!

No two "grounds" are EVER the same voltage!!

Sea-Level analogy:

You piss in the ocean. Does "sea level" rise?

In the large scale: no. Your pee is small, the ocean is large, and your pee comes from rain which comes from the ocean so it all averages-out.

In the very small scale: your pee causes millimeter waves which may submerge and expose small creatures at the surface. To them, sea level is changing violently.

And peeing off the USA coast will raise sea-level in France, but only milli-micrometers and not for many hours after the pee.

The ocean is big, but not infinitely big, and water does not move infinitely fast. So peeing in the ocean will cause local disturbance.

Same in electricity. There is no perfect ground. If you need to dump crap into ground, you have a "dirty" ground. If you also need a clean ground reference, you have to keep it separate and carefully consider where you bring the two grounds together.

And then there may be a Safety Ground, which should not normally have crap in it, but is SOLID so any large crap (like a short in the wiring) will blow the house-fuse before it starts a fire and maybe before anybody gets a shock from a "hot" chassis.

 

[NewYorkDave]

[quote author="PRR"]> You piss in the ocean. Does "sea level" rise? [/quote]

How many beers have I had?

And peeing off the USA coast will raise sea-level in France, but only milli-micrometers and not for many hours after the pee.

I wish you'd told me that before I wasted all those hours down at the beach last summer. I'll have to find some other way to get those Frogs.

:wink:

 

[Guest]

Thanks PRR for the expanded analogy. I can't really tell if you're making fun, but I am extremely slow (see signature).

In any case, can someone point me to an explanation of a "dirty" ground?

Also, how is it that ground is NEVER sea level? I understand that ground is always a reference, and that it's variable, but...

Anyway.

Thanks.

Kelly

 

[tmbg]

hm this may be a good example

For some reason, at my house, if I grab the metal ring (the screwy bit) on the cable TV line, and then touch something metal and earthed, like my PC case, I get a nasty shock. I throw a scope on it and discover that, as referenced to the 'ground' of my earthed PC case, the shield of my CATV line has 56VRMS AC on it.

BUT, as far as that connector is concerned, that 56VRMS is 'ground'

 

[Guest]

The more I think about it the more it makes sense. You're only measuring the relative voltage between two places. Ground is simply one of those reference points.

kb

 

[CJ]

This is another good article that has been floating around , err I mean grounding around, I mean.....


http://www.aikenamps.com/StarGround.html

 

[PRR]

> referenced to the 'ground' of my earthed PC case, the shield of my CATV line has 56VRMS AC on it.

Betcha the TV-cable really is grounded to dirt, and your 120V power is NOT grounded.

With a battery-powered AC voltmeter: what is the difference between the round-hole in a US-type 3-pin outlet, and a metal rod in the dirt? If it isn't very close to zero, you have a safety problem.

If the power-ground is pretty-near dirt-ground, it is possible the cable-TV line is connected funny. But they usually HAVE to drop a solid dirt-ground every 1,000 feet along the street, to drain lightning and stray static. And it isn't trivial to "float" a cable-TV ground. In my area, they also ground AT the entrance to the house.

 

[tmbg]

well, it's done it in TWO places I've lived, so maybe it's my PC!

 

[PRR]

> an explanation of a "dirty" ground?

You have an airport. Some pilot asks what elevation it is at (assumed relative to sea level), so she can descend through clouds without fear of hitting land on the way down.

i.e.: at LaGuardia, you can watch the altimeter down to 50 feet, then look out the window for tarmac. But at Denver, when the altimeter says 5,000 feet, you already crashed. Pilots like to know these things.

Sight a laser dead-level from the surface of your runway to the sea. Go to the sea, put a stick on top of the ocean, and see how far up the laser hits. (Ignore the fact the earth is curved.)

Some days the sea is calm and flat, no problem. But sometimes the sea has huge waves: where do you measure? That's a "dirty ground".

> how is it that ground is NEVER sea level?

Well, it will sometimes be "zero", the same way that giant waves rise and fall around average sea level. But mostly, "everything sloshes". Not much, in a good layout, but you have to be aware of these things. Don't dump big current in the wire you use as input reference for a sensitive input stage.

 

[nacho459]

My experience is that chassis doesn't ground as well as the AC ground, I built a Plexi Pre and connected my audio ground to the chassis and then the chassis to AC ground, but still had some hum. Once I connected the audio ground directly to the AC ground it went away, I guess this is what you mean by a "dirty ground" after all the chassis is picking up all sorts of crap from RF an the Transformer.

On some things however I have found out it is better to float the whole audio ground.

 

[Guest]

Sorry to bug, Nacho, but what do mean by float?

Thanks a lot!!!

kb