[CButterworth]

I expect to get the remaining parts of my Cornet2 installed over the weekend.  As my workbench it outside and located just E of Phoenix, Arizona my work-window is limited to a couple of hours each morning.  The resistors, diodes and lugs are already in place.  All I have to do is to install the remaining components and plug it in!

Now, it goes as said that voltages are measured by connecting the DMM between the GND lug and using the other probe on the test points.  I have done that numerous times with other projects.

The one thing that I always have a difficult time with is that the GND lug is also directly connect to the chassis.  The GND-lug PCB can read a voltage of up to 550VDC depending upon where the probe goes.  Yet, I still have difficulty getting the concept that I can safely touch the chassis because it is securely connected to mains GND.

Why does this work?  Is it simply because the resistive path from chassis to mains GND is far lower than chassis to GND via human?

Regards,
Charlie

[amandarae]

I expect to get the remaining parts of my Cornet2 installed over the weekend.  As my workbench it outside and located just E of Phoenix, Arizona my work-window is limited to a couple of hours each morning.  The resistors, diodes and lugs are already in place.  All I have to do is to install the remaining components and plug it in!

Now, it goes as said that voltages are measured by connecting the DMM between the GND lug and using the other probe on the test points.  I have done that numerous times with other projects.

The one thing that I always have a difficult time with is that the GND lug is also directly connect to the chassis.  The GND-lug PCB can read a voltage of up to 550VDC depending upon where the probe goes.  Yet, I still have difficulty getting the concept that I can safely touch the chassis because it is securely connected to mains GND.

Why does this work?  Is it simply because the resistive path from chassis to mains GND is far lower than chassis to GND via human?

Regards,
Charlie

Hello Charlie,

Hmmmm.....I am a little bit confuse about, you say, 550VDC on the ground?  Where is this reference to then?  The GND is a reference point so that all voltage can be measure from it, either above it(+) or below it (-), the in between (GND potential itself) should be zero because your test leads are electrically connected (shorted) to each other by then.

I am no expert Charlie but you should be able to measure the test point voltages with respect to any point that is ground as well as reference to the chassis because you will see that in the Cornet 2, the incoming GND wire (from the wall) connects directly to the chassis of the unit and the Ground lug terminal (Silver) located at the back of the chassis (i.e Green wires on the instruction manual in configuring the transformer wiring).

regards,

Abe

[CButterworth]

Abe,

Doesn't 550VAC come out of the transformer then get fed into the rectifier?

I am clear about the wiring, Jim has really designed a nice system, and I like the way that the unused wires from the transformer get tidied-up.

You have hit the nail on the head, when you said that the GND reference is that by which you measure voltage either above it or below it.  I always envisage a system whereby the voltage (B+) flows from positive into negative on the mains, rather like a pipe containing flowing water.  However, if you make a hole and stick in your finger, you'll get wet.  Isn't this like sticking your finger onto a grounded chassis?  In reality, you don't get a shock, but I find it difficult to appreciate why not (the reason why a metal chassis must be earthed).

Charlie

[samplesj]

No, you won't "get wet", because you aren't sticking your hand in it.  The current is flowing from hot to neutral.  Ground isn't involved at all (except in emergencies)

[CButterworth]

Samplesj,

Yes, I know.  It always makes my heart race a little, when I first get a project finished, and I know that all is well, but I still have to touch-test the chassis to check it!  Even though everything is well-grounded, it still racks my nerves a little.  I simply suppose that I have to accept that the hot-to-neutral works.

Charlie

[amandarae]

Abe,

Doesn't 550VAC come out of the transformer then get fed into the rectifier?

I am clear about the wiring, Jim has really designed a nice system, and I like the way that the unused wires from the transformer get tidied-up.

You have hit the nail on the head, when you said that the GND reference is that by which you measure voltage either above it or below it.  I always envisage a system whereby the voltage (B+) flows from positive into negative on the mains, rather like a pipe containing flowing water.  However, if you make a hole and stick in your finger, you'll get wet.  Isn't this like sticking your finger onto a grounded chassis?  In reality, you don't get a shock, but I find it difficult to appreciate why not (the reason why a metal chassis must be earthed).

Charlie

I see, you are talking about the secondary of the transformer (measured between red and yellow) before rectification.  The "VDC" on your first post derailed me.

Well actually, in the real world, the electrons flow from negative to positive and it is not the other way around.  You do not get shock because there is no path for the current to flow or you do not complete a circuit but the current "sees" that you are "open", very high resistance.

But then again, what do I know!

regards,

Abe

[samplesj]

Samplesj,

Yes, I know.  It always makes my heart race a little, when I first get a project finished, and I know that all is well, but I still have to touch-test the chassis to check it!  Even though everything is well-grounded, it still racks my nerves a little.  I simply suppose that I have to accept that the hot-to-neutral works.

Charlie

IF it is assembled correctly then you could touch it all day even if it weren't grounded.  NEVER NEVER use a metal chassis that isn't grounded just in case, but its just a bit of insurance in case something breaks loose. 

The chassis will never see voltage at all unless we mess something up, but yes if it does see voltage then the idea is that from chassis to outlet to panel to ground rod is much lower resistance than your chass to hand to earth.

[CButterworth]

Samplesj,

I really like to ensure that the metal chasses on my projects are always well grounded.  I tend to use a good bolt with a nice star washer to bite into the metal, then a lock washer, followed by a nut, then the lug, lock washer, nut.  This seems to ensure that over the years (hopefully) of use, the chassis ground never comes loose.  It may seem like overkill, but lock and star washers, and nuts are cheap.

Charlie