# Need help in understanding Electrically Common

#### beeson76

Joined Apr 19, 2010
211
On page 94 of the Ebook, I am having trouble with understanding what is Electrically Common in the top and bottom illustrations.

When you put in a temporary ground on the top line, don't you just make the circuit a parallel circuit, not protecting the person working on the load...when both switches are closed. But then I have problems figuring out if/or if not the ground connections are Electrically Common, because all illustrations show Electrically Common/Uncommon with a load. I will stop there as far as questions because the more I think about it, the more confused I get. I am also referring to sections 2.7 and 2.8 which deal with Electrically Common points in a circuit.

#### thatoneguy

Joined Feb 19, 2009
6,359
Electrical common are points at the same potential, or tied together. Usually the negative side of the battery is common or "ground", and the other side is V+.

Which book/section are you looking at? Can you post a link from the chapters above at the top of the screen (same info, just in a web format)?

#### thatoneguy

Joined Feb 19, 2009
6,359

All points on a wire are electrically common, such as the positive side points 1,2, and 3.

The negative side are also electrically common, points 4, 5, and 6.

The voltage at any of those points is equal, and the current through those points is also equal, as it is a series circuit.

Electrically common typically means equal voltage, but is also used to describe "ground" or common, which is the 0V measuring point for a circuit, the point from which other voltages are referenced from.

Joined Dec 26, 2010
2,148
On page 94 of the Ebook, I am having trouble with understanding what is Electrically Common in the top and bottom illustrations.

When you put in a temporary ground on the top line, don't you just make the circuit a parallel circuit, not protecting the person working on the load...when both switches are closed. But then I have problems figuring out if/or if not the ground connections are Electrically Common, because all illustrations show Electrically Common/Uncommon with a load. I will stop there as far as questions because the more I think about it, the more confused I get. I am also referring to sections 2.7 and 2.8 which deal with Electrically Common points in a circuit.
You have to assume that the temporary ground is effective: in practical terms, connected with a heavy gauge low resistance conductor so that a large rise of voltage cannot occur.

Although this is not indicated in the simple diagram, any practical supply system components would have some degree of internal resistances, as would the wiring, so that the current flow into a short-circuit although very great would not be infinite.

A responsibly designed system would also incorporate fault protection devices such as fuses or automatic circuit breakers, so that after a short time of excess current flow the supply would be cut off.

#### beeson76

Joined Apr 19, 2010
211
I made up a simple diagram to show what I am talking about.

In the text it says that the ground would protect someone working at the load. But if the Disconnect Switch and On/Off Switch were both closed, wouldnt that make the circuit into a parallel circuit thus not protecting the person on the load.

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Joined Dec 26, 2010
2,148
I made up a simple diagram to show what I am talking about.

In the text it says that the ground would protect someone working at the load. But if the Disconnect Switch and On/Off Switch were both closed, wouldnt that make the circuit into a parallel circuit thus not protecting the person on the load.
Did you read through my last post?

The person is in parallel with a connection to ground, which should be designed to have too little resistance to allow a dangerous voltage to be developed, even allowing for the large current which would flow in such a situation. They should not therefore experience a dangerous shock in a properly designed system.

Inevitably, some rise of voltage must occur, but in a well-protected system this will be removed in a fraction of a second when fuses blow or circuit breakers trip.

#### strantor

Joined Oct 3, 2010
6,743
In your circuit you have grounded both sides, making them electrically common. As you drew it, the entire circuit (all points 1-5) are electrically common and the battery is shorted out. The person should be safe, except for being hit by flying acid.