# Measuring 240v phase to ground

Discussion in 'General Electronics Chat' started by eels1212, Jul 2, 2008.

1. ### eels1212 Thread Starter New Member

Feb 21, 2008
7
0
Can somebody tell me why you don't measure 240 phase to ground? I know phase to phase you should see 240v. But say one one side of a coil you can see 120 phase to ground and zero across the coil. I know when the coil is energized you see 240v across it. But why when it is not energized to you see 120 phase to ground both sides of the coil?
Thanks Much
Ed:

2. ### thingmaker3 Retired Moderator

May 16, 2005
5,073
8

In a nutshell, E$_{line}$=$\sqrt{3}$*E$_{phase}$ for Wye, and E$_{line}$=E$_{phase}$ for Delta

3. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
For a typical household service drop in the USA, you get L1, L2 and Neutral.
Neutral is the center tap of the power company transformer's secondary winding.
L1 and L2 are the opposite ends of the winding.
Ideally, L1 and L2 are equal in amplitude, but 180° out of phase with each other.

Neutral is tied to earth ground at only one place; your service panel. This is necessary for safety, otherwise if there was a fault in the power company's transformer, your service could "float" to several thousand volts.

When you measure from L1 to L2, you'll get around 240V RMS.
Since Neutral is tied to earth ground, and L1 and L2 are opposite in phase to each other, you'll get about 120V RMS from either L1 or L2 to Neutral or ground.

Ground is there to protect humans from injury.
Neutral is the return path for 120V.

You should never attempt to use the ground wire as a return path. If there is current on the ground wiring, there is a problem somewhere that needs to be corrected.

Your neutral line should always measure within a few volts of the ground line. If it's more than that, you have a problem somewhere.

4. ### Pich Senior Member

Mar 11, 2008
119
5
When mesuring L1 or L2 on a 208 system to ground it will give you 120v. In your casse the reason you get 120v on both sides of the col is because the 240v is switched on on side only. Therefore when you mesure on the L1 side of the coil you read the unswiched part of the 240 volt power and when mesured on the other side of the coil you are reading the the 240 of the 240v that has not been switched. It is a good practice to switch both L1 and L2 when the system has a grounded neutral, for safety and and easier trouble shooting.

5. ### eels1212 Thread Starter New Member

Feb 21, 2008
7
0
Thanks for the help guys,
I don't know if I was being very clear with my example. Pich is that why I was seeing in a denergized state the coil had 120v to ground on each side(L1to ground, L2 to ground) of the coil but zero betwenn L1 and L2. When the coil was energized(or switched on) I saw 240v across the coil L1 to L2 and still 120v to ground each side of the coil(L1 to ground, L2 to ground). That's what confused me. And this is because only 1 leg is switched?
Thanks again
Ed

6. ### Pich Senior Member

Mar 11, 2008
119
5
Both L1 and L2 have a potintial of 120v to ground. If only one leg is switched you will read 120v to ground on the other leg on both sides of the coil becauses the coil just becomes a conductor for the unswitched leg. Yes I think only one leg is switched