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#128966 05/25/04 11:20 AM
Joined: May 2004
Posts: 3
Junior Member
Hello there. First time post. I am a CET and have studied electronics for years. My teachers could not answer these questions:
What is the nature of the service entrance neutral line vs. residence supplied earth ground? Older homes tie neutral and ground together. Newer panels have isolated neutral bus bars. If neutral is merely the center tap of a 240v service pole transformer, why is there no voltage potential between it and earth ground? More questions later.........


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#128967 05/25/04 11:56 AM
Joined: May 2004
Posts: 697
Frank,there is no voltage because it is bonded, as required, usually by a screw that bonds the neutral bar to the distribution panel enclosure.


#128968 05/25/04 11:57 AM
Joined: Aug 2001
Posts: 7,520
Hi Frank,

The incoming neutral is just a center-tap on the 240V transformer winding, but it is also the point which is grounded. Thus there is 120V RMS from either outside terminal to ground, but the AC sinewaves are 180 degrees out of phase, giving 240 between them.

As a CET you will be familiar with the common full-wave rectifier circuit using two diodes from outer ends of a secondary winding and the center-tap connected to chassis/common. It's a similar arrangement on the 120/240V power service, except that no rectification is involved.

As I understand it (I'm in England) all American services have the neutral busbar connected to earth-ground at the service entrance, either by using a single bar with both the incoming neutral and the grounding electrode conductor connected to it, or by having separate neutral and ground bars but with a bonding screw/jumper connecting them.

The place where the neutral and ground bars have to be separate is in a sub-panel. If they were bonded together at that point, then you would have a parallel path between the main panel and the sub-panel, and some of the neutral current would flow along the grounding conductors (and possibly also along conduits, trunking etc.).

#128969 05/25/04 12:28 PM
Joined: May 2004
Posts: 3
Junior Member
Thank you DAVE55 & PaulUK.
I will go back to the prototype board and examine the effects of a secondary winding output. I understand completely what you mean about the center tap, but here comes the rub......If the center tap is not grounded, it should conduct in both directions, therfor there would be voltage potential between it and ground. When it is earth grounded,( as opposed to chassis ground) there is no voltage potential. Why then doesnt the grounded center tap represent itself as a short circuit to earth ground ? How can current flow in the circuit if half of the cycle current is flowing to ground? Have i confused you yet??????

Thanks again,

ps.. Ill be back..........

#128970 05/25/04 12:49 PM
Joined: Apr 2004
Posts: 349
Hi Frank,

You need to be aware that the center (neutral) tap is also grounded at the transformer location (on the pole), so, in theory, even if the neutral to ground bond were lifted at the service panel, the neutral is still grounded (at the pole).

To follow up on something Paul said, within the premesis wiring, the neutral is bonded to ground at only one point (the main service panel), and remains separate at all points downstream.

Just an idea here - you can play around with this idea some at safer voltages by getting a 12/24V stepdown transformer, using the center connection as we use a neutral.


There are 10 types of people. Those who know binary, and those who don't.
#128971 05/25/04 01:19 PM
Joined: May 2004
Posts: 3
Junior Member
Thank you RADAR!! And again, thank you Dave55 and PaulUK!!
Ok, I have to wait to get to my experimenters box. I will set up a small model utilizing the 12/24 (or similar) transformer and build a scale model. This has been haunting me for so long. You guys are the best!!! Ill be back next week.
Thanks again,


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