why is the neutral and ground in subpanel separated?

Originally Posted By: jortor
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I do understant this is against code, I would like to know the reason. thanks in advance.


Originally Posted By: jtedesco
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Look here:


http://www.nachi.org/bbsystem/viewtopic.php?t=9595&highlight=isolated

![icon_smile.gif](upload://b6iczyK1ETUUqRUc4PAkX83GF2O.gif)


--
Joe Tedesco, NEC Consultant

www.nachi.org/tedescobook.htm

Originally Posted By: Greg Fretwell
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The neutral is grounded at the service and that is referenced as zero volts to everything around you. From that point down the neutral wire, as soon as you start drawing neutral current, the neutral voltage will start to rise in reference to ground (the inverse of voltage drop on the “hot”).


If you then attach an equipment ground to that distant neutral the voltage will be imposed on the case of the equipment you plug in.


Originally Posted By: bbadger
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As Greg has pointed out having the grounds and neutrals connected at a sub panel can in layman’s terms energize all the metal items that are connected to that sub panel.


Say you have a metal cased appliance like a refrigerator that is run from a sub-panel that has the neutral and grounds tied together. Now if you touch this refrigerators case while the other hand is on the sink you may get blasted.

Another issue is it causes parallel paths for current that should be carried only by the neutral conductor.

In a house with all metal plumbing if for example you run a dishwasher from a sub-panel that has the neutral and ground connected you will have current flowing through the metal piping.

Why?

The neutral is bonded to the grounding at the service panel

With the sub-panels neutral also bonded to ground and the grounds connected to the dishwashers frame you have created a connection from the sub-panels neutral to the dishwashers frame and therefore also to the metal plumbing feeding the dishwasher.

Now any neutral current at the sub-panel will split, some will flow back to the service panel on the neutral conductor but some will flow over the dishwashers grounding conductor to the dishwashers frame then through the houses plumbing and back to the service through the grounding electrode conductor.

This is all very clear if you draw it, it is difficult to explain.

This flow of current through the plumbing creates a fire and shock hazard.

If a plumber cuts the pipe to add something they have a good chance of receiving a shock.


--
Bob Badger
Electrical Construction & Maintenance
Moderator at ECN

Originally Posted By: lfranklin
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Bob and Greg that may be difficult to explain,


But you do a good job explaining it.