Been doing elec work for around 30 years and still not clear on why the neutral and ground are bonded together at the main service in a home, but a subpanel has to have the neutral and ground isolated.

If the neutral/grounded conductor and EGC are bonded together at a subpanel, the neutral's current flow back to the main panel will be partially carried on the EGC. This would be a potential shock hazard for someone coming in contact with the EGC.

steve

But it seems that if they are tied together at the main panel, it would act the same at the sub panel too.
And wouldn't there be a EGC potential shock hazard at the main panel too?

Only if the neutral opened up or at least didn't have a good connection with the source.

But if the neutral opened up at the sub-panel, it is still tied together to the ground at the main panel.

But with no N-EGC connection at the sub, there won't be current flow on the EGC. Current is always trying to get back to the source(transformer, generator). With the neutral open at the sub panel, the return current is now on the ungrounded conductors, with this causing erratic voltages on the sub panel's circuits.

But why would there be current flow on the SP EGC when it is tied to the neutral at the main panel.

Basic electrical theory: Electric current flows in _circuits_. When the equipment ground is bonded to the neutral at a single point only, there is no closed path, and thus no circuit. With no closed circuit, current cannot flow.

This ideal of single point grounding is only achieved when there is a single customer on the transformer. With multiple services connected to the same transformer, you have multiple point grounding of the electrical system, and thus closed circuits on the grounding electrodes and grounding electrode conductors, with current flowing through these parallel ground circuits. Plumbers have been shocked cutting water pipes after the neutral connection has failed.

-Jon

 
Find a new or used copy of IAEI/Soares’ Book on Grounding.

Well I am taking a deep breath and making a first post in reply to this. Take it easy on me this is my understanding of grounding.

1. grounding rod and netural. you never want to be able to get a shock from netural. So by grounding them at service entrance, all neturals throughout the system stay as close to netural as possible.

2. purpose of a ground wire is safty not "normal" operation, so you never want current in a ground wire under normal conditions. This begs the question, how would you get current in a ground wire? Hot wire shorts to something grounded. this creats a total short and will blow the breaker. Yes netural is at the same potential, but you do want current flowing in the netural.

3. ground loops. these are caused by current flowing in the ground wires. Your ground is no longer a true ground. You could get shocked by touching "grounded" metal. Main cause of these is netural and ground connected somewhere it should not be.

Well, at least this is my understanding of it.