Three phase powered offline SMPS failure

MaxHeadRoom

Joined Jul 18, 2013
30,684
You could be right. But wouldn't it be a good thing to do connecting neutral to earth at the electric post?
Here it is only permitted where the Neutral becomes isolated from Earth GND, e.g. a secondary of a transformer, it is then permitted to set up a new earthed neutral.
 

MisterBill2

Joined Jan 23, 2018
27,607
Here it is only permitted where the Neutral becomes isolated from Earth GND, e.g. a secondary of a transformer, it is then permitted to set up a new earthed neutral.
Connecting neutral to earth assures that contacting any phase will deliver a serious shock, while a truly floating circuit will not deliver a shock if only one point is contacted.
Next, given the implied importance of the low voltages, they must power the billing portion of the system. What could be more important than assuring that every milliwatt-second is charged to the customer?? (This is intended to be a bit of sarcasm, it does not reflect my vaues)
 

Ian0

Joined Aug 7, 2020
13,155
Connecting neutral to earth assures that contacting any phase will deliver a serious shock, while a truly floating circuit will not deliver a shock if only one point is contacted.
The problem with IT earthing is that it is single-fault tolerant.
The first live to earth fault goes unnoticed.
The second one delivers the serious shock.
That's why it is limited to systems where only one load is connected.
 

MisterBill2

Joined Jan 23, 2018
27,607
The problem with IT earthing is that it is single-fault tolerant.
The first live to earth fault goes unnoticed.
The second one delivers the serious shock.
That's why it is limited to systems where only one load is connected.
The smart person always assumes that every 400/480 volt wire is a hazard to touch, and always assumes no exceptions to that rule.
Natural Selection takes care of the others.
 

MisterBill2

Joined Jan 23, 2018
27,607
So What are you saying? That NFPA and NEC, CEC are wrong? :confused:
I did not say that at all. But the reality is that with one side of any voltage source solidly tied to actual GROUND, every live wire is a definite available source of shock. You KNOW THAT. The decision was made and that is how it is. Right or wrong is not part of the discussion for common mains power. Isolated private systems are a separate case, and often tradition prevails over logic.
 

nsaspook

Joined Aug 27, 2009
16,330
An old saying.

1) The voltage on the neutral won't HURT you.
2) The voltage on the neutral will KILL you.

Worked on Navy ships where all AC power is floating. It wasn't done because of safety. The single-fault tolerance capability is useful when power to equipment is more important than any one person being electrocuted.
What kind of ground do they have on ships? A floating ground!
 
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