Mike Holt Case Studies

[ATSman]

Found this interesting article on case studies of neutrals and grounding:

http://www.mikeholt.com/documents/technical/2008-01-Stray-Contact-Voltage-Case-Studies.pdf

 

[steve66]

I'm confused.

Case Study #1; Slide or page #37:

Question #1: Why does the primary voltage neutral have to be connected to the low voltage neutral?
Question #2: How does a "spark gap" or MOV count as a "connection"?
Question #3: On page 39, that wire looks really small, but again, I'm confused on what the MOV is supposed to be doing.

Seems to me like the solution would be to replace the cable.

I really like the fine print - "we don't replace cable due to neutral corrosion." Sounds like they are saying its too expensive to do it right.

 

[ghostbuster]

To Steve 66

http://ecmweb.com/power-quality/case-stray-voltage-lake

Link to ecm article explains neutral ground connections--utility and customer---hope this helps

 

[GoldDigger]

I'm confused.

Case Study #1; Slide or page #37:

Question #1: Why does the primary voltage neutral have to be connected to the low voltage neutral?
Question #2: How does a "spark gap" or MOV count as a "connection"?
Question #3: On page 39, that wire looks really small, but again, I'm confused on what the MOV is supposed to be doing.

Seems to me like the solution would be to replace the cable.

I really like the fine print - "we don't replace cable due to neutral corrosion." Sounds like they are saying its too expensive to do it right.

The NEC does not care whether the primary and secondary neutral are connected or not. That is strictly an NESC and POCO policy issue.
Some path is needed beyond just the earth rod resistance to handle bolted GF currents and lightning induced currents. The service neutral to transformer connection is perfectly good for the first purpose unless it is a primary hot that faults to secondary side. A connection with a high breakdown voltage such as a spark gap or MOV can service this purpose just fine.
Same for lightning surges.
Note that the voltage drop in the primary neutral from primary current is a very small percentage of the primary voltage and will not interfere with operation of primary OCPD.

 

[steve66]

The NEC does not care whether the primary and secondary neutral are connected or not. That is strictly an NESC and POCO policy issue.

That's what I was thinking, although the document states the opposite.

The service neutral to transformer connection is perfectly good for the first purpose unless it is a primary hot that faults to secondary side. A connection with a high breakdown voltage such as a spark gap or MOV can service this purpose just fine.
Same for lightning surges.

OK, so we really just need something to protect from high over voltages on the secondary. But I'm a little skeptical that MOV and the attached wiring (maybe #6?) can clear the upstream high voltage OCP on a primary to secondary transformer fault before the MOV self destructs.

I'm not a utility engineer by any means, and I haven't read the other 3 "case studies", but the whole thing seems like a Band-Aid fix to me.