Question on test.

[AC\DC]

A functionally grounded conductor is a special case of a non-solidly grounded conductor, which is a special case of a grounded conductor, but the nonsolidly grounded conductor color coding is defined clearly in 690.31(B)(2)(b), so I don't see why any further code work is needed.

I did not dive deep enough. It does modify 200 thanks

 

[Carultch]

So, exactly what is a functionally grounded conductor and where can it be found? With transformer coupled inverters one could say that the negative DC conductor was functionally grounded, but they have gone the way of the dodo.

On systems from 10 years ago, it was common practice for an inverter to ground one of the polarities thru a GFCI fuse or breaker. That polarity would be the grounded conductor, and the standard required you to identify it as you'd identify any other grounded conductor, and label the inverter to inform the user of the possibility of an ungrounded white wire during a ground fault. The code didn't have the term "functionally grounded" at the time. In concept, a system like that could be built today, and the grounded polarity would be the "functionally grounded conductor". The current standard requires you to identify it, as if it were an ungrounded conductor, in contrast from the pre-2017 standards.

Today, most inverters no longer isolate the DC and AC sides, so the two polarities "float" at symmetric DC voltages to ground, and use a different means of detecting ground faults. They still call this a functionally grounded system, as long as the AC side is connected to a grounded grid. I consider both of these conductors to be ungrounded in this case. The reason the NEC calls it functionally grounded, is that there is only one degree of freedom to set the two voltages relative to ground, rather than both of them being two independent ungrounded voltages. My preferred term for this, is "symmetry grounded system".

I like the term functionally grounded in general, but I dislike that the NEC assigns the same term for two wildly different kinds of systems.

 

[tortuga]

Interesting thanks @Carultch
I don't what a good term would be for the present systems but I would not include the word grounded at all.

 

[Carultch]

Interesting thanks @Carultch
I don't what a good term would be for the present systems but I would not include the word grounded at all.

What makes a system ungrounded, is that the source only establishes the voltages between/among the current-carrying conductors, and there is a degree of freedom that allows ground to be anywhere relative to any of the current-carrying conductor voltages. Ungrounded DC has a single degree of freedom (signed voltage only), while ungrounded AC has 2 degrees of freedom (magnitude and phase).

A grounded system has zero degrees of freedom, once the source establishes the voltages across each pair of conductors.

 

[ggunn]

On systems from 10 years ago, it was common practice for an inverter to ground one of the polarities thru a GFCI fuse or breaker. That polarity would be the grounded conductor, and the standard required you to identify it as you'd identify any other grounded conductor, and label the inverter to inform the user of the possibility of an ungrounded white wire during a ground fault. The code didn't have the term "functionally grounded" at the time. In concept, a system like that could be built today, and the grounded polarity would be the "functionally grounded conductor". The current standard requires you to identify it, as if it were an ungrounded conductor, in contrast from the pre-2017 standards.

I was designing PV systems back then and I do not remember using a white wire for DC negative. As best I can recall, DC conductors were red and black - red for positive and black for negative.

 

[Dennis Alwon]

I was designing PV systems back then and I do not remember using a white wire for DC negative. As best I can recall, DC conductors were red and black - red for positive and black for negative.

Just as it is in automobiles

 

[Carultch]

I was designing PV systems back then and I do not remember using a white wire for DC negative. As best I can recall, DC conductors were red and black - red for positive and black for negative.

It was a logical consequence of the rule requiring grounded conductors to be identified as white or gray. The norm back then in projects I had designed, was white for the grounded, and black for the ungrounded (regardless of sign). There was no code requirement until 2014 to follow the red & black color scheme, that is the standard for automotive wiring. It was most common to bond the negative polarity, unless you had a specific reason otherwise (e.g. bipolar, or modules requiring positive ground).

Oh wow, I realized 2014 was ten years ago. Anyway, replace ten with twelve, since I'm talking about pre-2014 systems when GFCI-grounding of one polarity and transformered inverters were the norm. 2014 was when I first started seeing non-isolated inverters in the US market.

 

[tortuga]

What makes a system ungrounded, is that the source only establishes the voltages between/among the current-carrying conductors, and there is a degree of freedom that allows ground to be anywhere relative to any of the current-carrying conductor voltages.

That sounds good to me, but I can also see how logically a service electrician would expect any conductor with 'grounded' in the name be eventually connected to a ground bar somewhere even If its a special oddball system with a fuse or a ground resistor. Anything else should be some type of ungrounded conductor.

 

[ggunn]

Circling back around...

Can anyone give contemporary examples of functionally grounded conductors?

 

[tortuga]

Circling back around...

Can anyone give contemporary examples of functionally grounded conductors?

I am no expert in this but perhaps
Page 4 of

https://files.sma.de/downloads/SC-HEM-UEN083710.pdf

mentions this:

The essential difference to the standard devices in the Sunny Central product range is that one of the
PV generator's poles is centrally grounded in the Sunny Central by a fuse. This system concept was designed
in the USA and has been used in the inverters for the American market since it was first commercialized.
There are no normative stipulations regarding this concept. Determination of the maximum leakage current,
which causes the GFDI to disconnect the system, occurs in accordance with UL1741.

 

[roger]

I think given the fact that there has been 50 posts to date we can absolutely conclude the author of the test needs to go back to the drawing board.

 

[ggunn]

I think given the fact that there has been 50 posts to date we can absolutely conclude the author of the test needs to go back to the drawing board.

Not all of us.

 

[jaggedben]

I think given the fact that there has been 50 posts to date we can absolutely conclude the author of the test needs to go back to the drawing board.

This. I was about to say something like this. Also Don is correct the code needs work. Or at least, the code does not directly answer the test question.

FWIW twelve years ago me and my co-workers were pulling white wire for the conductor that was then considered grounded but has since been redefined to being functionally grounded.

 

[ggunn]

So where might any of us encounter a functionally grounded conductor these days? Someone mentioned Sunny Central inverters; I don't know if that is factual but I would hazard a guess that even if true it would not be something most of us are likely to encounter.

 

[tortuga]

So where might any of us encounter a functionally grounded conductor these days?

How would we know? would it make a difference? It seems the code is saying you treat functionally grounded the same as ungrounded.

 

[tortuga]

the code does not directly answer the test question.

I thought solarken had the answer:

690.31(B)(2)(b)

 

[LarryFine]

I thought solarken had the answer:

Like that's gonna stop us!