Ungrounded Systems (was 600 VDC switches) [RE-wrenches]

[Allan Sindelar]

Wrenches,
I forwarded John's and Larry's post to John Wiles for his take on this, as
their points made sense to me, too. John's response is good, so I'm
forwarding it here.

Allan:
Post the following at your own risk.  You can tell them its from me if you
want.  Bill Brooks offered to talk with anyone about the 2005 NEC drafts in
a recent post.

The PV community and the code writers have long understood that both
grounded and ungrounded systems can be installed and operated safely.  There
are proposals that are being drafted and that will be submitted for the 2005
NEC that will "permit" ungrounded PV systems.  However to make them as safe
as they are and have been in Europe, the balance of systems complexity and
costs may go up.  Here is the current Draft.  It will be submitted to NFPA
before 1 November 2002.

690.35  Ungrounded Photovoltaic Power Systems (New)

Photovoltaic power systems operating at a maximum systems voltage of over
125 volts direct current shall be permitted to operate with ungrounded
circuit conductors where the system complies with (A) through (E).

(A)  All ungrounded photovoltaic source and output circuit conductors shall
have disconnects complying with 690 Part III.

(B) All ungrounded photovoltaic source and output circuit conductors shall
have overcurrent protection complying with 690.9.

(C) All ungrounded photovoltaic source and output circuit conductors shall
be provided  a ground fault protection device or system complying with
(1)-(4).

   (1)  Detect a ground fault current at least as low as 1.0 ampere for each
10 kilowatts of PV array (direct current output at Standard Test
Conditions).

(2)  Interrupt the flow of the detected ground fault current.

   (3)  Indicate that a ground fault has occurred.

(4)  Automatically disconnect the ungrounded faulted conductors and/or shut
off the utility-interactive inverter or charge controller for that portion
of the faulted array.

(D)  The ungrounded photovoltaic source and output conductors shall consist
of sheathed (jacketed) multi-conductor cables or shall be installed in
conduit .

(E)  Ungrounded circuit conductors shall be permitted to be used with
ungrounded battery systems complying with 690.71(G).

Substantiation

In many parts of the world, photovoltaic power systems are being installed
and operated safely with none of the PV source circuit or PV output
circuits.  Peer-reviewed IEEE technical papers have substantiated the fact
that grounded and ungrounded PV systems may be installed and operated
safely.

This proposal allows PV system operating at a maximum system voltage of over
125 volts to operate with the PV source and output circuits ungrounded
providing several conditions are met.  These conditions are similar to the
conditions under which ungrounded European PV systems are installed and
operated safely.

The maximum system voltage of 125 volts was selected to preclude the use of
ungrounded systems on the lower voltage 24 and 48-volt nominal systems
because much of the available hardware (switchgear, overcurrent devices,
ground fault devices, etc) for these lower voltage systems is designed for
use with grounded systems.  Utility-interactive systems operating at the
higher voltages will benefit from using ungrounded PV arrays in terms of
cost and performance.

Paragraphs (A) and (B) require that the ungrounded system complies with NEC
provisions found in other articles.

Paragraph (C) ensures that ground faults are detected, interrupted, and
disable the array or faulted portion of the PV array.  Turning off the
inverter or charge controller effectively meets the requirement to disable
the PV array and is allowed.

Paragraph (D) requiring sheathed or jacketed cables or conduit is consistent
with European practice requiring double insulation on PV conductors.  There
are no single-conductor cables in the US that are equivalent to the European
double insulated conductors.  For example, European USE-2 single conductor
cables have an insulation that is nearly twice as thick as the standard
USE-2 available in the US.  The use of a jacketed cable or conductors
installed in conduit provides the second layer of insulation and enhances
the safety of these higher voltage systems where the ungrounded conductors
are exposed to weathering, ultra violet exposure and possible decay over
time.

Paragraph (E) allows these higher voltage ungrounded PV systems to be used
with the higher voltage ungrounded battery systems.

----- Original Message -----
From: <jberdner at sma-america.com>
> The whole grounding paranoia we have here in the US makes no sense to
> me.  It seems like the regulatory and inspector community has had
> "ground everything" beaten into them for so long they have forgotten to
> ask why we do it in the first place. IMHO, the array should be floating,
> not grounded, at all times and certainly when you work on it.  Solidly
> grounding the array gives you a single fault lethal hazard which seems
> stupid to me.  Fortunately the Sunny Boy's ground fault fuse IS the
> connection to ground and it is allowed to be there by Code because it is
> part of the GFDI.

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