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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>All,<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>With all due respect to the many people who have been involved
in the review and comment on this fire event over the past 1.5 years, there is
more misinformation and misinterpretations about this particular fire than any
we have had to date. It stems from a variety of information that was posted
about this event including a good, but not complete, report from Pete Jackson, the
inspecting electrical authority.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>BAKERSFIELD FIRE not Target Fire:<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>The “Target Fire” I prefer we refer to as the “Bakersfield Fire”
since it happened near Bakersfield. The fact that it happened on a Target store
is unfortunate, but that retail establishment should not be referred to in
subsequent postings, since it has a lot of PV and has been very supportive of
the PV industry. SunPower, while allowing poor wire management for years, is
also not to blame ultimately for this fire.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>THE PROBLEM:<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>The problem with the Bakersfield Fire is endemic to all PV
systems over about 30 kW in size that have been built in the last 5 years.
Since 2005, large inverters have been employing ground fault protection
equipment that lifts the grounded conductor in a fault. Prior to 2005, the
Trace Technology 3-phase inverters sensed the grounded conductor to ground
connection, shutting the inverter down if the current got above 10 amps. The
2008 NEC required that most PV systems have a GFP and that the fault path be
interrupted, so subsequent designs started employing a ground fault fuse
similar to small residential inverters.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>NOT GOOD:<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>The presupposition of this ground fault protection (GFP) method
is that it must be the first fault. If any fault exists prior to opening the
ground fault fuse, the GFP system is ineffective and potentially worse than not
doing anything at all. If a fault occurs on a string-level grounded conductor
on a larger system (30 kW would do it), the ground fault current generated
cannot exceed a few amps. This is due to the fact that there is very little
voltage pushing the current. At best, we can probably run 3 amps through a
fully bolted fault in the source circuit grounded conductor. This is
problematic since the ground fault fuses on large PV inverters are set at 4
amps.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>PERFECT STORM:<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Therefore, since we can’t see the fault in the grounded conductor,
we proceed with operation although the status of the system has gone from “safe”
to “fire hazard”. If the next fault (and one fault always results in a second
fault eventually) occurs in a large feeder circuit (100 or more amps), then the
GFP fuse instantly opens removing the main ground connection on the array.
However, this is exactly what you don’t want to do if you already have a fault
in array. Now, instead of having a large equipment grounding conductor to carry
the fault current, a 10AWG wire (or smaller) gets the entire return current as the
short circuit now has to return at the point where the first undetected fault occurred.
Shortly that conductor gets so hot that the insulation on the conductor catches
on fire and then the fire is driven into whatever flammable material is next to
the wire (PV module underside).<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>THE BAKERSFIELD MISLEADING MESSAGES:<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Until the Bakersfield thermal event (fire), very few believed
that this could happen. Many still believe this to be a “two-fault” event that
we generally do not have to design for. I disagree. The likelihood that this
will happen multiple times in the future is not only possible but very likely. Since
the first fault is not visible to the GFP, it is a dangerous blind spot. The
first fault could exist at installation if proper commissioning tests are not
done (as pointed out by Matt). It is very easy to repeat experimentally. <o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Unfortunately, the current messages coming from the Bakersfield Fire
miss the point. The first common message is that we need to focus on expansion
joints for raceways. This is a good message, and improper techniques by both
installer and inspector probably resulted in the feeder fault that drove the
fire, all the best expansion joints in the world will not solve the fire
problem. Another common message is that disconnects would have solved the
problem. While I helped develop the code requirement for the 2011 NEC requirement
for disconnects in sight of combiner boxes, disconnects could not have stopped
the fire. Disconnects would have only helped with cleanup and overhaul operations.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>THE REAL MESSAGE—CHANGE GFP METHODS:<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>While expansion joints, disconnecting means, and proper wire
management are all excellent messages from this fire, the real message to the
codes and standards community is to change the way we design our GFP circuits. Good
array design and installation practices can certainly reduce the possibility of
faults, but it is impossible to eliminate these problems in a system that is intended
to operate for 25 years, regardless of how much O&M is done. The only way
to resolve this problem is with GFP devices that can detect faults in the source
circuit grounded conductor, or better yet—don’t have a grounded conductor at
all. Arc fault detectors are not the answer since most of these faults start as
ground faults—just as at Bakersfield. The only way to get GFP fault detection
below 1 amp for large PV systems is to UNGROUND or resistively ground the array
circuit—just as they do it in Europe and Japan. Currently, European inverters
can detect faults below 300 mA. That is an order of magnitude better than we
can with our solidly grounded systems. It is also low enough to detect faults
in just about all array types.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>RELATIVELY EASY FIX:<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Moving to ungrounded or resistively grounded arrays is relatively
easy and the NEC covers the requirements in 690.35. We already have several
transformerless designs from Power-One and SMA (new TL line in the U.S.), but there
is nothing holding back larger inverters using transformers with ungrounded
arrays. Existing arrays could be made much safer if they were to be retrofitted
with a circuit at the inverter that would lift the grounded conductor each morning
and test for ground faults on the grounded conductor prior to starting the
inverter. While not quite as safe as “European-style” systems, it is a large
improvement over where we are today. Don’t be surprised if things change about
how we do GFPs soon. If I have anything to do with it, it is going to happen.
The current hazardous situation, and bad information about the Bakersfield
fire, cannot continue.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Bill.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Bill Brooks, PE<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Principal<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Brooks Engineering<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>873 Kells Circle<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Vacaville, CA 95688<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>707-332-0761 (office and mobile)<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>707-451-7739 (fax)<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>bill@brooksolar.com (email)<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>www.brooksolar.com (web)<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
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