More than a century after the famous “War of the
Currents,” the relative safety of AC versus DC electrical systems is still a subject of discussion, particularly in today’s
photovoltaic (PV) power systems. The discussion is renewed due to the recent availability of AC modules, which for the first time give
PV system installers the option to install solar modules with no DC wiring.
An AC module is a PV module with an integrated
microinverter. In a true AC module such as that enabled by the SolarBridge PantheonTM, all DC wiring is protected. The combination
of the inverter and PV module is listed by the Nationally Recognized Testing Laboratory (NRTL), and the National Electric Code
(NEC) defines an AC module as follows:
“A complete, environmentally protected unit consisting of
solar cells, optics, inverter, and other components, exclusive of tracker, designed to generate AC power when
exposed to sunlight.”
In an AC module, the integrated microinverter takes in a
relatively small DC voltage (18 V to 36 V for most silicon modules) and transforms it to line-frequency AC power. AC modules are
connected through AC, rather than DC, distribution.
Due to the technical challenges of producing a
microinverter that is both cost-effective and reliable enough to coexist with a
PV module, true AC modules are only now being introduced to
the marketplace. In addition to the well-known energy harvest and installation cost benefits, AC modules also have
advantages in safety over DC-based PV systems.
There are three safety considerations reviewed in the
following sections: shock hazard, fire prevention/arc faults, and rescue worker safety.
The classic debate of whether DC power or AC power is
more dangerous is moot to this discussion. It is generally acknowledged that it takes less DC current to be lethal. However, the
amount of current that actually flows depends on many factors, including the amount of voltage driving that current. In the U.S.,
AC distribution with AC modules will be the standard 240 VAC or 208, whereas typical DC distribution ranges from about 150 V
to 600 V. In either case, there is plenty of voltage available to generate lethal currents.
The relevant issue is whether these voltages exist when
human contact is possible. A very important feature of AC modules is that the AC output automatically deactivates when
disconnected from the utility (for example, by opening a circuit breaker), as mandated by UL1741 (based on requirements in IEEE1547 and
NEC). The DC voltages within the AC assembly are only at the potential of a single PV module (much lower than from a
string of modules) and are suitably protected in the NRTL listed assembly.
Therefore, the modules themselves do not present live
electricity to their handlers.
On the other hand, conventional DC modules do not
inherently have a shut-off switch. As DC modules are wired together in series, they produce progressively higher voltages. All
but the smallest amount of light produces a potentially lethal voltage. There have been methods devised to attempt to provide a remote
shutoffbut so far with limited implementation. With AC modules, the shutoff is not only inherent, it is mandated by codes and
Shock is a hazard – primarily for installers and
maintainers of PV systems. However, as noted below, it is also an important
issue for rescue workers. During installation, there are many
opportunities for accidental cutting and stripping of high-voltage wires. During the nominal 25-year life of PV systems, there is
additional risk of degradation of insulation, nicks, scratches, etc. that can make maintenance of the PV system more dangerous.
With AC systems, PV practitioners must de-energize AC
distribution while installing or performing maintenance on a system, as part of normal protocol. This eliminates all live
electricity from the output of the AC module and therefore greatly reduces
Fire Prevention and Arc Faults
Arc faults are a major cause of PV fires-. Not only
are PV fires dangerous and expensive, they give a black eye to the entire PV industry. In PV systems, series and parallel arcs can
occur due to a variety of circumstances, such as resistive connections or damage to conductor insulation. Once an arc has started,
it can easily lead to a fire.
In both DC and AC systems, sufficiently high voltages
exist to produce arcs. The advantage of AC systems is that AC circuits are less likely to sustain arcs since the
current passes through zero twice per cycle and the internal DC voltage is onlythat of a single PV module instead of multiple modules.
It is a known fact that relatively smaller DC voltages can sustain an arc.
This problem has plagued telecom systems for decades,
even for nominal 48-V systems. A single PV module DC output voltage could also be as high as the telecom voltages, however,
it does not have the same risk of producing sustained arcs. The DC voltage from a single PV module is more likely to
collapse and prevent sustained arcing. The risk of sustained arcing and level
of damage from this arc go up with the number of DC modules
connected together in the system.
A variety of arc fault-detect devices exist on the market
today. However, since the DC power from a conventional PV module cannot be turned off (not even blanketing and foaming are
completely effective), there is more likely to be a sustained source of energy behind a fault. In an AC module system, if an
AC fault causes an abnormality in the grid voltage, the microinverter may detect it and shut off the PV power automatically.
The AC output distribution, the inherently limited
characteristics of a single PV module DC output combined with the investigations and follow-up inspections of the NRTL
listing the AC assembly make the overall AC system much safer with regard to arc faults.
Rescue Worker Safety
With the knowledge that conventional DC-based systems
have live voltage, the safety risk for firefighters and other rescue workers is increased. Shutting off power is one of the
first things that firefighters do. In the case of a DC system, shutting off AC power only shuts off the central inverter but there is
still live DC of hundreds of volts.
This poses risks to firefighters. First, firefighters may
be hesitant to spray water knowing that the conduction paths introduced by the water may actually make the fire worse. Second, it
may be more dangerous for safety workers to cut through roofs and walls (e.g., to gain entry or to let smoke out) since it could
result in chopping through live wires. As a result, there have been instances in which firefighters have simply let the building “burn
in a controlled manner” rather than try to extinguish the fire. Therefore, conventional DC systems may create inherent fire safety
concerns for all PV systems, which obviously is bad for the whole industry.
While working with electric power is always cause for
taking precautions, there are several safety advantages to using AC modules. In the past, PV systems installers had no choice
but to implement high-voltage DC systems. The SolarBridge Pantheon™ gives installers the freedom to choose a safer,
more reliable system. The “War of the Currents” may always continue, but for PV systems it looks like Westinghouse was right.
By Patrick Chapman, Ph.D. – Chief Technology Officer SolarBridge Technologies
Steve wurmlinger – Senior Regulatory Engineer SolarBridge Technologies