subject: Understanding & Using Solar DC-AC Inverters – Part 5 [print this page] Understanding & Using Solar DC-AC Inverters Part 5
Inverters & safety
Finally, there are a couple of important safety aspects to bear in mind whenever
You're using a DC-AC inverter. Many people assume that because an inverter is operating from a nominal 12V battery and it can't deliver as much output as a normal mains power outlet, its relatively safe. Nothing could be further from the truth.
Even a low power inverter rated at a mere 60 watts has an output which is
potentially fatal, if you should end up connected across it. Such an inverter can
typically deliver up to about 360mA at 230V, which is over TEN TIMES the
current level needed to stop your heart and cause fatal fibrillation!
Needless to say higher power inverters are even more dangerous.
There.s also another kind of safety risk associated with inverters, which arises
from the fact that in many inverters, there.s a direct electrical path between the
mains-voltage output circuit and the low voltage input circuitry (including the
battery leads). This path is usually via the auto turn-on sensing, and possibly
also the voltage or current sensing used for output regulation.
When the inverter is being used to power a single tool or appliance, this internal
current path normally doesn.t pose any safety risk because the complete batteryinverter-
appliance system .floats. above earth. However if the tool or appliance
is faulty and develops a short circuit or severe leakage between its mains wiring
and its external metal case or frame (which would normally be earthed, when
it.s plugged into a mains power outlet), there.s a risk that the battery connections can become dangerously .live..
How can this happen? Think about it: if the leakage path to the appliance.s
frame happens to be from the side of its mains wiring connected to the .active.
side of the inverter.s output, and the frame of the appliance becomes connected
to earth, this will immediately raise the .neutral. side of the inverter.s output to
the full output voltage above earth .
and with it, the low voltage side of the inverter.s circuitry and the battery terminals as well. So if an unsuspecting person who also happens to be earthed should touch one of those safe-looking 12V battery terminals, they can receive a potentially fatal shock.
It.s for this reason that inverter manufacturers and suppliers generally advise
strongly against connecting an inverter into the permanent wiring of a house,
office or factory . especially in a way where the appliances connected to its
output tend to become automatically connected to mains earth, and/or linked
back to mains neutral (which is ultimately earthed, with the MEN system).
If you want to run a number of appliances from a single high-power inverter,
and have the convenience of permanent wiring and mains-type power outlets,
the safest approach is to have a wiring system and outlets that are kept totally
separate from any wiring that is connected to the AC mains and mains earth. The
outlets driven from the inverter output are also best left unearthed, and clearly
marked as .
INVERTER POWER: FLOATING..
This totally separate, floating system not only reduces the risk of accidental
shocks due to faulty appliances, but also helps to remind users so they don.t
accidentally plug inappropriate appliances into the inverter outlets.
In any case, always think twice before touching the terminals of a battery that
you know is being used to power tools or appliances via a DC-AC inverter.
Remember that there.s always a risk those battery terminals could deliver you a
much greater shock than you.d ever get from 13.8 volts, if one of those tools or
appliances should develop a fault.
Understanding & Using Solar DC-AC Inverters Part 5
