Step-down transformers are commonly used to
convert the 220 volt electricity found in most parts of the world to
the 110 volts required by North American equipment. For example,
they are popular with American expatriates who don't want to throw
away their American appliances when moving to Europe.
Unfortunately, when used incorrectly, step-down
transformers can be dangerous. This article is a cautionary tale of
one incident that could have ended in disaster.
With Step-Down Transformers
Most step-down transformers (certainly all
inexpensive ones) are actually
autotransformers, meaning they have only a single winding with a
center tap rather than two separate, electrically isolated windings.
Schematic of step-down transformer, one plug orientation
this means is that the 110 volt output is not electrically isolated
from the 220 volt input. What's worse, even though the voltage
between hot and neutral on the output side is the desired 110 volts,
the voltage between neutral and ground will be 220 volts if
the power plug happens to be plugged into the outlet the wrong way.
European power plugs are not polarized, so there is a 50 percent
chance of this happening.
Schematic of step-down transformer, opposite plug orientation
The problem with
Usually, having 220 volts between neutral and
ground in an appliance designed for 110 volts is not a problem — the
insulation has a large safety margin. However, if you connect a
surge protector (or a piece of equipment with built-in surge
protection) on the 110 volt side, bad things can happen.
components which protect against surges by effectively shorting out
any excess voltage. Some surge protectors contain only a single varistor connected between hot and neutral; those will work fine
with a step-down transformer. However, many surge protectors have
additional varistors connected between hot and ground and between
neutral and ground. When a surge protector of this kind is used with
a step-down tranformer, one of these varistors can be subjected to
the full 220 volts. This is enough to trigger the varistor into its
conducting mode, effectively treating the 220 volts as a surge.
Varistors are designed to absorbed short-lived
surges, but they can't handle a persistent overvoltage. A varistor
subjected to twice its rated voltage will quickly be destroyed,
usually causing a short circuit and a blown fuse.
What Happened To
I had moved back to Europe from the U.S. and
brought with me some electrical appliances and a step-down
transformer. Because the transformer had only one outlet and I
needed to connect multiple appliances, I used a U.S. power strip
connected to the 110 V output. Like most power strips sold in the
U.S., it had built-in surge protection.
This worked fine for several weeks. Then I went
travelling and unuplugged everything just to be safe. When I
returned and plugged the step-down transformer back in, there was a
bang and the lights went out. A fuse had blown in my apartment's
breaker panel. I replaced the fuse and tried plugging in the
step-down transformer again; the fuse instantly blew for a second
I opened up the power strip and found that it
contained three varistors, one of which was charred. Measuring the
charred varistor with a multimeter showed that it was shorted out.
Innards of the surge protector, with charred varistor
Why It Happened?
This is what must have happened: When I
returned from my trip, I plugged in the surge protector the opposite
way from before the trip. Before the trip, there was no more than
110 volts over any of the hidden varistors in the power strips, but
with the plug oriented differently, the voltage rose to 220 volts,
causing the varistor to short out.
Schematic of connections at time of incident
What Could Have Happened?
It could have been much worse. For one thing,
if the varistors in the power strip had been sturdier, the one
getting the excess voltage might not have shorted out instantly and
blown the fuse, but instead slowly overheated and started a fire.
But what concerns me most is the following
scenario: After the initial incident, the blown varistor was
completely shorted out. This left the power strip with a short
between hot and ground, a very dangerous condition. If at that
point I had plugged the step-down transformer into an ungrounded
outlet, the chassis of any grounded 110 volt equipment connected
to the power strip would have been live with 220 volts, and I
would have stood a good chance of being electrocuted.
Never, ever use a surge protector on the 110
volt side of a 220-to-110 volt step-down transformer. It could kill
you. If you must use a power strip, make very sure it doesn't have
surge protection built in.