Never Use a Surge Protector with a Step-Down Transformer


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.

A step-down transformer

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.

The problem 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

What this means is that the 110 volt output is not electrically isolated from the 220 volt input. What's worse, in many European countries the power plugs can be plugged into the outlet two different ways, which leads to another problem: there is a 50 percent chance that the voltage between neutral and ground will be 220 volts.

Non-polarized "schuko" power plug
Schematic of step-down transformer, opposite plug orientation

The problem with surge protectors

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.

Surge protectors contain varistors, 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 absorb 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 me

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 time.

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 use a surge protector on the 110 volt side of a 220-to-110 volt step-down transformer if the power plug can be plugged in two different ways. It could kill you. If you must use a power strip, make very sure it doesn't have surge protection built in.

If you are in a country with polarized power plugs, such as the UK, this problem is less likely to affect you, but it's still possible if either the wall outlet or the power plug of the transformer has been wired with incorrect polarity. Exercise care.

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