Never use a surge protector when wiring a step-down transformer converting 220-volt electric current to 110-volt electric current. Alternately, use a step-down transformer that has electrically isolated windings kept entirely separate from one another.
When wiring a step-down transformer without separate, isolated windings, the circuit along which the 110-volt current flows is not isolated from the 220-volt input. The design of European-style 220-volt plugs leads to a 50 percent chance that a 220-volt difference in voltage occurs between the neutral and ground when using such a step-down transformer. While this is not a dangerous problem normally, the inclusion of a surge protector on the 110-volt side can destroy electrical appliances and even cause electrical fires to break out.
Surge protectors contain components called varistors that absorb short, sporadic bursts in voltage. They are not designed to cope with extended exposure to abnormally high voltage and may trigger as a result of the large difference in voltage between the ground and neutral of the step-down transformers' circuit.
The easiest way to determine whether a particular step-down transformer has electrically isolated windings is by looking at how many power outlets the device has. If the transformer has two outputs, a 110-volt output and a separate 220-volt output, it is dangerous to use in conjunction with a surge protector.