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http://www.gson.org/stepdown/ 

Never Use a Surge Protector with a Step-Down Transformer

BackgroundStep-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 eample! they are

 popular with American epatriates who don"t want to throw away their American appliances

when movin# to $urope.

A step-down transformer 

%nfortunately! when used incorrectly! step-down transformers can be dan#erous. &his article is a

cautionary tale of one incident that could have ended in disaster.

The pro!em with step-down transformers

'ost step-down transformers (certainly all inepensive ones) are actually autotransformers!

meanin# they have only a sin#le windin# with a center tap rather than two separate! electrically

isolated windin#s.

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Schematic of step-down transformer! one plu# orientation

*hat this means is that the 110 volt output is not electrically isolated from the 220 volt input.

*hat"s worse! in many $uropean countries the power plu#s can be plu##ed into the outlet twodifferent ways! which leads to another problem+ there is a 50 percent chance that the voltage

between neutral and ground will be 220 volts.

 Non-polari,ed schuo power plu#

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Schematic of step-down transformer! opposite plu# orientation

The pro!em with surge protectors

%sually! havin# 220 volts between neutral and #round in an appliance desi#ned for 110 volts is

not a problem / the insulation has a lar#e safety mar#in. owever! if you connect a sur#e

 protector (or a piece of equipment with built-in sur#e protection) on the 110 volt side! bad thin#scan happen.

Sur#e protectors contain varistors! components which protect a#ainst sur#es by effectively

shortin# out any ecess volta#e. Some sur#e protectors contain only a sin#le varistor connected

 between hot and neutral those will wor fine with a step-down transformer. owever! manysur#e protectors have additional varistors connected between hot and #round and between neutral

and #round. *hen a sur#e protector of this ind is used with a step-down tranformer! one of

these varistors can be subected to the full 220 volts. &his is enou#h to tri##er the varistor into itsconductin# mode! effectively treatin# the 220 volts as a sur#e.

3aristors are desi#ned to absorbed short-lived sur#es! but they can"t handle a persistent

overvolta#e. A varistor subected to twice its rated volta#e will quicly be destroyed! usually

causin# a short circuit and a blown fuse.

"hat happened to me

4 had moved bac to $urope from the %.S. and brou#ht with me some electrical appliances and a

step-down transformer. 5ecause the transformer had only one outlet and 4 needed to connectmultiple appliances! 4 used a %.S. power strip connected to the 110 3 output. 6ie most powerstrips sold in the %.S.! it had built-in sur#e protection.

&his wored fine for several wees. &hen 4 went travellin# and unuplu##ed everythin# ust to be

safe. *hen 4 returned and plu##ed the step-down transformer bac in! there was a ban# and the

li#hts went out. A fuse had blown in my apartment"s breaer panel. 4 replaced the fuse and tried plu##in# in the step-down transformer a#ain the fuse instantly blew for a second time.

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4 opened up the power strip and found that it contained three varistors! one of which was charred.

'easurin# the charred varistor with a multimeter showed that it was shorted out.

4nnards of the sur#e protector! with charred varistor 

"h# it happened

&his is what must have happened+ *hen 4 returned from my trip! 4 plu##ed in the sur#e protectorthe opposite way from before the trip. 5efore the trip! there was no more than 110 volts over any

of the hidden varistors in the power strips! but with the plu# oriented differently! the volta#e rose

to 220 volts! causin# the varistor to short out.

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Schematic of connections at time of incident

"hat cou!d have happened

4t could have been much worse. For one thin#! if the varistors in the power strip had been

sturdier! the one #ettin# the ecess volta#e mi#ht not have shorted out instantly and blown the

fuse! but instead slowly overheated and started a fire.

5ut what concerns me most is the followin# scenario+ After the initial incident! the blown

varistor was completely shorted out. &his left the power strip with a short between hot and

 ground ! a very dan#erous condition. 4f at that point 4 had plu##ed the step-down transformer into

an un#rounded outlet! the chassis of any grounded 110 volt equipment connected to the power strip would have been live with 220 volts! and 4 would have stood a #ood chance of bein#

electrocuted.

$onc!usion

 Never use a sur#e protector on the 110 volt side of a 220-to-110 volt step-down transformer ifthe power plu# can be plu##ed in two different ways. 4t could ill you. 4f you must use a power

strip! mae very sure it doesn"t have sur#e protection built in.

4f you are in a country with polari,ed power plu#s! such as the %7! this problem is less liely to

affect you! but it"s still possible if either the wall outlet or the power plu# of the transformer has been wired with incorrect polarity. $ercise care.

85ac to #son"s home pa#e9