Dielectrically Isolated?

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I'm in the market for a stepdown transformer, and in my research i am
finding the following....

The specs say: This is not dielectrically isolated. What does that mean, and
how do I know if I need it to be isolated?.


Re: Dielectrically Isolated?

"Spankalogical Protocol"
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** That means the unit is not a normal transformer with separate primary and
secondary but an "auto-transformer" with a single winding having a tap ( a
connection some way along it) at the desired voltage.

If you have some item of gear intended for the US or Japanese markets so
built for a 115 volt AC supply -  then you need to examine this item and
remove any caps connected from AC to ground or chassis.

When you care to " get real"  and post just what the stepdown unit is AND
your intended use is it  MAY become possible to give more advice.

...........    Phil

Re: Dielectrically Isolated?

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Not sure I realy understand this bit Phil, can you tell me why?  Inrush
current?  DC offset if load is negligently put between tap & active instead
of tap & neutral?


Re: Dielectrically Isolated?

 "Phil Allison"
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**  When using a 240/115 auto-transformer it must be assumed that either or
both supply leads to the 115 volt appliance will be at 240 volts AC relative
to ground.  This will happen if the active and neutral wires are swapped at
the AC supply outlet ( including any re-wired plugs, extension leads and
multi-outlets), the lead going to the step down or by simply a having a
missing neutral anywhere in that path ( eg an AC switch is in the neutral or
a ill fitting plug and socket).

Typically, the caps fitted from AC to chassis ground in 115 volt appliances
are not able to cope with 240 AC mains supply and will develop a dead short
at some time. Also, some old US appliances simply have no ground yet still
have a "ground cap" -  these become quite lethal if used with an

BTW :   There is no electrical standard covering auto-transformers of this
sort (ie those fitted with AC lead and US style outlets) so no way they can
be type approved. It is illegal for an electrician to install such a
transformer for general use -  it is a prohibited item under AS 3000.

..........   Phil

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