Re: Transformer question; safety
- From: "Jim Gregory" <jim.greg@xxxxxxxxxxxx>
- Date: Tue, 05 Apr 2005 18:17:11 GMT
Chris,
The fuse is there to protect the integrity of the *supply* feeding the
transformer and everything else - from non-behaving equipment connected to
the mains distribution network.
The fact that, in this case, it will rupture (blow) when a shorting or an
excessive current fault affects the secondary, and therefore the primary, is
beneficial to and noticed by the user when it halts further powering!
It is not unusual to have another, anti-surge, fuse protecting a secondary.
In this country, half-voltage temporary site or safety transformers handling
about 2kVA deliver 115V at their isolated secondary. They often have a
line-to-neutral leakage-sensing, fast-acting RCD resettable trip added.
And I have seen versions with a balanced secondary, ie, centre-tap to
ground/earth, giving only a 57-V potential if one line is accidentally
touched.
Jim
"ChrisG" <cgonz@xxxxxxxxxxx> wrote in message news:4252c2f9$1_2@xxxxxxxxxxxx
> I'm trying to get a better intuitive understanding of transformers.
>
> One text I have explains that an increase of current in the secondary
> causes an increase in back emf (of the secondary) which acts to reduce
> the back emf of the primary, which increases the current in the primary.
>
> Since power supplies frequently fuse the primary side, it seems that
> the secondary side can cause excessive currents in the primary. But
> what is the limit to which the primary current can be increased? In
> other words, what happens when the secondary is shorted?
>
> I have seen comments regarding isolation transformers - that they
> limit the current that can be delivered, and thus are safer. If output
> power is close to input power where does the energy go? Heat?
> Impedence from the coils?
>
> I have also seen things saying that a small voltage secondary is less
> dangerous (than high voltage), but isn't that low voltage secondary
> capable of delivering high current, which is usually described as more
> dangerous than voltage?
>
>
>
>
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