Hi Jeff:
Low-power transformers are difficult if not impossible to
protect by means of a primary fuse. As you have pointed
out, the change in current between rated load and short-
circuit reflected to the primary is too small to cause a
fuse to operate.
The safety issue is that of overheating as a result of a
fault in the load. I know of three schemes to protect a
transformer from overheating:
1) Use a fuse in the primary circuit.
2) Use a thermal protector wound into the transformer.
3) Use an "impedance-protected" (e.g., doorbell) transformer.
Clearly, the fuse won't do the job in this case.
A thermal protector is an effective means of preventing
overheating, although it may be relatively expensive for
a small transformer.
Most small transformers are "impedance-protected." This
means that the primary impedance is so high that the
transformer will not overheat under any condition of load,
including maximum power and short-circuit. A doorbell
transformer is a common example. Also, most direct-plug-
in transformers are impedance-protected. No fuse, no
thermal protector.
I suggest you contact the transformer manufacturer and
ask how the transformer is protected against overheating.
I'll bet it will be impedance-protected.
UL and others commonly certify impedance-protected
transformers. I would insist on a certified transformer.
If you are going to test the transformer, I suggest two
tests (and NOT the 210% of fuse rating test). Measure
the input current and the transformer temperature rise
under the following loads:
a) Maximum power (this will cause maximum E x I power
dissipation in the transformer), and
b) Output short-circuit (this will cause maximum
I x I x R power dissipation in the secondary).
The test data will tell you (1) whether the input current
under fault conditions is enough to operate the fuse, and
(2) whether the transformer will get unacceptably hot
under fault conditions.
As for the fuse voltage rating... yes, it is absolutely
necessary to use a fuse with a voltage rating equal or
greater than the supply voltage. When the fuse operates,
at the moment the element opens, an arc occurs between
the ends of the element. The heat of the arc continues
to melt the ends of the element back until the spacing
is too great to sustain the arc. Since the electric
strength of air is a function of voltage, it is possible
for a fuse used at a voltage exceeding its rating to
continue arcing until something else happens in the
circuit, usually an explosion or a fire! Been there,
done that! (Sometimes, in secondary circuits, you can
use a fuse beyond its voltage rating PROVIDED the source
is well below the fuse interrupt current rating.)
Best wishes for the holiday season,
Rich
-------------------------------------------------------------
Richard Nute Product Safety Engineer
Hewlett-Packard Company Product Regulations Group
AiO Division Tel : +1 619 655 3329
16399 West Bernardo Drive FAX : +1 619 655 4979
San Diego, California 92127 e-mail: [email protected]
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