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Common
Test Failures
Moe
Lamothe, P.Eng, Lamothe Approvals Inc.
We
often see the same failures and problems occurring in equipment we are testing.
The following failures and some suggested solutions to these failures should be
considered before you submit the equipment to us or any other testing agency.
Dielectric
Strength
The
purpose of the dielectric strength test (also
known as electric strength test or hi-pot test) is to verify adequate creepage
and clearance distances so that at some point in the future, the product will
not become a safety hazard due to a breakdown from primary to ground or
secondary.
In
theory, all of the necessary distances can be verified by inspection but due to
the large numbers of areas to be investigated this is not always practical and a
visual inspection does not always find production problems. Some of the older
standards only specify a dielectric strength test with no investigation of
creepage or clearance.
It
is essential that you verify the dielectric strength of your product prior to
submitting it for testing. The applicable standard will give you the appropriate
values to use. Common values are 1,500Vac (or 2121Vdc) primary to ground and
3,000Vac (or 4242Vdc) primary to secondary, but these will vary depending on the
safety standard used.
Transformer
Overload
The
secondary windings of mains transformers are short-circuited and subjected to
any overloads that may arise from any fault condition. Windings are tested in
turn, one at a time, to simulate short circuits or overloads. All other windings
are loaded or not loaded, whichever load condition is the least favourable.
Short-circuits or loads are applied on the
load side of any current-limiting impedance or over-current protective device
which is connected directly to the winding.
For
the short circuit test, any input fuse or protector is left in-place and in most
cases the protector opens. We have seen
cases where with a short on the output of the transformer, the input did not
draw enough current to open the fuse - it makes us wonder what the designer
thought he was protecting!
For
the overload test, the secondary of the transformer is loaded so that the
primary draws 135% (based on North American characterized protector) or 150%
(based on IEC characterized protector) of the input protector (fuse or circuit
breaker). The protector is removed for this test.
This
test frequently results in a failure, usually because the transformer flames. An
opened primary winding is acceptable as long as the allowable temperature of the
transformer winding during the fault are not exceeded before opening. With
multiple winding transformers it is usually necessary to include a thermal
protector in the primary winding or a fuse in each secondary winding.
Moe
Lamothe is the president of
Lamothe Approvals Inc. (Georgetown, ON, Canada). He can be reached at
info@lamotheapprovals.com.
The
copyright for all of the text, tables and illustrations remains with Lamothe Approvals Inc. Permission is granted to print or reproduce
this document provided that it properly attributed to Lamothe Approvals Inc.
Prepared
May 2003
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