How to overload a linear transformer with multiple protective devices

Peter Selvey

Staff member
Super Moderator
#11
You need to take into account the resistance of the windings to best estimate which protection will operate first.

For example, in your case you have a 24V rated at 1.0A with a fuse of 1.25A. Since the fuse is close to the rating, it is likely that the fuse based test will produce the lowest results (operate first).

However, for the 11A 0.2A winding has a fuse of 0.63A, so the test current (~1.3A) will be nearly 7 times the rating. If the manufacturer used the smallest gauge wire for possible for 0.2A, then due to the high resistance you might get very high temperatures causing the TCO to operate first. On the other hand, if large gauge wire is used, then the fuse based test will get the lowest results (operate first).

But the main point of your questions is whether the protective devices that did not operate in the test can be considered not safety relevant. This is correct. You do not need to apply for example Clause 4.8 of IEC 60601-1:2005 to components that do not provide a safety function.
 
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Roland chung

Trusted Information Resource
#12
..., then the fuse based test will get the lowest results (operate first).
Should we get the lowest or the highest results?

You do not need to apply for example Clause 4.8 of IEC 60601-1:2005 to components that do not provide a safety function.
That means if TCO did not operate during overload, then it is not safety relevant, then it is redundant, then no requirements apply to it. So this TCO can be removed in order to save money.
 

Peter Selvey

Staff member
Super Moderator
#13
In case there are two protective devices protecting against the same hazard, the one which gives the lowest (best) result is the one used in tests and also considered the safety component. The other one can be removed if desired.
 
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