Hi Richard:
> You mention a few times the 25 Amp test. The designs I have seen that try
> and use this approach (and I don't ever recall seeing a design I was happy
> with) were all switch mode power supplies where there was an input fuse of
> about 2A, meaning that the test would be performed at 3A in the past (and
> will be performed at 4A in the future, if the CDs go through un-changed).
I'm surprised that, today, a SMPS would used screen/
shield construction.
The effectiveness of a SMPS transformer is a function
of the physical proximity of primary and secondary
windings. All of the SMPS transformers that I have
been involved with over the past 10 years or so use
reinforced/double-insulation between primary and
secondary.
While I mentioned the 25-amp test, I fully concur
that the test current is a function of the relevant
overcurrent device.
> problem I am not convinced that it is the whole story. For me, another
> chapter in this story is the part played by the very high surge current
that
> flows for a very short period of time. It is here where we could usefully
I presume you are referring to the circuit prospective
current. (I am not familiar with the proposal to
WG8.)
I would agree that this current, at a very small
contact area, is the driver for the hole in the
screen/shield.
However, I believe that the circuit prospective
current is of little or no consequence when testing
a robust earthing circuit complying with the
constructional requirements. (Of course we should
test this hypothesis.)
Best regards,
Rich
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