Hi Eric:
You ask what spacings are necessary for a circuit
rated at less than 30 V and "isolated" for 250 V.
Unfortunately, you did not tell us what the circuit
is isolated from.
I am going to assume that your isolated circuit is
a "floating" circuit, meaning that it has no connection
to ground or any other circuit. I further assume that
the circuit is functionally derived from an SELV circuit,
e.g., transformer-coupled to an SELV circuit. I further
assume that it is isolated from all mains (primary)
circuits by the SELV circuit (so that we do not need to
consider spacings between the isolated circuit and the
mains).
The first thing we must decide is that this is a
secondary circuit being isolated from another secondary
circuit. In this case, we can ignore the spacings
required for mains circuits as secondary circuits are
not subject to transient overvoltages. Another way of
saying this is that the circuits are subject to
Installation Category I spacings requirements.
However, in IEC 1010-1, the spacings requirements apply
to Basic, Supplementary, and Reinforced insulations.
My definition of the problem says that the construction
only has functional (operational) insulation, since all
voltages are less than 30 V.
So, there are NO spacing requirements for the construction
as I have defined it. Use anything you want!
I would choose the value, for design purposes only, from
Table D.1, for 300 V, namely 0.5 mm clearance and 0.7 mm
creepage.
Best regards,
Rich
"The word 'impossible' is not in my dictionary."
-- Napolean Bonaparte
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Richard Nute Quality Department
Hewlett-Packard Company Product Regulations Group
San Diego Division (SDD) Tel : 619 655 3329
16399 West Bernardo Drive FAX : 619 655 4979
San Diego, California 92127 e-mail: [email protected]
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