On a related note, and referring to the document whose link I posted
previously,
http://www.ti.com/lit/ml/slup227/slup227.pdf
It seems odd to me that the functional (not safety) clearance
requirements at mains voltages (see table 2, and note that I'm in
Australia with 240V AC mains) are higher than the distance between the
pins for a typical mains voltage triac. For example,
http://www.farnell.com/datasheets/1726515.pdf
Anyone have any thoughts on that?
Sylvia.
The creepage distance in a basic safety isolation barrier and a
functional creepage distance are handled in pretty much the same way.
If insufficient, the distance is shorted to see if an unsafe condition
is created.
If it does not exhibit excessive operating leakage current, still
passes safety hipot, has no open traces resulting and no flame hazard
is created etc, then it's OK. This repeated for all single fault
abnormals.
Obviously, any short of an isolation barrier runs the risk of creating
excess leakage current or a hipot failure, even without the addition
of tracking due to destroyed internal components that may result. So
creepage shorctcomings in these areas are not acceptible.
Optocouplers can be used to couple across supplementary isolation -
effectively 2xbasic isolation, only if internal and external physical
dimensions and materials are controlled sufficiently to do so, by
demonstration to the certifying agency.
Recent consolidation of safety standards has removed any advantage
that clearance distances had over creapage distances, in safety
isolation measurements, so slots are a waste of effort from a
regulatory point of view. Practically, however their function remains
unaltered.
RL