> From: Jeff Seeger <[log in to unmask]>

>         Here I go again, no answers - just more questions....
>
>         Does anyone have at hand what spacings should be when the vol-
>         tage _does_ exceed 32VAC?  Is there a corrolary DC threshold
>         (my addled mind recalls 40v)?  Where does one look for such a
>         thin slice of a standard... ?

Depending upon the product into which these boards go into,
you could try UL-1950, 3rd Edition "Information Technology Equipment"
which has been coordinated as an agreement between the US and
Canada as well.  The associated European document is EN-60950
and the international standard is IEC-950, and last but not least
for our friends down under it's the Australian standard AS-3260.

These have to do with spacings between primary voltages and neutral,
primary voltages and ground, primary voltages and secondary voltages.
Primary voltage to mean 115vac, 230vac, 440vac, etc ...

Basically, UL-1950, 3rd Edition defines a hazardous voltage as

"Para. 1.2.8.3 HAZARDOUS VOLTAGE: A voltage exceeding  42.2 V peak,
 or 60 V d.c., existing in a circuit which does not meet the
 requirements for either a LIMITED CURRENT CIRCUIT or a TNV CIRCUIT."

LIMITED CURRENT CIRCUIT: Lots of criteria in the standard for this one.
TNV: Telephone Network Voltages.

It has charts for two type of spacings:

(1) Clearance - To mean the spacing between conductors through *air*.
    *Usually* means between wires but I have had to deal with the
    "clearance" between an insulated wire and a trace on a board.
    As the crow flies, so to speak.

(2) Creepage - To mean the spacing between traces of a pcb along
    the board surface.
    As the crow walks, so to speak.

There are a couple of other factors that go along with determining
this spacing.

One is the environment in which the product will be used.
There are three categories: Pollution Degree 1, 2, and 3.
Think of them this way: 1 = clean room environment, 2 = inside
house of office or building, 3 = outside.

Another has to do with the CTI rating of the board.
CTI - Comparative Tracking Index.

Still a third factor has to do with the "type" of voltage
in the trace. If it's DC voltage, then use that value.
If it's a nice sine wave like from a wall socket, use
the rms. value (115vac, 230vac, 440vac, etc ...). If it's
some oddball waveform like a triangular wave, then use
the highest value or peak value of the wave.

I think that's been enough for everyone ...

Regards,  Doug

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