We are working on a product that uses a terminal block inserted into a
metal
frame. See ASCII artwork below:
| ______________
| | |
_ |__________|_| |
/ || | |
| || | |<--terminal block (outside portion)
\_||__________|_ |
^ | barrel | | |
| |<5 mm min>| |______________|
| |<--solder |
| tab |
| |<--metal frame
screw |
head
Each connection from the outside goes to the inside through a hole in the
metal frame via the plastic barrel of the terminal block.
Under the screw on the inside is a metal tab that is soldered
into a printed circuit board. The distance from the tab to the frame is at
least 5 mm.
During Fast Transient Burst testing at 4 kV to IEC/EN 61000-4-4 (Edition 2,
year 2004), we see arcing between the solder tab and the frame along the
barrel of
the terminal block, but only when testing at the 100 kHz pulse rate. When
the same test is run at the 5 kHz pulse rate, we do not see any arcing. In
addition, a "hipot" test done at 5 kVACrms (7.07 kV peak) does not arc.
Waveform verification tests on the FTB generator with the standard 50 ohm
and 1000 ohm loads shows no significant difference in the waveform between
the two burst rates.
Questions:
1. What is the physical mechanism that causes the arc with the faster pulse
rate but not with the slower rate (or 60 Hz AC)?
2. It appears that the creepage (and clearance) distance that we used
(based on UL
standards for pollution degree 2) are not adequate for 100 kHz FTB. Are
there any spacing guidelines available that will allow us to pass the FTB
test reliably?
Don Borowski
Schweitzer Engineering Labs
Pullman, WA
-
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