Creepage and clearance question

[markp]

Hi All,

I have a pair of signals that can be +/-75V (it's actually a pair of
squarewave signals of opposite phase, when one signal is +75V the other is
0V, and when it is 0V the other is +75V). The question is, can I use a
connector with a floating pin between them, even if the creepage and
clearance calculations for each pin are, say, only 60V? My guess is you can,
but do you have to allow some kind of de-rating?

For a hypothetical example, let's say I have two signals of +/-75V. I work
out that ideally I could use two pads on the PCB, say a minimum of 1.5mm
apart. However, I have only got a 1mm connector, but I have got 3 pins on it
so the middle one can be left floating and not connected to anything. I'm
guessing it depends on the toital *clearance* between the two outer pads,
and so the width of the middle pad has to be subtracted from the distance
between the two outer pads before the calculation is made.

Any views?

Mark.

 

[Dan H]

Hi All,

I have a pair of signals that can be +/-75V (it's actually a pair of
squarewave signals of opposite phase, when one signal is +75V the other is
0V, and when it is 0V the other is +75V). The question is, can I use a
connector with a floating pin between them, even if the creepage and
clearance calculations for each pin are, say, only 60V? My guess is you can,
but do you have to allow some kind of de-rating?

For a hypothetical example, let's say I have two signals of +/-75V. I work
out that ideally I could use two pads on the PCB, say a minimum of 1.5mm
apart. However, I have only got a 1mm connector, but I have got 3 pins on it
so the middle one can be left floating and not connected to anything. I'm
guessing it depends on the toital *clearance* between the two outer pads,
and so the width of the middle pad has to be subtracted from the distance
between the two outer pads before the calculation is made.

Any views?

Mark.

Yes - the actual amount of non conductive material is what counts

Dan

 

[markp]

Yes - the actual amount of non conductive material is what counts

Dan

Thanks Dan, that's what I thought, it would make sense!

Mark

 

[John Larkin]

Hi All,

I have a pair of signals that can be +/-75V (it's actually a pair of
squarewave signals of opposite phase, when one signal is +75V the other is
0V, and when it is 0V the other is +75V). The question is, can I use a
connector with a floating pin between them, even if the creepage and
clearance calculations for each pin are, say, only 60V? My guess is you can,
but do you have to allow some kind of de-rating?

For a hypothetical example, let's say I have two signals of +/-75V. I work
out that ideally I could use two pads on the PCB, say a minimum of 1.5mm
apart. However, I have only got a 1mm connector, but I have got 3 pins on it
so the middle one can be left floating and not connected to anything. I'm
guessing it depends on the toital *clearance* between the two outer pads,
and so the width of the middle pad has to be subtracted from the distance
between the two outer pads before the calculation is made.

Any views?

Mark.

Do you really have a connector that's rated below 75 volts pin-to-pin?

John

 

[Paul Burke]

Do you really have a connector that's rated below 75 volts pin-to-pin?

LV Directive stuff I should think.

 

[markp]

Do you really have a connector that's rated below 75 volts pin-to-pin?

John

No, the connector's fine but is on a 1mm pitch. It's the PCB I'm concerned
about. The inter-gap spacing is around 0.4mm, given tolerances I'd like to
increase it somewhat by using no-connects on the connector.

Mark.

 

[markp]

LV Directive stuff I should think.

Not this time! This is for an EL backlight circuit, the EL has to get to the
PCB via a 1mm pitch FPC connector with several spare pins on it.

 

[John Larkin]

No, the connector's fine but is on a 1mm pitch. It's the PCB I'm concerned
about. The inter-gap spacing is around 0.4mm, given tolerances I'd like to
increase it somewhat by using no-connects on the connector.

Mark.

OK, skipping pins will certainly increase breakdown voltage, from
maybe 3000 to maybe 6000.

John

 

[Rich Grise]

Not this time! This is for an EL backlight circuit, the EL has to get to the
PCB via a 1mm pitch FPC connector with several spare pins on it.

Is it for a one-off, or production? If it's a one-off, I'd ask, is it
possible to physically pull that middle pin out of the connector body?

If it's production, I'd say order some connectors with that pin not
installed, maybe not even a hole there.

But, what do I know? The last time I saw anyone making connector headers,
they were .025 pins on .1" centers.

Cheers!
Rich

 

[Rich Grise]

No, the connector's fine but is on a 1mm pitch. It's the PCB I'm concerned
about. The inter-gap spacing is around 0.4mm, given tolerances I'd like to
increase it somewhat by using no-connects on the connector.

Ask your PCB maker how small of a pad he can make - or don't even put a
pad there at all, just the PTH, for the solder to stick to so it isn't
flapping in the breeze - but just let them etch all the copper off around
that hole.

Just an idea.

Thanks!
Rich

 

[markp]

OK, skipping pins will certainly increase breakdown voltage, from
maybe 3000 to maybe 6000.

Hi John,

I guess you're suggesting that I've over-estimated the problem! Where did
you get those breakdown figures BTW? I've always used *much* larger
distances than that for PCBs. I'm looking for a minimum of Pollution Degree
2 using Category 2 material (that, according to the table below, should be
need around 1mm total gap. With a 1mm pitch connector and 0.4mm insulation
between the pins we are taling about a minimum of 4 pins with the centre two
being no connect)

http://www.ce-mag.com/ce-mag.com/archive/01/03/ProductSafety.html

Mark.

 

[markp]

Ask your PCB maker how small of a pad he can make - or don't even put a
pad there at all, just the PTH, for the solder to stick to so it isn't
flapping in the breeze - but just let them etch all the copper off around
that hole.

Just an idea.

Thanks!
Rich

It's a surface mounted connector unfortunately but interesting idea anyway!

Mark.

 

[John Larkin]

Hi John,

I guess you're suggesting that I've over-estimated the problem! Where did
you get those breakdown figures BTW? I've always used *much* larger
distances than that for PCBs. I'm looking for a minimum of Pollution Degree
2 using Category 2 material (that, according to the table below, should be
need around 1mm total gap. With a 1mm pitch connector and 0.4mm insulation
between the pins we are taling about a minimum of 4 pins with the centre two
being no connect)

http://www.ce-mag.com/ce-mag.com/archive/01/03/ProductSafety.html

Mark.

I was guessing about *actual* breakdown voltages. UL seems to
generally want 1 mm on a pc board, up to 85 volts (electronic test
equipment), not too different from your numbers. If it's regulatory
compliance, you've got to do it, but it's gross overkill in real life.

John

 

[John Larkin]

Hi All,

I have a pair of signals that can be +/-75V (it's actually a pair of
squarewave signals of opposite phase, when one signal is +75V the other is
0V, and when it is 0V the other is +75V). The question is, can I use a
connector with a floating pin between them, even if the creepage and
clearance calculations for each pin are, say, only 60V? My guess is you can,
but do you have to allow some kind of de-rating?

For a hypothetical example, let's say I have two signals of +/-75V. I work
out that ideally I could use two pads on the PCB, say a minimum of 1.5mm
apart. However, I have only got a 1mm connector, but I have got 3 pins on it
so the middle one can be left floating and not connected to anything. I'm
guessing it depends on the toital *clearance* between the two outer pads,
and so the width of the middle pad has to be subtracted from the distance
between the two outer pads before the calculation is made.

Any views?

Mark.

Just saw an ad for some AVX high-voltage ceramic caps. 2KV in a 1206
package, which, using our standard footprint, gives a pad-pad
clearance of 52 mils, 1.3 mm!

John

 

[Roy L. Fuchs]

I have a pair of signals that can be +/-75V (it's actually a pair of
squarewave signals of opposite phase, when one signal is +75V the other is
0V, and when it is 0V the other is +75V). The question is, can I use a
connector with a floating pin between them, even if the creepage and
clearance calculations for each pin are, say, only 60V? My guess is you can,
but do you have to allow some kind of de-rating?

The Mfgr's spec on the connector will declare voltage isolation
specs for you.

 

[Pooh Bear]

Hi All,

I have a pair of signals that can be +/-75V (it's actually a pair of
squarewave signals of opposite phase, when one signal is +75V the other is
0V, and when it is 0V the other is +75V). The question is, can I use a
connector with a floating pin between them, even if the creepage and
clearance calculations for each pin are, say, only 60V? My guess is you can,
but do you have to allow some kind of de-rating?

For a hypothetical example, let's say I have two signals of +/-75V. I work
out that ideally I could use two pads on the PCB, say a minimum of 1.5mm
apart. However, I have only got a 1mm connector, but I have got 3 pins on it
so the middle one can be left floating and not connected to anything. I'm
guessing it depends on the toital *clearance* between the two outer pads,
and so the width of the middle pad has to be subtracted from the distance
between the two outer pads before the calculation is made.

Any views?

Mark.

IEC 60065 or whatever standard may be more appropriate to your product.

Graham