Load Near Earth Ground

[D from BC]

I'll just learning safety approvals and I'm uncertain about what to
do.
Here's the problem:

L ----
120VAC offline converter===load
N --- +
|
--- 2.4nf
--- Stray capacitance
| Also breakdowns ~350VDC
Gnd------------------------- +

I have a 'bizarre' load with stray capacitance to earth ground which
can breakdown at ~350VDC.

This does not look like it'll pass a typical hi-pot test for a
commercial product.

What's a nice way to make this passable?

Can I fix it with MOVs and fuses?
A MOV from L to G.
A MOV from N to G.


D from BC
British Columbia
Canada.

 

[Joerg]

I'll just learning safety approvals and I'm uncertain about what to
do.
Here's the problem:

L ----
120VAC offline converter===load
N --- +
|
--- 2.4nf
--- Stray capacitance
| Also breakdowns ~350VDC
Gnd------------------------- +

I have a 'bizarre' load with stray capacitance to earth ground which
can breakdown at ~350VDC.

This does not look like it'll pass a typical hi-pot test for a
commercial product.

What's a nice way to make this passable?

Can I fix it with MOVs and fuses?
A MOV from L to G.
A MOV from N to G.

The offline converter usually provide the isolation.

 

[D from BC]

The offline converter usually provide the isolation.

The converter doesn't have an isolation transformer.
For example, if the earth ground makes contact with either of the
converter outputs ... there's a big poof!


D from BC
British Columbia
Canada.

 

[Joerg]

The converter doesn't have an isolation transformer.
For example, if the earth ground makes contact with either of the
converter outputs ... there's a big poof!

Then I'd say that there is most likely a serious code violation. The
only caps that are allowed between line connections and PE are the
X-capacitors and those must usually be rated 4kV for breakdown:

http://www.tdk.co.jp/tefe02/e4b3_cd.pdf

And in applications such as medical you can't even use those.

A stray capacitance that arcs at 350V sounds scary. Probably there is a
clearance and/or creepage path violation.

 

[D from BC]

Then I'd say that there is most likely a serious code violation. The
only caps that are allowed between line connections and PE are the
X-capacitors and those must usually be rated 4kV for breakdown:

http://www.tdk.co.jp/tefe02/e4b3_cd.pdf

And in applications such as medical you can't even use those.

A stray capacitance that arcs at 350V sounds scary. Probably there is a
clearance and/or creepage path violation.

I think so too. I'm looking into that.
I'll probably have to change the converter design to use an isolation
transformer.


D from BC
British Columbia
Canada.

 

[Robert Baer]

I'll just learning safety approvals and I'm uncertain about what to
do.
Here's the problem:

L ----
120VAC offline converter===load
N --- +
|
--- 2.4nf
--- Stray capacitance
| Also breakdowns ~350VDC
Gnd------------------------- +

I have a 'bizarre' load with stray capacitance to earth ground which
can breakdown at ~350VDC.

This does not look like it'll pass a typical hi-pot test for a
commercial product.

What's a nice way to make this passable?

Can I fix it with MOVs and fuses?
A MOV from L to G.
A MOV from N to G.


D from BC
British Columbia
Canada.

Usually, capacitance from (either side of line) to ground comes from
transformer primary winding(s) to its core, which *should be* tied to
ground.
2.4nF is a bit large, tho..

 

[Robert Baer]

The converter doesn't have an isolation transformer.
For example, if the earth ground makes contact with either of the
converter outputs ... there's a big poof!


D from BC
British Columbia
Canada.

Then the converter was not designed to any standard and the
menufacturer is exposed to all kinds of legal actions including murder.

 

[D from BC]

Then the converter was not designed to any standard and the
menufacturer is exposed to all kinds of legal actions including murder.

Yikes...I'm the one who designed and made the converter.
I'm exploring if I can use a nonisolated converter (no transformer)
and pass hi-pot testing.
The load has a high surface area in close proximity to earth ground.
That's why the breakdown and the capacitance.
A believe a hipot test will make the load arc to ground.


D from BC
British Columbia
Canada.

 

[Joerg]

I think so too. I'm looking into that.
I'll probably have to change the converter design to use an isolation
transformer.

If you can't redesign the load I think that's you only option.