Re:Multiple Leakage/Touch Current Sources

[email protected] Wed, 17 Nov 2004 11:02:17 -0400

This is a multi-part message in MIME format.

Ron,

I don't quite sure if you are just trying to understand the problem or fix the
problem.


The leakage current is primarily caused by the "Y" caps in the line filters and
power supplies. Here is what you can do to reduce the problem.

1. Change the line filters to Medical Grade versions which have smaller "Y"
caps
and has lower leakage current.

2. Remove the "Y" caps from your switch mode power supplies if you can live
with
an increase in line conducted emissions. Many power supplies have one mounting
screw who's only purpose is to ground the "Y" caps. Float this one mounting
screw will take the caps out of the ground path and reduce the leakage current.

3. You said your product is Hardwired to the building power. This would make
your product "Permanently Connected Equipment", not  "Portable Equipment".  The
leakage current or Touch Safety requirement of 3.5mA is a Fault Test for
Portable Equipment because the Safety Earth Ground can fail. In Permanently
Connected Equipment it is assumed that the equipment is wired by an electrician
and the earth ground is verified to be good. Failures in the Earth Ground
Conductor is not likely to occur in Permanently Connected Equipment so higher
leakage currents for such equipment is usually not a concern.

4. You could provide a second Earth Ground Conductor in the form of a Grounding
Strap or wire. I have seen this on equipment where the leakage current exceeds
the 3.5mA requirement. Again, since this is a Single Fault Test, by providing a
second Earth Ground Conductor, no hazard can occur in a Single Fault
Condition. Some equipment that uses this method usually has some kind of
caution label on
the equipment or right on the power cord itself to inform the installer of the
need for the Earth Ground Conduction. Make sure this information is in the
Installation Manual as well.

As far as the relationship between different leakage sources, you are correct,
they do not seem to directly add. You have to test to know for sure and you
will
see differences between units, so plan for some margin.

Hope this information is helpful.
Brian Kunde
LECO



____________________Reply Separator____________________
Subject:    Multiple Leakage/Touch Current Sources
Author: [email protected]
List-Post: [email protected]
Date:       11/17/04 8:26 AM

Howdy all,

I have come across a problem where a product is exceeding 3.5 ma of touch
current. The product in question has 2 line filters and 4 Switching Mode
Power Supplies. Because of the addition of the fourth power supply the
overall touch current can be as much as 4.0 ma. Ideally, we do not want to
make the product so it has to be hard-wired to the building mains.

Because of the high touch current, does anyone know what the relationships
are between different touch current sources? I've heard that touch current
is not linear when you add up the different sources and phase shifts between
L-G and N-G might be a problem. However, I'm looking for some detailed
information on this issue so I can provide ways to investigate the problem
and hopefully reduce the overall touch current.

BTW, the product is not medical equipment and all touch current tests are
done with the product operating at 264 V AC, 50 Hz.

Best regards,
Ron Wellman


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This message is from the IEEE Product Safety Engineering Society
emc-pstc discussion list.    Website:  http://www.ieee-pses.org/

To post a message to the list, send your e-mail to [email protected]

Instructions:  http://listserv.ieee.org/listserv/request/user-guide.html

List rules: http://www.ieee-pses.org/listrules.html

For help, send mail to the list administrators:

     Ron Pickard:              [email protected]
     Scott Douglas             [email protected]

For policy questions, send mail to:

     Richard Nute:           [email protected]
     Jim Bacher:             [email protected]

All emc-pstc postings are archived and searchable on the web at:

    http://www.ieeecommunities.org/emc-pstc

Title: Re:Multiple Leakage/Touch Current Sources

Ron,

I don't quite sure if you are just trying to understand the problem or fix the
problem.

The leakage current is primarily caused by the "Y" caps in the line filters and
power supplies. Here is what you can do to reduce the problem.

1. Change the line filters to Medical Grade versions which have smaller "Y" caps
and has lower leakage current.

2. Remove the "Y" caps from your switch mode power supplies if you can live with
an increase in line conducted emissions. Many power supplies have one mounting
screw who's only purpose is to ground the "Y" caps. Float this one mounting
screw will take the caps out of the ground path and reduce the leakage current.

3. You said your product is Hardwired to the building power. This would make
your product "Permanently Connected Equipment", not "Portable Equipment". The
leakage current or Touch Safety requirement of 3.5mA is a Fault Test for
Portable Equipment because the Safety Earth Ground can fail. In Permanently
Connected Equipment it is assumed that the equipment is wired by an electrician
and the earth ground is verified to be good. Failures in the Earth Ground
Conductor is not likely to occur in Permanently Connected Equipment so higher
leakage currents for such equipment is usually not a concern.

4. You could provide a second Earth Ground Conductor in the form of a Grounding
Strap or wire. I have seen this on equipment where the leakage current exceeds
the 3.5mA requirement. Again, since this is a Single Fault Test, by providing a
second Earth Ground Conductor, no hazard can occur in a Single Fault Condition. Some equipment that uses this method usually has some kind of caution label on

the equipment or right on the power cord itself to inform the installer of the
need for the Earth Ground Conduction. Make sure this information is in the
Installation Manual as well.

As far as the relationship between different leakage sources, you are correct,
they do not seem to directly add. You have to test to know for sure and you will
see differences between units, so plan for some margin.

Hope this information is helpful.
Brian Kunde
LECO

____________________Reply Separator____________________
Subject: Multiple Leakage/Touch Current Sources
Author: [email protected]
Date: 11/17/04 8:26 AM

Howdy all,

I have come across a problem where a product is exceeding 3.5 ma of touch
current. The product in question has 2 line filters and 4 Switching Mode
Power Supplies. Because of the addition of the fourth power supply the
overall touch current can be as much as 4.0 ma. Ideally, we do not want to
make the product so it has to be hard-wired to the building mains.

Because of the high touch current, does anyone know what the relationships
are between different touch current sources? I've heard that touch current
is not linear when you add up the different sources and phase shifts between
L-G and N-G might be a problem. However, I'm looking for some detailed
information on this issue so I can provide ways to investigate the problem
and hopefully reduce the overall touch current.

BTW, the product is not medical equipment and all touch current tests are
done with the product operating at 264 V AC, 50 Hz.

Best regards,
Ron Wellman

----------------------------------------------------------------
This message is from the IEEE Product Safety Engineering Society
emc-pstc discussion list. Website: http://www.ieee-pses.org/

To post a message to the list, send your e-mail to [email protected]

Instructions: http://listserv.ieee.org/listserv/request/user-guide.html

List rules: http://www.ieee-pses.org/listrules.html

For help, send mail to the list administrators:

Ron Pickard: [email protected]
Scott Douglas [email protected]

For policy questions, send mail to:

Richard Nute: [email protected]
Jim Bacher: [email protected]

All emc-pstc postings are archived and searchable on the web at:

http://www.ieeecommunities.org/emc-pstc