Hi Matthew:
In answer to your question how to distinguish between functional earth
circuits and protective earth circuits:
Protective earth (PE) circuit carries fault current to earth. Presumably,
the fault current is from a mains-to-earth fault.
Functional earth (FE) circuit does not carry mains fault current to earth.
I think of functional earth circuits as carrying functional signal returns
and functional faults, although they may also be for establishing an
equipotential environment.
PE and FE are usually connected together. In this case, the sites of the
possible mains-to-earth faults must be identified and the resulting current
path must be identified to determine which conductors are PE and which are
FE.
Hope these comments help.
Best regards,
Rich
From: Matthew D. Varas <m...@wrmed.com>
Sent: Tuesday, October 18, 2022 9:53 AM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: [PSES] Confusion about IEC 60601-1-11 Power Cord Requirements and
Functional vs Protective Earth Terminals
Hello,
We are converting a product to comply with 60601-1-11 for home use and I was
hoping to get some clarification on the power cord requirements.
The device is a mains-powered heating tank which uses snap-action
thermostats to control mains power through the internal heating pad. It is
currently intended for professional healthcare environments and has a metal
tank that is treated as an applied part. A three-prong power cord is used to
provide a protective earth connection as a MOP which, as I understand it, is
not allowable per clause 6 of 60601-1-11 ("shall be class II or internally
powered" and "shall not have a functional earth terminal").
The sales and marketing folks do not want to switch to a two-prong cord out
of concern that it will look unprofessional when used in a professional
healthcare environment, regardless of the fact that it will be redesigned to
be double insulated through other MOP's. In trying to respond to them a
coworker and I are in disagreement about how to interpret clause 6 and could
use some help. Does this mean that :
1. The power cord must be two prong, or
2. The power cord can be three prong but can have the earth terminal
not connected internally
If it were to not be connected internally, I assume that a C18 connector on
the device (while still using the three-prong cord) would be the correct
path, however I interpret this to mean option 1 is correct.
Which leads to the second related part of my question:
I am unclear on the differences between a functional earth terminal and a
protective earth terminal. After looking at Figure 2 in 60601-1, in this
particular application it doesn't seem like there would be a difference
between the two since it uses only mains power. Is this correct and can
someone help me understand what the difference would be between the two,
especially as it relates to home use?
Thank you,
Matthew Varas
Electrical Engineer
WR MEDICAL ELECTRONICS CO.
direct 651.604.8473 | cell 763.222.6900
email m...@wrmed.com <mailto:m...@wrmed.com>
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