Protective vs. Functional Earth Differences

U

username10

#1
Please see the grounding scheme below.

In order for the device to be a class 1 device, it needs to have the chassis as protective earth. This is accomplished by running a conductor from the AC MAINS input to a ground stud on the chassis.

However, if I also have my analog ground also connected to the chassis, doesn't this make my protective earth functional ground?

How should this be wired to meet class 1? Should all the ground be tied to the chassis or only the earth from AC?

Thanks!
 

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B

bassa

#3
However, if I also have my analog ground also connected to the chassis, doesn't this make my protective earth functional ground?
No it doesn't. A PE is PE because it's used to Protect the operator/patient, by means of a reliable ground connection, to make sure the touch current won't exceed certain values.
Connecting a FE to it doesn't change this.

8.6.8 states that it is not allowed to use a functional ground for Protective Earth.

The other way around (like you described) is allowed.
 

Peter Selvey

Staff member
Super Moderator
#4
It is a bit confusing. The term "functional earth" as used in the standard is only for cases where a special terminal is provided for the user or installer to connect to an external earth, usually for the purpose of noise reduction.

It is rare to see such a terminal, so in most cases, there are no "functional earth" connections, at least as defined in the standard. Of course in day to day conversation, we often refer to something as being "functionally" earthed just to make it clear it is not protectively earthed.

The standard actually uses two terms "earthed" and "protectively earthed".

The term "earthed", when used in the standard, includes all parts which are earthed, regardless of the reason.

The term "protective earth" covers only those parts which need to be earthed
for protection against electric shock.

Throughout the standard, you will see some requirements which refer to "earthed" parts and others to "protectively earthed" parts. But if you see a requirement for "functionally earthed" parts, ignore it unless you really have a functional earth terminal, for external connection.
 
S

sankui

#5
The term "earthed", when used in the standard, includes all parts which are earthed, regardless of the reason.
In this case, did you mean that the reliability of connection of earth need not to be investigated? And therefore such earth connection does not provide ONE MOP?
 

Peter Selvey

Staff member
Super Moderator
#6
Exactly. If you look at the definition of a MOP, you will find it is limited to protection against electric shock.

The earthing may be there for other safety reasons (e.g. sensor noise reduction, EMC immunity, which are also safety issues), but in the context of IEC 60601-1 the term "protective earth" is used for MOP only which in turn means it is just for electrical safety.

Any part which is earthed to prevent electric shock is an MOP and must meet the relevant requirements to confirm the protection is reliable, like being <0.1ohm @ 25A.

Other parts which are earthed for other reasons are not MOP and don't need to meet these tests. But there may be some requirements which refer to earthed parts in general (e.g. the green/yellow marking for conductors is allowed for these other parts).
 
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