Protective earth for Class I ME equipment

#1
Hi All,

Below is the High level depiction of our Class I Medical electrical equipment, powered with an external Medical grade power supply.
The power supply for this medical device is medical graded and has a protective earth at its mains power chord.
The medical graded power supply is power Via an UPS.
Inside the box named Medical Device is operated from the 18V DC sourced by the medical power supply
Does the block below named as Medical Device needs a protective earth line connected to it.

Some more insight:
The Box is made of Sheet metal and powder coated.
The digital ground of the circuit is connected to the metal box via the mechanical mounting hole
The device has applied parts both of type CF and BF.
The PC interface to this device is via Fibre optic cable.
The PC and the monitor is not placed inside the patient room. Only the medical device is placed in the patient room.
Please let me know if you have any additional queries...
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Benjamin Weber

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#2
First of all: You may have the power supply classified as class I and the medical device as class II.

You as the manufacturer decide wether the medical device is class I or II. It depends on your insulation concept.

If you want the medical device to be class I: Is the PE conductor from the power supply input available at the output? I am not talking about the ground line I am almost sure, it is not. If I am right, you have to make the medical device class II compliant, i.e. double or reinforced insulation!
 
#3
First of all: You may have the power supply classified as class I and the medical device as class II.

You as the manufacturer decide wether the medical device is class I or II. It depends on your insulation concept.

If you want the medical device to be class I: Is the PE conductor from the power supply input available at the output? I am not talking about the ground line I am almost sure, it is not. If I am right, you have to make the medical device class II compliant, i.e. double or reinforced insulation!
Thank you very much for your quick response.
For your first question... The answer is... as you mentioned. the PE is not available at the output of the power supply. Output is only the DC +V & GND.
The existing design is a old design and has the digital ground connected to the metal enclosure.
All the Leakage currents and isolation satisfies the standard requirement
Is it not that when we classify a ME equipment... Power supply with the Medical device together is considered as an ME equipment? and in that case this device in question falls under Class I right? This is my understanding. Please comment...
 

michael Cejnar

Involved In Discussions
#4
If your power supply (PSU) is a specified component in UIM and preferably on device decal and has a unique connector to prevent foreseeable misuse of customer connecting a different non-medical PSU, then I believe it is all one medical device and is Class I, with functionally earthed SELV secondary circuit with accessible metal parts, and you already have double / reinforced insulation from mains in the medical PSU.
However, operating from SELF (e.g. 18V) alone is not protection enough - see Cl. 6.2 of IEC60601-1, for why it does not recognize SELV operated Class III.

Once you have accessible metal parts, that are only functionally earthed, then you have problems.

Key issue is what other equipment may be connected to your medical device, and whether faults in other equipment could make your metal case live - failing touch current per Figure 17 or Fig 18 of IEC 60601-1:2005+AMD1:2012 (Ed3.1).

Consider also if your ME is in Patient Environment - if user can touch patient, especially if any open tissue or saline flowing into veins, and touch your metal chassis - then your allowable touch leakage current becomes much lower - at CF level.

We have had this design for over a decade, but we are an installed system, run the whole medical system from a medical isolation transformer and require in UIM that any connected equipment be suitably medically isolated to IEC60601-1 and system leakage tested by hospital biomeds prior to use.

In our latest redesign, we have protectively earthed chassis in one version and have no accessible metal parts in another, including a plastic ECG socket and recessed HDMI an USB sockets.

Hope that helps.
 
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