Appearance of an External Voltage on SIP/SOP (Medical Devices)

Roland chung

Trusted Information Resource
#11
"Earthing" of Signal circuits

Usually we talk about "protective earth" to mean <0.1 ohm. This very low resistance is necessary to carry fault currents.

However, in the context of sinking leakage currents in the order of 1mA, we don't need to worry about 0.1ohm. In a Class I device, most signal circuits are "earthed" in the sense that they can sink a few mA of leakage current. An impedance of the signal circuit to earth of even 10,000ohms (resistive or capacitive) is unlikely to cause any increase in the voltage of accessible parts.

This is still considered "protective earthing" but is a special case. Both 2nd and 3rd edition allow for this special case in the clause for earthing.
It is common that the DC RETURN of intermediate circuit is connected back to earth. But, this is normally for EMC purpose. That said, the DC RETURN which the signal circuit connected to would normally be considered as functional earth and interruption of the functional earth is a normal condition.

The rationale described in the annex A for 8.6.4b) seems not the case which mentioned above.
 
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Peter Selvey

Staff member
Super Moderator
#12
We really need a white board for this one ...

A "functional earth conductor" is an external connection (see Figure 2, item (7) in IEC 60601-1:2005. You can imagine noise sensitive equipment that might be provided with an external terminal for grounding for EMC purpose. Obviously having nothing connected to such a terminal is normal condition.

But this has nothing to do with internal connections. Breaking an internal connection is a single fault condition.

I think the case here is that in practice this internal connection is often used sink current from IT equipment, but it is not really well recognized by the standard or industry.

I think it is a valid under IEC 60601-1:2005, but if you don't like it feel free to offer another solution.
 

Roland chung

Trusted Information Resource
#13
I just asked our R&D engineer. He said the main purpose for connecting the intermediate circuit (SIP/SOPs circuit) to earth is:

1) provide a path for noise (i.e. EMC purpose);

2) provide a common reference for signal circuit when such circuit is connected to external equipment via SIP/SOP.

Figure 2 or 3 or 4 in IEC 60601-1:2005 indeed shows the functional earth terminal is external. However, I would think it is not the only connecting form. The standard also allows a internal functional earth connection for class II equipment (clause 8.6.9).
 

Peter Selvey

Staff member
Super Moderator
#14
I am happy to agree that the standard doesn't handle the situation well. However, from basic risk management we are talking about the following hazardous situation:

1) earth opens on a 950 device, results in leakage of 1-2mA into a connected medical device via a SIP/SOP
2) internal functional connection(s) opens in the medical device
3) the leakage gets to an accessible part
4) resulting electric shock from 1-2mA causes harm

A current of 1-2mA is not really that serious, and the sequence of events is also highly improbable.

It is reasonable to assume a functional earth is open if we are talking about a direct mains electric shock. But in this case we are talking about a current limited to 1-2mA by the capacitors in the 950 device.

The alternate would be to designate the internal connections as protective earth connection. This means, they would need to comply with the marking requirements (green/yellow) but not the 25A/0.1ohm.

I would oppose the green/yellow marking on the grounds that it would lead to unnecessarily costs and confusion for a subject that is not really a high risk situation.
 
S

sankui

#15
Hi Peter,

Do you think it is possible to appear a MAINS on the SIP/SOPs directly(e.g. because of breakdown of the insulation inside 60950 device)? For type B equipment, it could be hazardous if the internal earth of SIP/SOPs circuit open at the same time (the internal earth can not meet 25 A/0.1 ohm, open is therefore normal).
 

Peter Selvey

Staff member
Super Moderator
#16
The assumption of insulation failure in a 950 device leading to high fault current in a SIP/SOP of a medical device is not realistic.

First, it is a double fault condition (earth open, basic insulation fails).

Second, if it were to occur the operator and people in the environment around the 950 device would be placed at a serious and clearly unacceptable risk (far higher than the patient). Anybody who touches the case of the 950 device would be subject to direct mains voltage.

So, it's not a realistic scenario to be considered. The only thing we should be talking about are leakage currents in the order 1 - 5mA.
 
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