MOPP required for SIP when considering Single Fault Condition (SFC) in 60601-1

Peter Selvey

Leader
Super Moderator
Also for leakage, you need about 40mA to have even a 1% probability of stopping the heart.

A current of 1mA has essentially no probability of stopping the heart applied external to the body, and would only be a concern if it is applied in open heart surgery via a a small contact area just to the right point in the heart itself. Also the value is too low to cause any I²R heating.

Thus, the discussion on whether 0.1, 0.25, 0.5 or 0.75mA is reasonable hinges on our reaction to the sense of current. As mentioned, a high open circuit voltage can cause a spark prior to contact which concentrates the current and can make easier to sense or even painful. This in turn can cause a person to move in an unintended way. A person on top of a ladder drilling a hole that gets a 1mA shock might react in a way that causes them to fall off the ladder and break their back. So, lower is better. But even at 0.1mA it is possible to sense. I had a bass guitar with 0.05mA coming through a double insulated amp and I could sense this if I brushed the strings lightly, especially with the sensitive part of my forearm. With that in mind, I do agree to try and meet 0.1mA for a medical environment, to limit the possibility a nurse or surgeon making an unintended action. That said, most IT equipment stays under 0.1mA in normal condition to avoid a heap of customer complaints that "I felt a shock from your device".
 

ian44

Registered
The "2MOP" way of writing is relatively new but there has always been either "double/reinforced insulation" or "basic insulation + earth" since the 1970s at least, it doesn't matter if it is an electric fence, TV, laptop or washing machine. This is common to all standards. The 2MOP is just a more generalistic way of writing and allows esoteric implementations.

The only difference is the limits in the standards. Years ago, the insulation limits in 950 were considered for medical, but it was actually the 950 committee said they couldn't guarantee the probabilities were low enough if you had for example electrodes on the patient, catheters, infusion liquids, they were unconscious (no pain based retraction from contact) and also had a nice solid return path to earth. In the real world that kind of perfect path through the heart is pretty rare, but the medical standard wanted to assume it as normal condition. So for example, 950 limits might be based on a probability of insulation failing of 1/100,000 times per year, which is OK when combine with a further factor of 1/10 that this causes actual serious harm in the real world, achieving the proverbial one in a million. For 601 this second factor was assumed to be 1 (i.e. 100% sure to kill if the insulation breaks down), so they raised the limits for creepage and clearance, dielectric strength to be one in a million on their own.

These days though, any device with intimate contact with the patient has an additional barrier (the F-type barrier), which is equivalent to 1MOPP. This additional barrier makes all the above moot, since clearly there is no longer a perfect path to the patient. So it is perfectly fine to have 2MOP in the 950 device plus the 1MOPP to the patient. The 3rd edition allows this, although there continues to be misconceptions that IT equipment must be assumed to be dangerous and have only 1MOOP or even no MOP.

Thanks for the detailed info - I have a question about the following statement: “..it is perfectly fine to have 2MOP in the 950 device plus the 1MOPP to the patient. The 3rd edition allows this…”.

This statement makes complete sense to me, but I cannot figure out where in the 60601-1 standard I can reference something to support that statement. Can you point me to it? Or do I need a copy of 60590/62368 to find the relevant dielectric and creepage/clearances values for 2 MOP as defined by 60950/62368?

(Note: my thinking goes as follows:
As per 8.1 a) the NORMAL CONDITION includes short-circuiting of any insulation/creepage/clearance that does not meet sufficient MOP. Because we are considering the PATIENT, we need to consider MOPP. So I would think that in order for "2 MOP in the 950 device plus 1 MOPP to the patient" to be sufficient, I "think" that would require that 2 MOP in the 950 device be equal to 1 MOPP. So to echo MThomas' question, does 2 MOP in 950 = 1 MOPP? Or am I misunderstanding something?)
 

Peter Selvey

Leader
Super Moderator
Yes, for a simplistic approach we could assume 2MOP(950) = 1MOPP, although a more nuanced way would be 2MOP(950) >>>≥1MOPP but not quite 2 MOPP. Which is to say, if we equate 2MOP(950) = 1 MOPP it makes it sound like 2MOP(950) is fairly weak, but it is not. It's just that it doesn't quite reach the level of 2MOPP. It like saying 1.95 is not quite 2.0. Do you relegate the 1.95 to 1.0 just to make it simple?
 

ian44

Registered
Yes, for a simplistic approach we could assume 2MOP(950) = 1MOPP, although a more nuanced way would be 2MOP(950) >>>≥1MOPP but not quite 2 MOPP. Which is to say, if we equate 2MOP(950) = 1 MOPP it makes it sound like 2MOP(950) is fairly weak, but it is not. It's just that it doesn't quite reach the level of 2MOPP. It like saying 1.95 is not quite 2.0. Do you relegate the 1.95 to 1.0 just to make it simple?

Thanks for the overview. However, I feel like I am missing something in terms of how 2MOP(950) >>>≥1MOPP, particularly in terms of creepage and clearance distances.

For example, if we consider 120 VA, overvoltage Category II, 60601-1 has a single MOPP requiring 1.6 mm clearance and 3.0 mm creepage. I do not have a copy of 62368, but according to a link I found (but cannot include in this post...), 2 MOP (a.k.a. reinforced insulation) at 120 VAC requires 1.5 mm clearance and between 0.56 mm and 4.8 mm creepage (depending on the pollution degree and material group). Those numbers put 2 MOP(62368) < 1 MOPP for clearance and = 1 MOPP for creepage. What numbers are you looking at to get 2MOP(950) >>>≥1MOPP?
 

Peter Selvey

Leader
Super Moderator
There's two factors in the MOPP limits, one is the larger distances to address the higher risk of having electrodes, catheters and the like, and the second is a desire to keep it simple and have just one limit fits all approach. So in 601, MOPP clearance is determined by a a few paragraphs, a single table with one number for each voltage, while in 62368 it is covered by 12 pages and multiple tables and parameters to consider. So it's apples and oranges.

There's also something weird in 601 distances that skew larger as the voltage goes down. For a 230Vac device, using "typical" 2MOOP for IT is 3.6mm while in 601 1MOPP is 2.5mm, so 2MOOP > 1MOPP. However, statistically the extra 44% is huge, it could be 1 or 2 orders of magnitude less likely to breakdown (e.g. 0.1% @ 2.5mm vs 0.01% or 0.001% @ 3.6mm). It's not a linear relationship.

In contrast the values for 120V are essentially the same 1.52mm vs 1.6mm, so as pointed out 2MOOP < 1MOPP.

If we go further and consider a battery powered device with 2 x AA and no connection to mains, IT standards treat this is requiring no clearance (which is entirely reasonable) while 601 says it still needs 1.6mm clearance. Which is absolutely crazy. It is more than 1MOOP at 120Vac for IT. So, I don't give much credence to the values in 601 especially at the lower end. There's no connection to reality.

<additional thought>
I suspect the values at 230Vac were reasonably considered (Med ≈ 1.4 IT) , while the huge values at low voltages (5V, 12V etc) were driven by some mixed up logic and worst case assumptions. The values at 120V (Med ≈ 2.1 IT) seem to be caught in between in order to avoid making the craziness of the lower voltages stand out.
 
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VinceTech

Involved In Discussions
Figure J4 shows 2MOPP for working voltage, which maybe different from mains voltage. For example 5V. However, the 1MOPP of main voltage is prevent the risk in SFC where AP is connected to mains voltage. 2MOPP of 5V is not sufficient for 1MOPP mains voltage.
Yes, it's right. 2MOOP is sufficient if it's working voltage of mains. There is not need to specify another 1MOPP of mains voltage.
 
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