Informational How to consider the Working Voltage for splitting 2MOPP

[Roland chung]

I don't think the determination approach for W.V deviates from the standard.

The logic is that if a SFC results in additional faults all are considered related to the original fault. Therefore, if a fault did cause a higher potential but the Hi-Pot test and spacing is based on the NC voltage, the spacing/insulation could fail (but still as a result of the original fault).

 

[Peter Selvey]

Lets assume in normal condition CO2 laser supply has 10,000V. Insulation between the circuit and the operator is designed for 10,000Vdc.

In SFC (voltage feedback trouble) supply raises to 13,000V.

Case 1: insulation to operator breaks down
Case 2: insulation to operator does not break down

In case 1, it's a clear failure.

In case 2, what is the result?

I think Case 2 needs to re-inspect dielectric strength, creepage and clearance for the new voltage (13,000V).

Why? Because in this case we cannot rely on a single test to be representative of production and the real world. There are too many variables. Maybe one day it passes, next day it fails because the humidity is a little higher, or the test sample is slightly different.

Limits in standards have those variables built in so we can use just a single test and still be confident.

 

[Roland chung]

Your last sentence baffled me. It seems to be in contradiction to your point (reinspection for Case 2 is needed).

Limits in standards have those variables built in so we can use just a single test and still be confident.

Hum, a little bit confusing.

 

[Peter Selvey]

Sorry, not too clear. Mainly I was replying to:

The logic is that if a SFC results in additional faults all are considered related to the original fault. Therefore, if a fault did cause a higher potential but the Hi-Pot test and spacing is based on the NC voltage, the spacing/insulation could fail (but still as a result of the original fault).

I thought it means ignore any increase in voltage from the SFC, just decide the outcome based on the fault test. I was trying to explain a test alone is not enough, the design has to be reliable, therefore cr/cl for the new voltage is reasonable.

Maybe you did mean the same thing as me, just different terminology.

Sometimes a whiteboard would be good ...

 

[ca_moni]

Hello all
I'm resuming this thread from the bottom of the forum history (not so old, however) to share with the community my poor understanding of the isolation requirements of the IEC 60601-1 (ed. 3 - but perhaps even in the 2nd version) and my limited English capabilities ...hoping someone could help in solve the doubts.
Let's pass from the HV case here discussed to the opposite: we have a device (actually, a medical electric system composed by an interface and a device with BF-type patient connections (electrodes) that is powered from the USB port of a IEC 60950-compliant PC. There is electric separation both in the interface and in the amplifier (to get the BF classification for the electrodes).
Hence we have the following separations
- MAINS // USB - earthed (the internal PC power supply),
- USB // interface out – (DC/DC converter+opto, floating secondary, )
- interface out // BF type electrodes (DC/DC converter+opto, obviously floating ).
“Interface out” circuit is not accessible (plastic enclosures, cables with touch-proof plastic connectors, no openings in the case) . This is true also for the device, hence isolation to the enclosure (accessible parts) is always guaranteed , the only issue is the isolation of the patient connections from the other parts.
It seems that these separation can be consider in this way [I’M NOT SAYING THAT THIS IS THE ACTUAL DEVICE! ]:
- MAINS // USB - obtaining a secondary 5V earthed circuit separated from mains with 1 MOOP @250V [supposed, but I think it is required by IEC60950]
- USB // interface out – 1st MOPP , W.V=(5+5)10V, required isolation 500V
- interface out // BF type electrodes (obviously floating ) – 2nd MOPP , W.V. (5+5)10V, required isolation 500V

BUT , for clause 8.5.2.1, floating patient connection must be separated with 1 MOPP @ 250V [1.5kV] from all parts.
Then, requirements for interface out/electrodes, electrodes/case, electrodes/SIP/SOP become 1.5kV at least.

this however does not fit with the required separation from the mains (table 6, 2MOPP @250V=4000V )

1)should I consider the 5V from the USB as if they are actually the 250VAC from the mains? Can I use the MOOP in the PC as a part of the overall isolation?

2)And, if we consider each separation as 1MOPP @250V the required dielectric strength is 1.5kV AC for 1 minute. But to test 2MOPP @250V I must apply 4kV (table 6) from the shorted USB connector to the patient applied parts… I don’t think the devices will like the test very much! (1.5kV+1.5kV=3kV maximum, in the best case that the voltage will be equally divided). So, which is the meaning of describing this test as 2MOPP? The requirement is actually higher…

3) (technical question ) if a double isolation is composed of two elements each one rated 1.5kV , how much is the overall isolation ? this isolation in NC can bear 3kV … but in case of a single fault (one of the two elements), the other will bear all the voltage drop and will likely fail too… however, they are two separated isolation so they are effectively a double isolation.
…
sorry for the length, but I’m a little bit confused

Thank you for any help or comments

 

[Peter Selvey]

Assuming the PC complies with IEC 60950-1, we can start with the assumption that the USB circuit is 2 MOOP from mains parts. We don't know the internal construction (whether it is double insulation, reinforced insulation or basic + earthing), but we can assume there is 2 MOOP.

Between the USB and interface there is no isolation requirement as such. A voltage of 5V with 2 MOOP is safe for the operator to touch, and would only become a concern if there was a way to get the 5V to the patient via a low impedance path (which is pretty tough to do unless you are really trying hard). Formally, a warning and risk assessment is required (see Clause 8.4.2 c)).

You might have isolated the interface from the USB for functional reasons (e.g. noise from the PC, prevent ground loops), but you don't need to identify this as a MOP. I'm assuming here the interface is just a normal signal circuit, not connecting to TNV (telecom networks) where isolation might be reasonable.

Between the interface circuit and the BF circuit the analysis is correct: 1MOPP for 250V (1.5kV) has greater requirement than 2 MOPP for 5V (500V) so usually we only worry about the 1 MOPP for 250V.

Between mains parts (in the PC) and patient, the 2 MOOP (in the PC) + 1 MOPP (for BF) is greater than 2 MOPP.

For example, assume the PC is reinforced insulation for 230V:

2 MOOP (IEC 60950) = 3.0kV isolation
1 MOPP (BF isolation) = 1.5kV isolation
Total = 4.5kV, which exceeds the 4.0kV isolation needed for 2 MOPP.

No matter what the internal construction of the PC is this will always be the case, as long as the PC complies with IEC 60950-1.

 

[ca_moni]

Thanks Peter, I appreciate your contribution... they help very much in understanding.
Now, let's say that for the same device the 5V interface input is not from a PC compliant with IEC 60950 (we can state this as a requirement, in case the PC is not provided by us, but cannot be sure the customer will fulfill it - reasonably misuse). We want to guarantee the same level of safety and hence have to increase the isolation level.

- I still do not understand why 1 MOPP@250 V =1.5kV but 2MOPP@250V must be 4kV. (I'm not questioning the values, only the rationale or wording ... 4kV are not "2" MOPP, they are MOPP at another isolation level )

- if we have 2 separated MOPP in series (like the example, one in the interface and one in the device) and one will fail, this means that its isolation level has been overpassed. how can another isolation being an additional protective means? is not reasonably probable that also the second will fail if subject to the entire voltage drop that before was sustained by two isolation stages?

thank you again

 

[Peter Selvey]

Yes, this is a reasonable approach to worry about the actual 950 compliance (good risk management). But make sure this is under risk management, not part of formal compliance testing, so to keep things flexible and reasonable.

Obviously there will always be a degree of protection in the PC, otherwise means the operator is at serious risk (and there would be a lot of dead people on the planet from PC use). So we don't need to go to the extreme of assuming the PC has no protection, but maybe somewhere between 1-2 MOOP as a result of "misuse" (not following instructions).

The most realistic case will be a 950 compliant PC, but the user does not connect the earth pin, so 1 MOOP is missing (it's common in parts of Asia to use 3pin to 2pin adaptors because many wall sockets only have 2 pins, the earth connection is broken).

In this case you would have 1 MOOP + 1 MOPP as a result of misuse. Personally, I would be happy to accept this, but mainly because of a deeper knowledge and experience of where the requirements and limits come from, that would be a bit too long to explain here.

Some manufacturers do simply ignore the PC and put a full 4kV isolation barrier between the BF circuit and all other parts. Actually, because parts are easily available with 4kV isolation, it's not a bad option, because it saves any arguments (and could be a sales point). Sticky point might be the dc-dc converter, but 4kV devices do exist.

The 1 MOPP = 1.5kV and 2 MOPP = 4kV is probably driven by practicality (the same issue exists for MOOP, 1kV and 3kV is not balanced). In an earthed system there can be lots of parts between mains and earth (EMC capacitors, heaters, motors etc), which is 1 MOOP. If for example, 2kV was required a lot of parts could not be used. However, parts with 2 MOOP is limited (transformers, optocouplers). So 4kV there is practical.

For the final point: dielectric strength testing is an ageing test. That's why the test values are many times higher than working voltage: a 1.5kV test proves the insulation can handle 230V for 15 years. A 4kV test proves the insulation can handle 230V for 50 years (these are just rough figures for concept only). With that in mind, it's OK to put insulation in series: if one fails, it does not mean the other is going to fail immediately.

 

[Roland chung]

In this case you would have 1 MOOP + 1 MOPP as a result of misuse. Personally, I would be happy to accept this, but mainly because of a deeper knowledge and experience of where the requirements and limits come from, that would be a bit too long to explain here.

Such misuse (in this case, without earth pin) would only be considered as a normal condition since the misuse is frequent event (>1 event/year/device).

Anyway, 1 MOOP + 1 MOPP is already sufficient (6.5/ 4.5mm, 3kV) according to the IEC 60664-1.

 

[Roland chung]

Hello,

It is an old question but still not clear.
Many switching power supplies have been 60601-1 3rd Ed approved. But many of them have the insulation between the primary and secondary designed to 2 MOOP only (due to the higher cost for 2 MOPP).

An insulation path for ECG equipment consists of a 2 MOOP SMPS and an additional insulation with 1 MOPP from secondary circuit to applied part (This 1 MOPP insulation is also required for F type applied part). The maximum working voltage of the approved SMPS can be derived from the CB report, says 288Vrms/ 598Vpeak. Should the working voltage of 1 MOPP insulation which between secondary circuit and applied part also be 288Vrms? Or 250Vrms is sufficient?

That said there are two options:
1) 2 MOOP @ 288Vrms/ 598Vpeak + 1 MOPP @ 288Vrms
2) 2 MOOP @ 288Vrms/ 598Vpeak + 1 MOPP @ 250Vrms

It is really now common that medical device to employ 2 MOOP SMPS. Many manufacturers are indeed facing the same problem. Please share your experience.

Thanks,
Roland