How to test the Split MOPP

[Chandan N]

I think I got some clarity after revisiting the figure J5 from the standard. I was more confused on barrier F in the image above attached.
I was thinking barrier F shall be tested considering the total voltage of Barrier A and Barrier B.

I think Barrier F and A should be for Mains working voltage ? Barrier B is for HV working voltage - (white box testing). And all barriers shall be tested separately. Is it correct ?

 

[Peter Selvey]

In a complex situation like this, you should first systematically identify each "serious" hazardous source (e.g. mains, HV) and then the different points of accessible parts (operator contact at main enclosure; controls; screen; cabling; SIP/SOP, patient contact at cabling; application point); then determine all the different paths in between, next determine how there is 2MOP for each path, and then finally look to identify the parts involved and the specified requirements for each part (not view as the whole device).

For example, the HV can reach the patient in the hand held part, via the cabling, and via the 24V circuit (at least 3 possible paths, which may have three completely different MOP designs). Note that even if the 2kV is intended to be applied to the patient (and hence requires no insulation at the application site itself), application to sites outside of the intended zone or at times when the application site should not be energized (e.g. setting up, end of treatment) still requires 2MOP.

In a normal design with seriously dangerous parts (like mains), we put a ring barrier around the dangerous stuff and keep it to a small area. This makes it easy to analyze and show in a block diagram. However, in some cases, this is not possible, and the dangerous stuff needs to go different places in the device (like the 2kV part). In this case, the block diagram can be misleading, since it appears to lump all the different paths together, when they really need individual analysis. However, there can still be simplifications. For example, the 2MOPP in the cabling for the patient will also cover the 2MOOP for the operator. Or if there is 2MOOP between the 2kV and 24V, then there is no need to worry about the different paths between to the operator from the 24V circuit (via controls, touch screen, signal I/O etc). So although an initial analysis might find e.g. 20 different paths this might collapse down to say 10 paths for a device like this.

Finally, MOP can be a mix of things, so you need to drill down and find out exactly what the MOP is and make a list as part of the analysis. If it is solid insulation (wiring, optocouplers, capacitors), then the dielectric strength applies, which should in principle be applied just to that insulation (not an assembled device, a common misconception). If it is spacings e.g. on a PCB, connectors, then creepage/clearance applies. Again, if it is complicated, you really need to be systematic. So for example, if the above analysis ends up with 10 paths, then for each of those 10 paths, list the exact parts that are involved in the isolation, and the criteria that applies.

For HV circuits, the use of current limits can be useful. For example, if the HV part is a dc/dc converter that can only supply at most 2mA, and it shares a common ground to the 24V circuit which is also earthed, then shorting the HV circuit to the 24V circuit or earth should result in no hazardous situation due to the current limit. In this case, the current limit is 1MOP, and the "relative" low impedance of the other part (24V circuit, ground) being the other 1MOP (2MOP in total). Since the source it only 2mA, even 50Ω would be "low impedance" for patient limits (0.1V), and for operator even 30kΩ is OK (=60Vdc). It does not need to meet the 0.1Ω limits in 8.6.

 

[Chandan N]

Hi Peter,

Thank you for your time and writing the detailed explanation on motive for the MOPs in designs.

It is bit long post, but I felt it is better to provide more details about the device and my thought process.
Please bear with me.

Based on the studying the standards again and your explanations, I have updated and attached the block diagram.

Would like to add more details about the device, so that it give more clarity to the people helping me to understand better:

Application is similar to the application discussed in the thread "Insulation requirements of HV pulse circuits" - Insulation requirements of HV pulse circuits.
We are producing 2kV pulses to apply to patient from 500V DC by charging the capacitors. So 2kV is from capacitors.
2kV is delivered to patient thru active and return cables (wires). Wires are rated for 10kV insulation - provides 2 MOPP at 2kV.
By application, this device does not comes under IEC 60601-2-2 standard's scope.
Lets assume only IEC 60601-1 is applicable and this General Standard does not speak about the pulse voltages.

More on MOPs:
Since the peak voltage is 2kV, then the peak working voltage in the secondary circuit would be 2kV. Then to protect the patient and operator from mains, I will have to design AC to DC converter (or alternatively an Isolation transformer) which supports the 2MOPP at 2kV protection from primary circuit. Also maintain 2 MOPP Creepage and Clearances from enclosure to all secondary circuits (this could be difficult).

To avoid this and to utilize the commercially available medical grade power supplies, would like to split the barriers. A commercially available IEC 60601-1 certified AC to DC adapter provides 2 MOOP and 2 MOPP at mains voltage.
24V to 24V isolator provides - 2 MOOP at 2kV. 24V in the secondary is floating (not connected to earth), using Class II (2 pin) adapter. HV circuit is also floating.

Hand Held part - It has 2kV pulses(Just wires from HV circuit) and other control signals. Both are separated physically and by the wire insulations which provides 2 MOOP at 2kV. Also 24V to 24V isolator provides 2 MOOP at 2kV from secondary circuits.
HV circuit is separated from the other secondary circuits & enclosure by creepage and clearances applicable for 2 MOOP and 24V isolator continues to provide 2 MOOP.

From above understanding, In Normal condition MOPs shall be as follows:
1. AC to DC converter - Barrier A - Provides 2 MOPP / 2 MOOP at mains working voltage
2. DC to DC Converter - Barrier B - Provides 2 MOOP.
3. Creepage and Clearances or other solid insulations (like insulation sheet) between HV circuits and secondary circuits, enclosure - Barrier C - 2 MOOP
4. Creepage and Clearances or other solid insulations (like insulation sheet) between HV wires and secondary circuits, enclosure of hand held device - Barrier D - 2 MOOP
5. Wire Insulation provides 2 MOPP at 2kV.
6. Barrier E - Mains to Applied part - 2 MOPP at mains voltage

Query 1: Please correct if any thing is wrong in my understanding.

Fault Conditions / Special Conditions:
Like you mentioned, we will have to evaluate the paths other than intended current path which could be harmful. ex: if return wire is disconnected, then current may find path thru
Path 1: Source --> Patient --> Earth --> Source. Other
Path 2: Source --> Patient --> Operator -->Earth --> Source.
Path 3: Source --> Patient --> Operator -->Accessible parts --> Source.

Considering the above fault conditions, we need to design the 24V to 24V isolator to provide 2 MOPP at 2kV protection from secondary circuits. Because these paths carry patient leakage currents
Now, the above listed MOPs 2. 3. and 4. points shall be 2 MOPP instead of 2 MOOP.

With the identified paths, MOPs List will be :

1. AC to DC converter - Barrier A - Provides 2 MOPP / 2 MOOP at mains working voltage
2. DC to DC Converter - Barrier B - Provides 2 MOPP.
3. Creepage and Clearances or other solid insulations (like insulation sheet) between HV circuits and secondary circuits, enclosure - Barrier C -
2 MOPP
4. Creepage and Clearances or other solid insulations (like insulation sheet) between HV wires and secondary circuits, enclosure of hand held device - Barrier D - 2 MOPP
5. Wire Insulation provides 2 MOPP at 2kV.
6. Barrier E - Mains to Applied part - 2 MOPP at mains voltage

Query 2: Is my understanding correct ?

I have few more questions, but would like to know my basic understanding is correct or not.

Once again, thanks for your time

 

[Peter Selvey]

In a situation like this, the only way to assess is to have a look at the actual design, it's too complicated to discuss/cover here.

When I mentioned a "path" before, I did not mean external path, I meant individual paths internal to the equipment.

For example, one path from HV to the operator could be through the 24V/24V isolator to the main 24V secondary, which then goes to a touch screen and knobs, as well as signal I/O. For this path, the designer says the 24/24V isolator is the "formal" barrier (not, for example the touch screen plastic). The 24/24V isolator has will likely have a transformer, EMC caps and optocouplers bridging the two circuits, as well as spacings on the PCB. So now we get to the parts and we can then measure and assess the working voltage (including in normal/fault condition) and then derive specifications for each part (dielectric strength, cr/cl, sealed joints and so on).

Another path might be via analogue/digital interface with the HV circuit for monitoring and control, for example a series of 10MΩ resistors used to sense the actual voltage in the HV, opto-coupler for controlling the pulse signal to the patient. This kind of interface might be in a completely different location from the 24/24V isolation, and may have different voltages and different specifications.

Another path may be via a connector that has the pulsed HV and non-isolated secondary circuit, again different voltages and specifications for that point.

And so on.

Also note that if there are any switching regulators (such as for generating the HV) or pulsed outputs, it's never really "floating". The stray capacitance and EMC caps will normally be low impedance at switching frequencies.

You might be thinking ... how can I design in advance, but like most things the first step is a kind of dummy prototype and then look at the internal paths and see where the weak points are, and then improve, re-check and so on until the insulation design is robust, just like any engineering task.

 

[Chandan N]

Yes, we have to consider these points.
Was trying to identify whether we need MOOP or MOPP (by considering the internal and external paths). Then look into the design with appropriate components to achieve the required isolation as per MOPs (MOOP / MOPP).
With the above mentioned approach, we are identifying the MOPs - MOOP or MOPP.

For example, one path from HV to the operator could be through the 24V/24V isolator to the main 24V secondary, which then goes to a touch screen and knobs, as well as signal I/O. For this path, the designer says the 24/24V isolator is the "formal" barrier (not, for example the touch screen plastic). The 24/24V isolator has will likely have a transformer, EMC caps and optocouplers bridging the two circuits, as well as spacings on the PCB. So now we get to the parts and we can then measure and assess the working voltage (including in normal/fault condition) and then derive specifications for each part (dielectric strength, cr/cl, sealed joints and so on).

Yes, we have considered these and working on it. But had confusion on MOPP or MOOP. Now working on MOPP requirements based above external paths.

Another path might be via analogue/digital interface with the HV circuit for monitoring and control, for example a series of 10MΩ resistors used to sense the actual voltage in the HV, opto-coupler for controlling the pulse signal to the patient. This kind of interface might be in a completely different location from the 24/24V isolation, and may have different voltages and different specifications.

Yes, we have considered these and working on it. But had confusion on MOPP or MOOP. Now working on MOPP requirements based above external paths.

Also note that if there are any switching regulators (such as for generating the HV) or pulsed outputs, it's never really "floating". The stray capacitance and EMC caps will normally be low impedance at switching frequencies.

Yes.

You might be thinking. how can I design in advance, but like most things the first step is a kind of dummy prototype and then look at the internal paths and see where the weak points are, and then improve, re-check and so on until the insulation design is robust, just like any engineering task.

I think this would be the best approach to realize the paths.

Thanks Peter