Informational How to consider the Working Voltage for splitting 2MOPP

Peter Selvey

Leader
Super Moderator
I had exactly the same situation with 275Vrms / 750Vp in power supply, with F-type applied part. The mains parts could not meet 2 MOPP so the construction was split for 1 MOPP in the power supply and 1 MOPP in the patient isolation.

Fortunately, the patient isolation could meet this. Also noting that since interpolation is allowed, the increase in creepage/clearance is only small for the additional voltage from 250V to 275Vrms (4.0 to 4.4mm).

However, it is possible that the equipment with F-type insulation is designed exactly for 250V (1.5kVrms, 4.0mm). In this case it may not pass this approach.

The next approach could be to treat the complete insulation path (pri-sec + F-type insulation) as a single item of reinforced insulation. IN that case, the creepage is sure to be OK, but for dielectric strength and insulation thickness it can get messy depending on the construction.

Another approach could be to recognize that if pri-sec fail, the working voltage would fall to mains voltage, so the additional MOPP can meet 4.0mm only. But again a test lab might not accept this.

Finally, general RM/equivalence could be used. The combination of 2 MOOP + 1 MOPP is clearly equivalent to 2 MOPP.
 

Roland chung

Trusted Information Resource
Fortunately, the patient isolation could meet this. Also noting that since interpolation is allowed, the increase in creepage/clearance is only small for the additional voltage from 250V to 275Vrms (4.0 to 4.4mm).

The creepage can be interpolated, but the clearance is not allowed unless the PEAK WORKING VOLTAGES exceed 2 800 V peak or d.c.

But it is also fine as long as the working voltage < 400 Vrms. Since the clearance 3.5 mm is still less than the creepage, says 4.4 mm.

The next approach could be to treat the complete insulation path (pri-sec + F-type insulation) as a single item of reinforced insulation. IN that case, the creepage is sure to be OK, but for dielectric strength and insulation thickness it can get messy depending on the construction.

I also think this approach could be messy and easily make mistake.

Another approach could be to recognize that if pri-sec fail, the working voltage would fall to mains voltage, so the additional MOPP can meet 4.0mm only. But again a test lab might not accept this.

As we discussed before, we need to do a lot of fault tests to verify that only the mains voltage would appear on the secondary circuit when the pri-sec fail.

Finally, general RM/equivalence could be used. The combination of 2 MOOP + 1 MOPP is clearly equivalent to 2 MOPP.

If we use RM, we need to provide objective evidence (data, verification record, etc) to support this equivalent approach.

Peter, have you proven that the 275 Vrms would die when pri-sec short?
 

Roland chung

Trusted Information Resource
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.

Let's assume that reinforced insulation is required in normal condition (reference voltage is 10,000V).

In case 2, what insulation level should be considered for new voltage (13,000V)? Basic insulation or reinforced insulation? Because the 13,000V is generated under single fault condition, in theory basic insulation for reinspection is enough. If so, the reinspection is meaningless since the reinforced insulation @10,000V (NC) is more severe than basic insulation @13,000V (SFC).
 

Peter Selvey

Leader
Super Moderator
Sounds like a reasonable rationale, as long as the fault condition is considered rare (i.e. reasonable quality component and reliable design used in the voltage control circuit).

Note that in some cases for very high voltages, the assumption that 2MOP@10kV > 1 MOP@13kV might not be correct, need to check the values.
 

Roland chung

Trusted Information Resource
Could I use the concept "undetected fault is considered normal" in the 60601-1:2005?

That means if the fault in voltage control circuit is undetected (equipment works normally, no alarms, no blank display, etc), 13kV is regarded as normal voltage. Reinforced insulation @13kV is required accordingly.
 

Peter Selvey

Leader
Super Moderator
Yes, correct according to the standard, but the "undetected fault" theory has serious flaws, and cannot be applied universally. It's already unclearly written in the 3rd edition and will undoubted be revoked in future versions of the standard.

Instead, it should be replaced with basic risk management decisions, using the overall probability of the full sequence of events leading to harm. In some cases this will show that undetected faults are critical (such as an undetected fault in a protection system of an infant incubator) whereas other undetected faults are not as critical, such as the case here. But that is a bigger subject ...
 
J

Joho Zollo

I had exactly the same situation with 275Vrms / 750Vp in power supply, with F-type applied part. The mains parts could not meet 2 MOPP so the construction was split for 1 MOPP in the power supply and 1 MOPP in the patient isolation.
Do you mean if the power supply can meet 2 MOPP, then the 1 MOPP for F-type AP is not needed?

Our product consists of a laptop PC and a BF type AP. The connection between the laptop and the AP is through USB cable. We can replace the original IEC 60950 power supply with an IEC 60601 medical grade one providing 2 MOPP. In this case do we still need the isolation on the USB for 1 MOPP? There will be total 3 MOPP from mains to AP if the answer is yes.
 

Peter Selvey

Leader
Super Moderator
No, the F-type is there for a different reason. Since it is there it can also be utilised to meet the requirements for the mains isolation.
 
J

Joho Zollo

Thanks, Peter. Just want to make sure I understand correctly - even the mains part can provide 2 MOPP already, the additional 1 MOPP for F-type AP is still required, right?
 

Peter Selvey

Leader
Super Moderator
Yes, F-Type is between applied part and accessible parts, it is not related to mains from inside the device. The theory is we assume that the patient is somehow raised to mains voltage from an external source, hence we need a one MOPP to prevent dangerous currents flowing into the device to earth, which may be by via accessible parts.

It's hypothetical situation that was first raised in the 1970s when the original ECGs used to firmly ground/earth the patient via electrodes, raising concerns if say the patient say reached over to their radio and got an electric shock from the frame. Normally, most people would survive such as shock as they are usually mildly insulated from earth, which limits the currents. But if the ECG electrode is providing a solid path to earth, the probability of fibrillation increases to near certain levels. Hence, it is better to have the ECG on an isolated circuit. This has evolved over time to the 1 MOPP requirement between earth and accessible parts we have today. Although there is no evidence the hypothetical situation has ever happened, it, has become firmly established in medical standards. As a nice side effect, it has helped noise immunity, ESU immunity, and cross effects if the patient has multiple sensors, so the isolation still has a lot of value.
 
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