Why 1 MOPP to main voltage but 2 MOPP to working voltage

Gerald Leo

Registered
In IEC 60601-1: 8.5.2.1: The PATIENT CONNECTION(S) of any F-TYPE APPLIED PART shall be separated from all other parts, including the PATIENT CONNECTION(S) of other APPLIED PARTS, by means equivalent to one MEANS OF PATIENT PROTECTION for a WORKING VOLTAGE equal to the MAXIMUM MAINS VOLTAGE and shall comply with the specified limit for PATIENT LEAKAGE CURRENT with 110 % of the MAXIMUM MAINS VOLTAGE applied.
from my understanding mail voltage should higher than working voltage(most case), why only 1 MOPP to mainvoltage, but 2MOPP to working voltage.

how can 1 MOPP ensure the safety in case of single fault?
Why 1 MOPP to main voltage but 2 MOPP to working voltage
 

Peter Selvey

Leader
Super Moderator
Type F (BF or CF) insulation is intended to protect the patient against a hypothetical mains voltage source applied to the patient, which is external to the device under assessment.

Let's go back in time to the 1960s: image a patient with ECG electrodes across their chest, connected to a patient monitor. Around that time, ECGs could be an earthed circuit to reduce noise. The patient leans across to a radio to the latest updates on the Apollo missions (or the Ashes cricket score). The radio is poorly made, and the metal case is live with mains voltage.

An average person might touch the case, get a decent shock but survive because they are not solidly grounded, the shoes, floor cover etc help to reduce the current to <40mArms (the threshold of problems).

But the patient has this wonderful, low impedance electrode on their chest, directly grounding them to earth. The shock from the radio is more than 200mA, easily causing cardiac arrest.

From this scenario, F-Type circuits were born. These circuits were "floating" so they didn't ground/earth the patient.

Originally fairly light on requirements (just 500V isolation was OK, no cr/cl), they have evolved to requiring 1MOPP @ rated mains which typically means 1.5kV/4mm/2.5mm. The reason this is only 1 MOPP is due to the fact that the patient being expose to external mains voltage should already be rare.

For BF parts, I think this is overkill, since rogue mains voltage such as a defective radio should rarely be found in the real world. It should be floating but say 500Vrms, 1 or 2mm should be enough, not 4mm.

For CF there is a stronger case to be argued as simple broken earth wire can source leakage currents of 0.5mA or more, which can be dangerous if applied directly to the heart. So, 1.5kV/4mm makes sense there.
 

Gerald Leo

Registered
Type F (BF or CF) insulation is intended to protect the patient against a hypothetical mains voltage source applied to the patient, which is external to the device under assessment.

Let's go back in time to the 1960s: image a patient with ECG electrodes across their chest, connected to a patient monitor. Around that time, ECGs could be an earthed circuit to reduce noise. The patient leans across to a radio to the latest updates on the Apollo missions (or the Ashes cricket score). The radio is poorly made, and the metal case is live with mains voltage.

An average person might touch the case, get a decent shock but survive because they are not solidly grounded, the shoes, floor cover etc help to reduce the current to <40mArms (the threshold of problems).

But the patient has this wonderful, low impedance electrode on their chest, directly grounding them to earth. The shock from the radio is more than 200mA, easily causing cardiac arrest.

From this scenario, F-Type circuits were born. These circuits were "floating" so they didn't ground/earth the patient.

Originally fairly light on requirements (just 500V isolation was OK, no cr/cl), they have evolved to requiring 1MOPP @ rated mains which typically means 1.5kV/4mm/2.5mm. The reason this is only 1 MOPP is due to the fact that the patient being expose to external mains voltage should already be rare.

For BF parts, I think this is overkill, since rogue mains voltage such as a defective radio should rarely be found in the real world. It should be floating but say 500Vrms, 1 or 2mm should be enough, not 4mm.

For CF there is a stronger case to be argued as simple broken earth wire can source leakage currents of 0.5mA or more, which can be dangerous if applied directly to the heart. So, 1.5kV/4mm makes sense there.

Does 1 MOPP single fault safe? or just because it's really low probability to exposed to main voltage.
 

Peter Selvey

Leader
Super Moderator
BF and CF ratings are not part of the single fault system. Think of them more like a waterproof rating (IPX3). If you mark the device IPX3, it need to meet the IPX3 waterproof test. It does not matter if IPX3 is important or not. If it's marked on the label, you have to comply.

BF/CF are the same. Use the BF or CF symbol, then it has to meet at least 1 MOPP @ mains voltage between applied part and other parts. It's not relevant whether it's needed or not for single fault safety. It's not relevant what risk management says. It's not relevant what the working voltage is. If the symbol is there, it needs 1MOPP@mains.

Above I'm trying to explain the history to understand the background.
 

Gerald Leo

Registered
BF and CF ratings are not part of the single fault system. Think of them more like a waterproof rating (IPX3). If you mark the device IPX3, it need to meet the IPX3 waterproof test. It does not matter if IPX3 is important or not. If it's marked on the label, you have to comply.

BF/CF are the same. Use the BF or CF symbol, then it has to meet at least 1 MOPP @ mains voltage between applied part and other parts. It's not relevant whether it's needed or not for single fault safety. It's not relevant what risk management says. It's not relevant what the working voltage is. If the symbol is there, it needs 1MOPP@mains.

Above I'm trying to explain the history to understand the background.

@ peter Selvey:
thanks for your explanation. it's clear for me :)
 

jake_D

Registered
Hoping to piggyback on this discussion, as it pertains to my situation as well. I am designing a prosthetic device, which does have electronics (hence 60601) but the electronics do not directly connect to the patient. However, the case/ structural frame of the prosthetic device is ALUMINUM, and it physically connects to the patient via metal componentry and a carbon fiber socket. All internal electronics are insulated from the frame of the device with creepage/clearance in mind.

We are arguing internally whether or not the frame is an APPLIED PART, but I think we have to assume it is an APPLIED PART and since the patient wears the device for extended period of time, it is a BF type part.

This device is battery powered, although, the patient *could* plug it into the DC wall charger while wearing the device.

Is the DC Wall charger (or lack there of if disconnected) count as a MOPP?

It seems that the DC wall charger (medical grade, certified, double insulated, etc) is the MOPP in the case, because it protects the mains from going through the patient, back into the wall and hitting ground. If the Charger is NOT connected, there is no direct path to ground. Does this seem right?
 

atfarsoni

Registered
Type F (BF or CF) insulation is intended to protect the patient against a hypothetical mains voltage source applied to the patient, which is external to the device under assessment.

Let's go back in time to the 1960s: image a patient with ECG electrodes across their chest, connected to a patient monitor. Around that time, ECGs could be an earthed circuit to reduce noise. The patient leans across to a radio to the latest updates on the Apollo missions (or the Ashes cricket score). The radio is poorly made, and the metal case is live with mains voltage.

An average person might touch the case, get a decent shock but survive because they are not solidly grounded, the shoes, floor cover etc help to reduce the current to <40mArms (the threshold of problems).

But the patient has this wonderful, low impedance electrode on their chest, directly grounding them to earth. The shock from the radio is more than 200mA, easily causing cardiac arrest.

From this scenario, F-Type circuits were born. These circuits were "floating" so they didn't ground/earth the patient.

Originally fairly light on requirements (just 500V isolation was OK, no cr/cl), they have evolved to requiring 1MOPP @ rated mains which typically means 1.5kV/4mm/2.5mm. The reason this is only 1 MOPP is due to the fact that the patient being expose to external mains voltage should already be rare.

For BF parts, I think this is overkill, since rogue mains voltage such as a defective radio should rarely be found in the real world. It should be floating but say 500Vrms, 1 or 2mm should be enough, not 4mm.

For CF there is a stronger case to be argued as simple broken earth wire can source leakage currents of 0.5mA or more, which can be dangerous if applied directly to the heart. So, 1.5kV/4mm makes sense there.


Hi,

I have a similar situation and I thought this thread is a good place to ask my questions.

I'm working on a wireless, battery-powered (3V, non-rechargeable) medical device to be used intraoperatively by a surgeon. The device has no possible electrical connection such as connectors to outside world. First, let me explain the physical aspects of the device then my questions will be followed:
The shape and dimensions of the device are similar to those of an electrical tooth brush. The enclosure is made by two parts: a plastic enclosure and a metallic head. The plastic enclosure contains a battery and a PCB. The metallic head (floating and identified as a F-Type applied part) contains a small sensor. The insulation diagram is attached.

1- As you see in the insulation diagram, per CL 8.5.2.1, at least 1 MOPP should be used between the applied part and other internal parts. I understand this requirement for devices powered by main and/or connected to earth. What I do not understand is how a current can flow from a patient (connected accidently to a main) to ground through a battery-powered device like this ? If this situation is harmful for any reason, from this perspective, a stainless still surgery knife or scissor used by a surgeon can be harmful too.

2- 1 MOPP means 1500V/4mm/2.5mm separation between the applied part and other internal parts. Because we have very limited space in the metallic head, a 4mm/2.5mm separation between wires/sensor and applied part is not possible. Using 1500V-rated wires in the small metallic head is not possible since these types of wires are too thick. The question is: can we use lower rated wires (say 300V) and ask the lab not to test the dielectric strength? This can be justified by the exclusion of CL 8.6, 8.7, 8.8, and 8.9 based on Clause 8.4.2 (for voltages below 60Vdc).

Thanks for any help!
 

Attachments

  • Insulation _Diagram_1.pdf
    69.1 KB · Views: 102
Last edited:
Top Bottom