Defib Protected Applied Parts: 4mm clearance/creepage?

[HenryC]

I hope this can be useful to others too:

According to 60601-1 applied parts are considered defibrillator protected if, upon testing with a 5KV defib-like pulse on the part, no transient above 1V appears on the enclosure and other accessible parts of the instrument. Obviously the instrument should also withstand this pulse, i.e. should work properly and safely after the pulse, to be considered defib protected.

BUT, 60601-1 also states in 8.9.1.15 that defib protected applied parts require >4mm clearance/creepage.

Here come the 2 questions:

1. >4mm is needed according to 60601-1, but between the applied part and what? which parts/elements of the front-end circuit should be separated by at least 4mm to achieve defib protection? what is the rational for this in the context of defib protection?

2. I am assuming that often a front-end (for example for an ECG) is designed with components such as GDTs, resistors, capacitors, TVS, etc. so that the defib transient is shunted very close to the input port where the electrode is connected. why 4mm then? and again where? only if physical separation is deliverately/by-design used as a MOPP/MOOP in the defib circuit?

Thanks in advance

[MedMartin]

There is a rationale for impulse test voltage in one of the appendices of 60601-1. The following is from Appendix A in an older version of UL60601-1: "When a defibrillation voltage is applied to the thorax of a PATIENT, via externally applied paddles (or defibrillation electrodes), the body tissue of the PATIENT in the vicinity of the paddles and between the paddles becomes a voltage dividing system.
The voltage distribution can be gauged roughly using three-dimensional field theory but is modified by local tissue conductivity which is far from uniform.
If the electrode of another item of MEDICAL ELECTRICAL EQUIPMENT is applied to the PATIENT, roughly within the compass of the defibrillator paddles, the voltage to which such an electrode is subjected depends on its position but will generally be less than the on-load defibrillation voltage.
Unfortunately it is not possible to say how much less as the electrode in question may be placed anywhere in this area, including immediately adjacent to one of the defibrillator paddles. In the absence of a relevant
Particular Standard, it must therefore be required that such an electrode and the EQUIPMENT to which it is connected will be able to withstand the full defibrillation voltage, and this must be the no-load voltage as one of the defibrillator paddles may not be making good contact with the PATIENT.
This amendment to the General Standard therefore specifies 5 kV as the appropriate value in the absence of a relevant Particular Standard."

It also clarifies the requirement for the 4 mm:
"From IEC 664, table II, a distance of 4 mm is adequate for pulses of 5 kV having a short duration of less than 10 ms, such voltages arising typically from the use of a defibrillator, with a reasonable safety margin.
The validity of this margin, which has been retained to ensure that the EQUIPMENT pass the defibrillator test, and not only remain safe afterwards but also function normally, comes from three factors:
– The values in IEC 664 already have an inherent safety margin;
– in practice the applied voltage even on the PATIENT’S thorax will be much less than the assumed open-circuit voltage of 5 kV, as the defibrillator will be on load, and it has an appreciable internal impedance and a series inductor which adds to this impedance;
– IEC 664 allows for heavily contaminated surfaces, whereas in MEDICAL ELECTRICAL EQUIPMENT internal surfaces are clean."

And for your question 2 please consider also that there is a common mode test, where the defibrillator protection devices typically do not come into action but the voltage is on the isolation distance to primary.
Does this help?

Best regards,
Martin

[HenryC]

Thank you Martin. I really appreciate your message and that you went out of your way to find the exact text from the standard.

The rational behind the need for defib protection of up to 5Kv (e.g. gas discharge tubes or semiconductor alternatives like MAX30034) as in 60601-1 was clear to me. No question about that.

The question was more about the why and how of the 4mm clearance. I assume that 60601-1 wants to minimise the risk of those 5Kv coupling to a nearby metal part before a protection device at the input can shunt the high voltage and bring it down to a low risk value. Then common sense seems to dictate that the front-end should be designed with the incoming wires from the electrodes and their PCB traces, up to the first safety energy-rated resistor, remaining at 4 mm from any metal part/or nearby traces.

Is my interpretation consistent with the standard? 4mm between any point that would be at 5Kv during defib test and any other trace or metal part of the enclosure?

As for the second part of the question, you say "consider also that there is a common mode test, where the defibrillator protection devices typically do not come into action".

But wouldn't you always expect to find a protection from each electrode to ground to protect the Instrumentation Amplifier IC? Otherwise the Applied Part would not be defib protected. And then, if each electrode is protected with respect to ground, a common mode test would trigger the protection on all electrodes simultaneously (only the differential protections will not be activated). Is this what you had in mind?

Again thank you for taking the time to answer.

[MedMartin]

Hi HenryC,

I agree with your interpretation and that is also the way we handle it in our designs. A distance of 4 mm (or more) from patient inputs to other parts
before the defibrillation protection resistors.

And the 4 mm is also required to the primary side of the circuit, e.g. the mains power supply or a USB connection. This is for the test with all patient leads connected together during common mode test. In this test the defibrillator protection does not draw current but the voltage is applied across the isolation barrier. The secondary ground from e.g. the ECG amplifier is floating together with the patient leads. Is this concept clear?

Best regards,
Martin

[HenryC]

Martin,

Yes, it is clear that during common mode ESD or defib test, the electrodes (e.g. ECG) will be connected together and driven by the pulse generator.

In this configuration several Kilovolts will fall on the input protection Resistor, which is in series with a MAXIM300034 device to ground, which is also in series with the stray capacitance between the (isolated) secondary and primary circuits.

Here I think the defib protection (MAX300034 between electrode and floating ground) should draw current, correct? (whereas in your message you were probably thinking of a GDT placed differentially between the input electrodes, in which case the common mode test will not trigger the GDT, and therefore it will not draw current during the common mode test).

Did I understand your comments correctly?

In any case, you note that 4mm clearance is needed also for USB lines and mains wires. I suppose 4mm will be needed for any trace/wire from its entry point into the enclosure to the first element of protection that drops the possible KVolts voltage to low values (the protection resistor or ESD protection semiconductor).

Henry

[HenryC]

Just a quick additional comment to correct myself:

I said the defib protection between electrode and ground would draw current during a common-mode test. But only transiently I should have added, to charge the stray capacitance between the primary-seconday isolation. After that all the common-mode test (kilovolts) will fall on the primary-secondary isolation. And given that the stray CAP between primary and secondary circuits is small, this transient flow might not be very relevant for safety purposes.

[MedMartin]

Hi HenryC,

yes, I think we have the same understanding.

Best Regards,
Martin

[siggenejst]

Hi HenryC and MedMartin,

I followed your discussion with great interest, and I have an additional question.

Is the 4 mm air clearance also required within one applied part with several patient connections? ECG will have 3-10 patient connections within the same applied part, so do we need 4 mm clearance between all pins in the connector, or just between all pins and any other conductive part.

Thanks!