IEC 60601 Applied Part - Probes intended to be covered by sheaths

#1
Hello,

Can anyone help me clarify the following?

We are developing a medical electrical, portable, handheld, probe device which is invasive in a natural bodily orifice of patients.

The probe is powered by a rechargeable battery and is IP54 rated.

The probe is intended to be disinfected after each use.

The probe is only intended for normal use in conjunction with a thin, single-use, non-conductive, biocompatible sheath (a separate medical device).

The "insertable portion" of the unsheathed probe is clad in silicone rubber, insulating any electrical components.

In normal use, the probe is always covered with the sheath and is never intended for direct contact with patient tissue.

The probability of occurrence of sheath rupture during use is very low.

BS EN 60601-1:2006+A12:2014, sub-clause 3.8 defines APPLIED PART as:

"part of ME EQUIPMENT that in NORMAL USE necessarily comes into physical contact with the PATIENT for ME EQUIPMENT or an ME SYSTEM to perform its function".
Would I be correct in stating in the device technical documentation that the probe device does not have an applied part, and as such, requirements for applied parts do not apply?

If I'm not correct, what would the rationale be?

Thanks in advance.
 
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Peter Selvey

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#2
It's a bit of an annoying scenario as the electronics in the probe is unlikely to be dangerous (battery operated device, assume 12V or less). However, if you imagined that the probe contained parts at 1kV (just to be extreme), then it's clear that insulation is needed, and it makes sense to have this in the probe rather than the sheath.

Anyway, IEC 60601-1 considers even 12V to be live, so the system of the probe + sheath needs provide 2 MOPP insulation from voltages inside the probe. From memory, in this case it would at least 500Vdc dielectric strength, maybe cr/cl, maybe sealant test, waterproof tests etc.

In a system made by two seperate medical devices (probe + sheath), system level specifications such as 500Vdc should be split based on one of the following:
  • "as reasonable" (whichever device makes more engineering sense), or
  • "as agreed" (e.g. for two devices by the same manufacturer, by contract between different manufacturers), or
  • "by convention" (if such a historical convention exists)
For example, it makes sense that all biocompatibility and sterility requirements are covered by the sheath, and can be N/A for the probe.

For MOP requirements like 500Vdc, I think it makes sense for this to be covered by the probe, however it could be covered by the sheath if there is agreement or well established convention.

If it is the probe, then for the purpose of MOPP evaluation and testing, the sheath is ignored. Strictly speaking, this does not mean the probe is the "applied part", the sheath remains the applied part. Rather, it works via Clause 8.1a, which says that any insulation that is not tested for MOP is ignored. So if the sheath is not designed/tested for 500Vdc, it gets ignored and all the tests apply to the probe.

Because the sheath is ignored it can be useful to think of the probe as the "effective applied part" for the purpose of MOP testing.

Other cases might also apply in a similar way, for example, the probe temperature in normal and single fault condition technically apply to the system of the probe + sheath. Using the "as reasonable" option above, it makes sense to allocate all requirements to the probe. This means, designing and verifying the probe can never exceed 41°C (or as appropriate depending on the application). This makes more sense then including the sheath which may have negligible or variable impact on the true patient temperature. Since the sheath is ignored, the probe again becomes the "effective applied part" for temperature related specifications.
 
#3
Hello Peter,

Thanks for taking the time to reply – I really appreciate your clear breakdown.

Both the sheath and probe device will be manufactured by ourselves as separate medical devices, and I agree, as you say, that it makes sense for the bulk of the 60601-1 testing to be focussed on the probe rather than on the sheath.

My takeaway from your reply is that:
  • MOPP evaluation/testing effectively still applies, as even though by the letter of the standard the probe device doesn't make direct patient contact with the patient, the intended use of the probe relies on the sheath always being used, so it is therefore part of the same "system".
  • To ensure that MOPP is covered (as something in the "system" is contacting the patient during normal use), instead of the sheath being tested, the probe is tested, for practical purposes (especially considering the sheath may not be completely replied on as a means of insulation).
  • The insertable portion of the probe is the "effective" applied part.
Thanks also for the clause 8.1a reference – did you mean clause 8.8.1?:
Only insulation that is relied upon as a MEANS OF PROTECTION, including REINFORCED INSULATION, shall be subject to testing.
Regards,
Jan
 

Peter Selvey

Leader
Super Moderator
#4
The reference to 8.1a is "short circuit of any or all insulation that does not comply with the requirements of 8.8 [or 8.9]".

"does not comply with" can also be read as "has not been tested for".

Consider a device that has a mains supply, mains transformer, secondary circuit that supplies a touch screen. The touch screen has a clear 0.2mm plastic sheet between the operator and touch screen electronics. Obviously it makes sense to test the transformer for 2MOP @ 240V rather than the plastic sheet, especially as the makers of the touch screen never designed it for mains insulation.

Clause 8.1a is the "legalistic" mechanism to make this work. By shorting any insulation that has not been tested (often referred to as "functional insulation") it effectively makes the MOP in contact with the operator or patient. This is bread and butter in electrical safety, nothing special, it's needed to avoid touch screens getting destroyed with 4000Vac. It's not bending the standard, it is the standard.

The curve ball in your case is the system is made of two devices. That invokes the boundary kind of thinking e.g. my device doesn't directly contact the patient so I don't need to worry about patient issues. That approach is fundamentally wrong. The reason is any part of a system can be the source or harm or fails to reasonably prevent harm.

The correct approach is to consider the system requirements and then allocate them to the parts of the system where it makes sense.

So, in the system of probe+sheath, if the sheath has not been tested to 8.8, it is shorted according to 8.1a. Then the probe casing then becomes the part that contacts the patient for the purpose of Clause 8.

I should return to the original point which is that the probe probably doesn't contain any dangerous voltages, which is is the real issue. IEC 60601-1 is overkill in this case. The boundary approach might be your attempt to get some common sense into the situation, but really it's just that IEC 60601-1 has gone a bit crazy on 5V and 12V parts. There are definitely scenarios where 5V applied to a patient via an electrode or catheter could be an issue, but that's pretty rare. Anyway, take care not to use the boundary thinking in other cases, it's fundamentally wrong.
 
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