Additional Isolation Required? Doppler Probe Desktop Device

[Fred1977]

Hi All,
I'm working on a medical device and am a bit lost at sea as to whether isolation is required or not. The device consists of a small desktop unit with TFT display, buttons and USB socket for PC connect. The desktop unit is battery powered (4x1.5V), or can get 5V from USB. A doppler ultrasound probe (huntleigh) is plugged into the desktop unit via a cable. Typically the operator holds the probe against patients neck for about 30 seconds at a time to capture a signal.

The device presently contains no isolation. I gather the probe can be classified as a type B applied part, and in 60601-1 2006 I read this:

8.5.2.2 Type B applied parts:
...Type B part that is NOT protectively earthed shall be seperated by one MOPP from metal accessible parts that are nont protecticely earthed, unless:
- Metal accesible part is physically contiguous with applied part, or
- the risk that the metal accesible part will make contact with a source of voltage or leakage current above permitted limits is acceptably low.

Does the PC connection mean quite high leakage current is possible, and thus neither exclusion paragraph applies and I need that one MOPP isolation? From some reading I gather the USB port already offers one MOPP, in which case I would need no further isolation... but I am not certain that the 'PC USB port already offers one MOPP' statement is true or not.

Secondly, if I read further into the 'components and wiring' section, I find this:
8.10.4.1 Limitation of operating voltages:
Cord-connected hand held control devices of ME equipment and their associated cords shall contain only conductors operating at voltages =< 60Vdc, in circuits isolated from MAINS part by two MOPPs.

In the case of the above, does this mean I need two MOPPS? I would think that the doppler probe is not a 'hand held control', or is it? If this paragraph does apply to me, it just leads to more questions. The doppler probe itself allegedly includes 1 MOPP of seperation by virtue of its plastic casing design (I am awaiting confirmation from Huntleigh), in which case if the probe has a MOPP, and the PC USB port already offers a MOPP, I might be safe.. however the above paragraph says the conductors within the cord are also subjkect to the 2 MOPPs isolation, which means I need to isolate the doppler inside my desktop case housing.

Any guidance will be most appreciated, I fail to see any risk in the product's current un-isolated configuration, and would hate to have to add layers of expensive isolation if I don't need to.

Many thanks!

 

[Peter Selvey]

Generally for ultrasound probes the plastic cover (acoustic coupling lens with waterproofing) is considered the isolating part which provides the Type BF isolation (1500Vrms) and also protects against the internal voltages. Type BF is not required but most manufacturers use Type BF and it makes sense because you also have to isolate against the internal transducer drive voltages.

These drive voltages can be up to 200Vp, although the rms value is low at around 5Vrms. Some might argue that presence of 200V means that 4000Vrms isolation but this is not correct due to the low rms voltage. It's a bit complicated, but the 1500Vrms for the Type BF is far in excess of what is reasonable for the situation given the low rms.

The Type BF isolation also provides additional isolation for use with a PC. The PC provides 2MOOP but only 1 MOPP. With the probe insulation 2MOPP is provided to the patient.

The clause mentioned (8.10.4.1) only requires 2 MOP (not 2 MOPP). It practice this could means 2 MOOP or 2 MOPP depending on who uses the control (operator or patient). In your case, it would be 2 MOOP, which the PC already provides, so no further action required.

I think it all fits with common sense and the standard.

 

[Fred1977]

Peter,
Thank you for a crystal clear reply and one which I am very pleased to hear.
Regards,
Fred

 

[Roland chung]

Generally for ultrasound probes the plastic cover (acoustic coupling lens with waterproofing) is considered the isolating part which provides the Type BF isolation (1500Vrms)

Thermal cycling test specified in clause 8.9.3 applies to the ultrasound probe. However, I am a little bit confused about the sub-clause 8.9.3.2 and 8.9.3.3. Which one do you think we should carry out?

8.9.3.2 Insulating compound forming solid insulation between conductive parts

8.9.3.3 Insulating compound forming a cemented joint with other insulating parts

Thank you,
Roland

 

[Peter Selvey]

Yes, it's confusing. For a previous situation (also an ultrasound) we concluded 8.9.3.3 was appropriate, but it was not clear. Perhaps someone with an IEC 60950 background might be able to help (the tests seem to be borrowed from 950).

Of interest, we found that multiplying the voltages by 1.6 was too much for a typical medical ultrasound, especially for endoscopic applications. Already in the 1500Vrms there is a number of worst case assumptions which don't exist in the real world, it's overkill but it's possible to comply. However, with the 1.6 factor the voltage can be too much.

 

[Roland chung]

Yes, the thermal cycling test in IEC 60601-1 is derived from IEC 60950-1. Both standards treat the cemented joints in the same way. But for insulating compound used as solid insulation, 950 does not require the electric strength test voltage to be multiplied by 1.6.

Why do you think subclause 8.9.3.3 is appropriate for the ultrasound probes?

 

[Peter Selvey]

Again, not really sure on this one ... it seems like 8.9.3.2 is for cases where an for example epoxy is used to fill a gap with the test to make sure there are no bubbles etc, whereas 8.9.3.3 is for cases where an adhesive is used in a joint between two pieces of solid insulation, to make sure the seal is effective in the long term.

I could imagine the case where an epoxy is used on the surface of a PCB where the spacings are way too small (less than clearance limits). In that case both clauses could apply: 8.9.3.2 to make sure that the epoxy has no bubbles, and 8.9.3.3 to make sure the seal on the surface of the PCB is OK.

But, if the original spacing is OK for clearance, and only fails creepage, then only 8.9.3.3 is required (bubbles are not a problem, but the seal needs to be checked).

 

[raymond02302]

The ultrasound probe generally consist of different materials, plastic cover/housing and ultrasound transmitting/receiving window. They are cementeded by chemical reagent such as epoxy and other kinds of reagent. The gap filled with such reagent makes a cemented joint. This is why clause 8.9.3.3 is applicable. The thermal cycling test is to evaluate reliability of this cemented joint.