Separation of battery powered FB type applied part

[xavier.beaule]

Dear community,

I would like to start off by saying that I really appreciate everything that is done here. This forum has been an incredible source of information.

I have a battery-powered medical system with a BF-type applied part. According to 8.5.2.1 F-Type Applied parts, a F-type applied part should be separated from all other parts.

I have noticed that in some cases (ref: Separation of F-type applied part and remaining parts, 1 MOPP and 1500V/4mm/2.5mm separation), it was suggested that only separation from other applied parts/accessible parts is necessary:

F-Type insulation is between ECG circuit and all accessible parts. The accessible parts are more than just the USB/SD card, it is all plastic parts as well, such as displays, on/off switches, metal screw and the like.

The 1MOPP @ 250V (1500V/4/2.5) is related to the Type F classification (BF or CF) and only applies between the applied parts and earth or accessible parts.

In other cases (ref: Isolation of secondary grounded circuits), it was suggested to both isolate from the secondary circuit and other applied parts/accessible parts:

But you have to isolate your BF applied part from (cl. 8.5.2.1):
- the secondary circuit by 2 MOPP based upon the working voltage,
- all other parts, including the PE by 1 MOPP based on the maximum mains voltage (usually this is more than the 2 MOPP vs. secondary).

I don't seem to understand in which case secondary circuit isolation is needed and in which case it is not necessary. Can anyone help?

 

[Peter Selvey]

From the point of view of the standard, isolation to "all other parts" refers to parts which are accessible to the operator.

Thus, the question of the secondary (battery circuit) needs to be isolated depends on the isolation plan. In principle there is

Accessible parts < Isolation barrier #1 > Secondary < Isolation barrier #2 > Applied part

You can choose to make #1 have 1MOPP (1.5kV./4mm), in which case #2 does not matter. Or you could beef up #2 to 1MOPP and then #1 doesn't matter. Or in complicated cases, it could be that #1 + #2 in total is 1MOPP. Up to the designer.

In practice, #1 can have battery contacts, switches, touch screens, case screws, on/off switches, USB connectors etc etc which makes it difficult to guarantee 1MOPP in all these parts, not just for the type test, but in regular production and considering future design changes.

So having a small, controllable isolation barrier at #2 can be more cost effective and simple in the long run, even though it has some up front parts cost (dc/dc converter, optocouplers).

 

[xavier.beaule]

Thank you very much for the reply. It is highly appreciated.

According to our supplier, no access is possible to the conductive contacts of our battery connectors (normalized test finger IEC-61032).

If I understand correctly, it means that no isolation is required between the battery connectors and the applied part. If that is the case, is it required to perform a dielectric strength test between the conductive contacts of the battery connectors and the applied part?

I assume further risk analysis could justify the addition of a isolation barrier or protective measures between the secondary circuit and the applied part?

 

[Peter Selvey]

If according to the standard an MOP is required, then it needs to be decided where that is. For example, 1MOPP @250V = 1500Vrms, 4mm creepage, 2.5mm clearance. What parts are going to provide this barrier?

OK, test finger cannot touch the pins. Even so, where is 1MOPP?

Designer could say, I want 1MOPP to be in the gap between the test finger and the battery pins. If so, then test the full 1500V/4mm/2.5mm in the gap. But then, gap is only 0.5mm. So this gap cannot pass 1MOPP. Then, designer still needs to find 1MOPP somewhere else.