How to test 8.7.4.7 d) ? What "not protectively earthed accessible part" means?

eldercosta

Starting to get Involved
#1
First off, my device's Applied Part type is BF.

The question about the meaning of "not protectively earthed accessible part" is in the context of this clause, i.e. 8.7.4.7 d) .

My impression is this clause applies to accessible parts that may be in contact with the patient (per the rationale). However, I am not 100% sure about that, the text of the clause and rationale, to me, is in a gray area.

My device does have has a metallic cover that is fixed with screws to the back of the device (the cover is to protect connectors so its area is around 20% of the back. Both, cover and back are metallic and painted (powder coating). The cover is not protectively earthed, unlike the back part, which meets the protectively earthed requirements. Despite not being protectively earthed, the cover may be earthed by the screws. Depending on how the clause is interpreted, the test laboratory may apply V2 of Figure 18 to the cover that may be incidentally earthed. Or I am misinterpreting the clause and figure?

Thank you in advance for your comments.
 
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Benjamin Weber

Trusted Information Resource
#2
Clause 8.7.4.7 d) is about measuring the patient leakge current, when an external voltage is present at a metal accessible part that is not protectively earthed (this is NOT the applied part).

If you have accessible parts which are not applied parts per definition but could be subject to the same requirements per cl. 4.6, this applies also to those accessblie parts.

If your metal cover is not intentionally protectively earthed I assume the tester would either
  1. apply the external voltage (max. mains) to the metal cover during the test per fig. 18
    OR
  2. consider the cover as PE connected and then test the PE impedance (with removing the powder coating at the test location) and not apply the external voltage durign the test per fig. 18.
In the first case the tester would very likely have a problem with his test equipment, due a (more or less) short circuit of the test voltage V2 and earth via the metal cover. Actually I think he would have seen this coming and ask you wether this part should be PE connected or not.

If you are lucky, the PE impedance measured at the metal cover is OK. But in this case you should really consider this part as intentionally PE connected. You can do this by taking care of a good conductive connection via the metal screws or by an additional dedicated PE connection (a cable).
If you don't want to have the metal cover to be PE connected, then you have to seperate is clearly from PE, e.g. by using plastic screws.
 

Peter Selvey

Leader
Super Moderator
#3
This is interesting ... if you look in IEC 60601-1 ed 2, Clause 19.2 this test is generally not applicable. It is excluded from F-type parts, and also excludes Type B parts "unless inspection of the circuits and physical arrangements shows that a SAFETY HAZARD exists".

In practice this was rarely applied, as Benjamin indicated it's a potentially destructive test.

In Ed 3, if you read 8.7.4.7 d) it looks applicable to both B and BF. However, 8.5.2.2 indicates an exception for contiguous metal and also risk based option.

It looks like mistake in that that there is no tie between 8.7.4.7d and 8.5.2.2. For example the last sentence should read:

This test need not be conducted:
- if it can be demonstrated that there is adequate separation of the parts involved, or
- if the the exceptions in 8.5.2.2 apply.
 

eldercosta

Starting to get Involved
#4
Benjamin, Peter, first off, thank you for your comments.

@Benjamin
About the cover, getting it protectively earthed would require modifying a PC (medical/industrial - the cover just protects the connectors on the back of the PC), which I would rather not if I can.

@Peter
The cover is not close to the patient, quite the opposite. Per 8.5.2.2, there is a MOPP between this cover and the AP (actually there are two, as creepage/air clearance and insulation components meet the 2xMOPP for 240V mains). So my design meets the first part of the subclause.

After I posted, I noticed the third paragraph (This test need not be conducted if it can be demonstrated that there is adequate separation of the parts involved.); it seems to me that, based on this statement, this test does not need to be applied in my case.

About the last paragraph of your post, the amendment 2 of IEC 60601-1 3.0 says "Replace, in the existing third paragraph of list item d), "parts involved." with "parts involved or if the RISK is acceptably low according to 8.5.2.2."
 
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