Is any part of earthed (grounded) enclosure required to meet, say, 25A/0.1ohm?

[Roland chung]

Hello again,

The standard requires any part that is PROTECTIVELY EARTHED shall meet the low impedance and high current-carrying capability (e.g. 25A/ 0.1ohm). Does this mean that any part of the earthed enclosure shall fulfill such requirement?

I would think the impedance test does not apply to the part without mains voltage around, even that part is earthed.

Any comments is appreciated.

Roland

 

[Peter Selvey]

Yes, you are correct. Only parts that can become live if BASIC insulation fails (1 MOP) require the 0.1ohm test. This can be derived from the definition of protectively earthed (2.6.7 in 2nd ed, 3.96 in the 3rd). In general, it is important to distinguish between parts which are "protectively earthed" and just "earthed".

But there is a problem.

Biomedical technicians in hospitals are often required to perform safety tests on a periodic basis, which usually include the 0.1ohm/25A test.

The technician cannot know which parts are just earthed and which parts are protectively earthed.

Therefore, it is advisable to keep "solid" external metal parts <0.1ohm even if not actually required in IEC 60601-1.

 

[Marcelo]

Well, but if the part is accessible and not protectively earthed, remember that you still need 2 MOPs.

The technician cannot know which parts are just earthed and which parts are protectively earthed.

Unless you instruct them, which might be a good thing.

 

[Roland chung]

Peter, you are so experienced.

We are suffering from the situation you mentioned. The hospital technician told us that the product failed to meet the 25 A/0.1 ohm test. But the metal part being measured is far away the MAINS. Although all accessible metal parts are earthed, it is not necessary to have them protectively earthed.

The technician also argued that the internal conductors may break free at their joint and are therefore capable of touching any parts. I think the fixing of wiring has been considered in clause 8.10.

Any further comments?

 

[Roland chung]

Well, but if the part is accessible and not protectively earthed, remember that you still need 2 MOPs.

I think it is not true. The barrier between pri and sec is double insulation already. There is no hazardous voltage around such earthed accessible part (SELV only), why 2 MOP is needed?

 

[Peter Selvey]

It would not be the first time for this problem to happen (it also happens in test labs to, lab technicians burning out the 0V line of 24AWG signal circuit in parts of the equipment that don't need protective earthing).

In this case there is no easy solution. Although IEC 60601-1 is clear on this point, the biomed may be working to internal procedures that simply say to test any external metal part.

As Marcelo pointed out, if earthed parts should not be tested, it really should be in the instruction (service) manual. Formalizing this in the manual will help to take responsibility away from the biomed.

I think the comment about 2 MOP is correct. Parts which are only 1 MOP from mains parts should be earthed with 0.1ohm. Parts which are 2 MOP (or more) don't need to be earthed.

The confusing point may be that some parts are so well removed from mains that we don't think in terms of MOP anymore. But the 2 MOP are there, just back in the power supply.

 

[Roland chung]

I think the comment about 2 MOP is correct. Parts which are only 1 MOP from mains parts should be earthed with 0.1ohm. Parts which are 2 MOP (or more) don't need to be earthed.

The part that just earthed is connected to the part that protectively earthed. If we declare the earthed part is 2 MOP, I am sure the technician will conduct the Hi-Pot test with the value for 2 MOP. The test will definitely fail. And the technician will say the Hi-Pot test failed, not the impedance test this time.

 

[Peter Selvey]

Under 2nd ed this is covered by situation 17 g 3).

But a biomed should not be trying to re-test 601 in the hospital.

Any the 4kV test for 2 MOP should NEVER EVER EVER (EVER) (EVER) be done on a fully assembled Class I device. Even in a type test laboratory (yet everyone still does ... ).

 

[Roland chung]

From my side, I totally understand.

But the logic sounds strange. If you require parts which are 1 MOP from mains parts should be earthed with 0.1ohm, why do you not allow to conduct the 2 MOP Hi-Pot test for parts can not meet the 0.1ohm. The impedance test is a failure, but the Hi-Pot test is also a failure.

I mean just let's try to analyze if there is a real electric shock risk.

 

[Peter Selvey]

Dielectric strength is required only for solid insulation, not for the "equipment". The test must done carefully to ensure that actual safety insulation (planned point of insulation) is stressed, while at the same time other parts are not stressed.

For double/reinforced this generally requires dis-assembly of the equipment to access the actual point of insulation. For example, directly testing the transformer, or removing the power supply PCB from the frame so that only pri-sec are stressed, and not pri-earth. Any attempt to test an assembled device, especially Class I equipment, usually results in the voltage stressing the wrong areas, and can also result in the real insulation not being tested.

This requirement is specified in the standard (both editions). The 3rd edition makes it even clearer under 8.8.3 c).