Issues with leakage current testing for a CLASS II device since no PE and FE is present

muhammad sadiq

Starting to get Involved
#1
HELLO,
I am working on a APD dialysis ME , which is powered by 2 prong class II medical grade power supply that provides 24 volts at the output end but no earth

so I was going through all the leakage testing that the 60601-1 tells us to do , touch leakage , patient leakage etc
but since 60601-1-11 states that the CLASS II device for potable home use medical device cannot have a protective earth and functional earth in the system, my question is how these leakage test are performed in practical in the absence for earth?

The confusion is that it seems like leakage test are mostly taken with respect to ground eg for patient leakages its current passing through applied part to the earth, but since we don't have a PE or FE, how do I check practically this leakage current

1) DO I provide earth from some other means. (our medical grade power supply is only 2 probe and is class 2 so no earth) OR
2) I make the connection from applied part to the neutral of the mains ??
3) i make the connection from applied part to -ve of DC supply?

a figure is attached for visuals
 

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lauxt

Starting to get Involved
#3
In the original diagram from IEC60601-1, you can see that the point defined as "earth" (with respect to the test) is the neutral connection after the transformer T1. In a building wiring system, the neutral is tied to earth at the fuse-panel/breaker-box. So there is always a leakage path that exists from ME's applied part, through measurement network, through earth, back through neutral, back through the parasitic capacitance of the transformer in the medical grade power supply, back to the applied part.

The worst case leakage current is basically when the ME is plugged into mains really close to the buildings panel, then the earth to neutral path is short. So, for the IEC60601-1 leakage current test setup..connect the MD between output/applied part and the neutral of the mains power input after T1.
 

muhammad sadiq

Starting to get Involved
#4
The worst case leakage current is basically when the ME is plugged into mains really close to the buildings panel, then the earth to neutral path is short. So, for the IEC60601-1 leakage current test setup..connect the MD between output/applied part and the neutral of the mains power input after T1.
This T1 transformer, does this transformer exist in the tester ?? or is it a isolation transformer that we have before the ME machine is connected so that we can give it 110% rated voltage?

8.7.4.2 * Measuring supply circuits ME EQUIPMENT specified for connection to a SUPPLY MAINS is connected to an appropriate power source. For single-phase ME EQUIPMENT, the polarity of the supply is reversible and tests are conducted at both polarities. INTERNALLY POWERED ME EQUIPMENT is tested without any connection to a measuring supply circuit. NOTE Figure F.1 to Figure F.5 (inclusive) show some suitable arrangements but do not cover all possibilities, for example, delta-connected 3-phase supplies.


mains 230V ===> Isolation transformer ===> our ME device

so I need to connect our applied part of the ME device to this isolation transformer neutral ??
 

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Benjamin Weber

Trusted Information Resource
#5
T1 is an an adjustable isolation transformer, that
  1. separates the supply circuit for the test from the mains supply and
  2. allows to give the 110% supply voltage.
We actually use two separate devices for this: A programmable AC source with sufficient power rating and an isolation transformer between the AC source and the equipment under test. Additionally we use a multimeter to monitor the actual supply voltage at the equipment under test, which is usually a little bit different from the AC source output due the isolation transformer in between.

Your figure is missing the reference ground which is connected to the neutral line of the isolation transformer output. Even though you don't have PE or FE, you might need this reference ground, e.g. for earthing of patient connections (see fig. 18 of 60601-1) or the metal plate under an isolating enclosure (S15 in the figs. 15 and 16 of IEC 60601) or simply the enclosure to earth during touch currents measurements (see fig. 14 od IEC 60601-1).
And you also have to test wirh reverce polarity of the supply voltage. The IEC 60601-1 figures have the switch S5 for this, but you actually can simply turn the plug of the power supply by 180°.
 
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