ECG lead leakage currents - How to specify ECG leads during electrical safety testing

[Michael Ayers]

I was wondering if anyone can help regarding how to specify ECG leads during electrical safety testing.

If I have an ECG device with 3 leads (RA, LA and LL), should it matter how these are specified on a 60601 safety tester? For example, provided that I am defining all leads as CF, should it matter which inputs (RA, RL, LA, LL etc...) I am plugging them into on the tester?

I am finding different patient auxiliary current measurements depending upon how they are being specified, but cannot work out why this should be the case.

Many thanks,

 

[Peter Selvey]

There might be a small amount of patient auxiliary current coming from the "right leg drive", this is kind of like noise cancellation.

For 3 lead systems, this can be set up to be the non-used lead. For example, if the patient monitor is displaying "Lead I", this is actually the voltage between Lead electrode LA and RA, and LL is then used for noise cancellation. So you might measure a small amount of current from LL.

If you switch to Lead II on the display, this is LL-RA which means LA has the noise cancelling function.

Another possibility is the use of higher frequency currents for detecting "leads off" condition. If your safety tester has the function to turn off the 1kHz filter, turn it off and see if there are higher currents indicated.

Anyway, I would discuss with the designer or manufacturer to find the source of the auxiliary currents if you want to determine the worst case.

The mains sourced leakage (capacitive leakage from mains parts) should be the same for all leads (and also all leads together). They are all on the same circuit so there's no difference.

For the mains on applied part test, in theory it is the same as for other mains related currents. But the position of the cable can change the result. So you might be getting different results due to moving the cable around around without realising.

 

[Roland chung]

For the mains on applied part test, in theory it is the same as for other mains related currents. But the position of the cable can change the result. So you might be getting different results due to moving the cable around around without realising.

Could you please explain more about the effect of the cable? Thank you!

 

[RichardY]

There might be a small amount of patient auxiliary current coming from the "right leg drive", this is kind of like noise cancellation.

For 3 lead systems, this can be set up to be the non-used lead. For example, if the patient monitor is displaying "Lead I", this is actually the voltage between Lead electrode LA and RA, and LL is then used for noise cancellation. So you might measure a small amount of current from LL.

If you switch to Lead II on the display, this is LL-RA which means LA has the noise cancelling function.

Another possibility is the use of higher frequency currents for detecting "leads off" condition. If your safety tester has the function to turn off the 1kHz filter, turn it off and see if there are higher currents indicated.

Anyway, I would discuss with the designer or manufacturer to find the source of the auxiliary currents if you want to determine the worst case.

The mains sourced leakage (capacitive leakage from mains parts) should be the same for all leads (and also all leads together). They are all on the same circuit so there's no difference.

For the mains on applied part test, in theory it is the same as for other mains related currents. But the position of the cable can change the result. So you might be getting different results due to moving the cable around around without realising.

Hi Peter,

I have a question. For a battery powered device, should we measure the the auxiliary between the battery terminal and a real patient connection port in single fault condition? For example, a device powered by 2 AA batteries and also have a real patient connection port. In single fault condition, a return loop may form between the patient and one AA battery. As the patient resistance is 1K ohm, the patient auxiliary current will be 1.5V/1000Ohm =1.5mA (This exceeds the limit in 60601-1 Ed. 3.2)

Thanks and regards,

 

[Peter Selvey]

The path should be considered but in practice not tested because the battery terminals should be reasonably insulated from the patient's applied parts/patient circuit. It's not simply a matter of measuring from the terminals because they are there, there has to be a plausible scenario involved to get the voltage to the patient. It could happen, but it's not difficult to avoid by sensible design.

I know that 601-1 has some crazy limits for cr/cl like 3.4mm, 500V which could see some designs fail even though they are perfectly safe. The 500V/3.4mm are secondary circuits that are derived from mains circuits, the standard is being lazy for MOPP and just wants one limit for all. In that case it might require some bending of the standard to get common sense e.g. 1mm/50V is fine for a battery operated device, and less if the distance is fixed e.g. on a PCB.

 

[RichardY]

The path should be considered but in practice not tested because the battery terminals should be reasonably insulated from the patient's applied parts/patient circuit. It's not simply a matter of measuring from the terminals because they are there, there has to be a plausible scenario involved to get the voltage to the patient. It could happen, but it's not difficult to avoid by sensible design.

I know that 601-1 has some crazy limits for cr/cl like 3.4mm, 500V which could see some designs fail even though they are perfectly safe. The 500V/3.4mm are secondary circuits that are derived from mains circuits, the standard is being lazy for MOPP and just wants one limit for all. In that case it might require some bending of the standard to get common sense e.g. 1mm/50V is fine for a battery operated device, and less if the distance is fixed e.g. on a PCB.

Thank you Peter for your prompt reply. This is really helpful.

 

[PeterStaaf]

Hi all (RichardY and Peter S). I can see that we could have currents flowing between the three ECG electrodes. But does IEC 60601-1 require this auxilliary currents to be measured? Are the electrodes not regarded as ONE PATIENT CONNECTION= They are tied togheter during tests and in this case there would be no other patient connection. Test Not Applicable.
Please refer to IEC 60601-1 :2006 +A1:2013(Figure E.4, clause 8.7.4.8 and clause 8.7.4.9)

 

[Peter Selvey]

No, this is not written in the standard. For "leakage" currents, sometimes they are measured from a single function shorted together, either as specified in the standard or because it's obvious by design (common circuit for the function, leakage current results will be the same regardless of measuring from one electrode or all tied together).

"auxiliary" currents are different. They are specifically intended to measured from a single patient connection (i.e. a single ECG lead or electrode).

Figure E4 is showing an example for measurement from a single lead to all other leads (tied together). That is, if there are four leads A, B, C and D, then there are four tests for auxiliary currents:

From A to BCD
From B to ACD
From C to ACD
From D to ABD

Figure E4 is just showing the first combination as an example.