Body worn ECG monitor leakage tests

[ShellyH]

Hi all!
I'm not sure if I'm completely over thinking this, or if I'm missing something. We are self certifying to the 60601 standard. Our current product is a bluetooth, lithium ion battery operated ECG monitor. The user would wear snap electrodes which our product would attach to. There is a dongle that plugs into our recorder units for data recording. The charging station is USB powered, and the ECG monitor can NOT be used in the charging station. We also specify the charging station should be connected to a computer for charging. Therefore, there is no real connection to mains, especially while in use. I suppose I could do a touch current test while it's charging, even though I don't think I really have to. As for patient auxiliary, patient leakage, earth leakage, and dielectric withstand my understanding is that these tests are not applicable since there is no possible connection to mains while the product is in use. Am I correct or am I missing something?

One other thing I would like to double check, our monitor would fall under type BF applied part since it's "floating" from earth, correct?

Thanks in advance!

 

[Al_Z1]

Hm, design of your battery prevents any current leakage from it to patient? Or possible leakage current is too low to be dangerous, which is justified in RMF? Do not know about other parts of your question, but this are first questions, concerning excluding some tests from 60601, which came to me. HTH, I guess, more experienced forum members will help.

 

[Peter Selvey]

By definition, leakage currents are currents that can flow via earth. While the "mains supply" is of course the critical source for these leakages, this is not the only source and secondary voltages such as 5Vdc can also be a source for leakage currents (including touch currents). However, for secondary voltages that are double insulated from mains (2MOPP) and less than 60Vdc, and only touched by the operator, special rules apply and generally they are not seen as dangerous.

For a device that is "body worn" (fully on the patient), there are no "leakage" currents as there is no real possibility of the current flowing via earth. However, the potential for patient auxiliary currents still exists. This is particularly true for devices with electrodes such as an ECG. Even small dc currents flowing over long periods (hours) can result in tissue necrosis, so it's important to check this carefully. Typically, an ECG will have relatively high series resistance e.g. 100kΩ in each lead which prevents any significant currents flowing. The limit is 10µA in normal condition, which is usually easily met without the series resistance due to the use of high impedance op-amps at the front end. The limit in singe fault condition is 50µA, which usually does require some inspection and test. Examples of a single fault condition is short of an ESD diode, and in this case the high series resistance is often the only limiting factor (e.g. 3V/100k = 30µA < 50µA). Also the PCB traces after the high series resistance (on the patient side) should keep a good space away from all other parts in the circuit, otherwise, it may be a valid to short traces in fault condition. In actual case, a spacing of 0.5mm is safe but to avoid discussion (with test agencies) 1.7mm is recommended. It's also good to have a spacing that easy to inspect visually, so from that point I think 1.7mm is good as well.

Touch currents do apply in charging mode but they are usually taken care of by the charger manufacturer (e.g. PC or USB charger). In IEC 60601-1, this is taken care of under Clause 16 which covers systems. In this clause, devices that are non-medical should comply with their normal IEC standard (not IEC 60601-1) and there are special controls when use in or connecting to devices inside the patient environment. However, since charging is outside the patient environment (there is no patient) then pretty much the only rule that remains is making sure the charger meets the relevant standards for the charger. In the real world, it's unrealistic to expect the end user to verify this, however these devices are normally subject to their own regulations which the public is relying on in general and should also be able to continue to rely on in this instance, the charging mode being nothing special. My practical suggestion is to add warning to make sure it is charged from device from reputable manufacturers and purchased from reputable sources (places where you can expect they will follow the regulations). In other words, don't use a 5V USB charger from Paddy's Market in Sydney!

 

[ShellyH]

By definition, leakage currents are currents that can flow via earth. While the "mains supply" is of course the critical source for these leakages, this is not the only source and secondary voltages such as 5Vdc can also be a source for leakage currents (including touch currents). However, for secondary voltages that are double insulated from mains (2MOPP) and less than 60Vdc, and only touched by the operator, special rules apply and generally they are not seen as dangerous.

For a device that is "body worn" (fully on the patient), there are no "leakage" currents as there is no real possibility of the current flowing via earth. However, the potential for patient auxiliary currents still exists. This is particularly true for devices with electrodes such as an ECG. Even small dc currents flowing over long periods (hours) can result in tissue necrosis, so it's important to check this carefully. Typically, an ECG will have relatively high series resistance e.g. 100kΩ in each lead which prevents any significant currents flowing. The limit is 10µA in normal condition, which is usually easily met without the series resistance due to the use of high impedance op-amps at the front end. The limit in singe fault condition is 50µA, which usually does require some inspection and test. Examples of a single fault condition is short of an ESD diode, and in this case the high series resistance is often the only limiting factor (e.g. 3V/100k = 30µA < 50µA). Also the PCB traces after the high series resistance (on the patient side) should keep a good space away from all other parts in the circuit, otherwise, it may be a valid to short traces in fault condition. In actual case, a spacing of 0.5mm is safe but to avoid discussion (with test agencies) 1.7mm is recommended. It's also good to have a spacing that easy to inspect visually, so from that point I think 1.7mm is good as well.

Touch currents do apply in charging mode but they are usually taken care of by the charger manufacturer (e.g. PC or USB charger). In IEC 60601-1, this is taken care of under Clause 16 which covers systems. In this clause, devices that are non-medical should comply with their normal IEC standard (not IEC 60601-1) and there are special controls when use in or connecting to devices inside the patient environment. However, since charging is outside the patient environment (there is no patient) then pretty much the only rule that remains is making sure the charger meets the relevant standards for the charger. In the real world, it's unrealistic to expect the end user to verify this, however these devices are normally subject to their own regulations which the public is relying on in general and should also be able to continue to rely on in this instance, the charging mode being nothing special. My practical suggestion is to add warning to make sure it is charged from device from reputable manufacturers and purchased from reputable sources (places where you can expect they will follow the regulations). In other words, don't use a 5V USB charger from Paddy's Market in Sydney!

Thank you so much, this is all very helpful!