Leakage current measurement without isolating transformer

[Benjamin Weber]

Dear leakage current experts ;-)

if the use of an isolating transformer is not possible (e.g. for devices with high power input) 60601-1 says, the device shall be supplied directly from mains and the reference earth of the measuring circuit shall be connected to the PE of the mains supply, see cl. 8.7.4.3 d) 2).

This is the common reference point, which is usuallly connected to one ot the putput poles of the isolating transformer "T1". In case of the direct supply from the mains, this connection must be disconnected and conneted to the supply PE - otherwise you have 50% chance of short-circuit between supply mains phase condutcor and PE - boom, circuit breaker operates. Don't ask, maybe this happened



So far, everything is fine, I get it.

Here's my problem:
We need to apply an additional external voltage in some cases, e.g. at the SIP/SOP during touch current measurement or at an F-type applied part. This is usually done via a second isolating transformer "T2". According to the figures 14, 16, 17, 18 one output pole of "T2" is connected to the commom reference earth. But in contrast to "T1" this is done after the commutator switch S9 to reverse the polarity. At "T1" this is a fixed connection before the corresponding commutator swtich S5!.

If the device is supplied directly (i.e. without "T1") and the common reference earth is connected to the mains PE: Do I need to or can I omit "T2" for the external voltage?

If yes: Depending on the position of S9 I will again create a short circuit between mains phase and PE.
If no: "T2" will prevent any current flowing to earth (here mains PE), hence I will never measure any leakage current caused by the external voltage.

Am I missing something here or am I understanding something completely wrong?!?

I would be happy about any input!

Best regards,
Benjamin

 

[Peter Selvey]

It is theoretically possible that the polarity of the mains supply can affect the result for the "mains on X" test, but this is extremely rare in practice.

If the design is like most medical systems, whenever there is a test with "mains on X" it is usually testing a system where "X" is separated from mains by an intermediate circuit which itself is earthed, and/or the relative capacitance is such that the polarity of the mains supply is pretty much irrelevant.

In practice there are small differences but this is far more likely to be due to CMRR of the measurement system. In other words, trying to measure less than a mV differential mode on a whopping 264V common mode voltage and expecting it to be all OK. So for a mains on applied part you might measure 17µA one mains polarity and 13µA on the other, but both results are probably a bit of junk, only roughly related to the real leakage flowing into applied part. But as long as it's well below the limit, no need to worry.

Upshot is: if you are a confident engineer, and know how the insulation system is implemented, then you can take some common sense short cuts and avoid loud explosions. This is actually allowed in the standard, somewhere in Clause 4 or 5 it gives some flexibility when deciding where and how to test as justified. It's also common practice, for example you don't measure enclosure leakage from every part, or measure the temperature of every part, there's always a bit of looking at the design and deciding where to test and what conditions to use. The problem with leakage currents is the CB scheme which has a pre-filled table that suggests that certain combinations HAVE TO BE TESTED or you will get a rejection from your reviewer or certification body. Which is where the confidence part comes in

 

[Benjamin Weber]

Hi @Peter Selvey ,
again a very competent answer - as I expected from you ;-)

Actually I also think that an analysis of the isolation system should be performed before simply testing all possible (but not necessary) combinations. And my guess is, that for devices where an isolation transformer cannot be used the relevant combinations are quite straightforward and that the polarity of the external voltage is irrelevant. My question was more a general one, just to be prepared.

By the way: IEC 60990 is always a good ressource when it come to leakage current basics! Unfortunately it does not say anything about tests with external voltages.