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Connecting system ground to mains earth

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FreeThinker:
Neutral conductors can ( and do! ) carry large currents and float well above Zero volts (Earth potential ).I started a thread about this and the problems it can cause. Truth is the mains supplyis dirty and getting dirtier by the week.The only answer is better filtering either at the incoming mains or at the polluting source, neither is likley to happen soon so if it is a problem you need to filter your equipment.Does your mains really float 90v to earth? seems very high to me.

saturation:
Thanks jahonen and alm, I'll withdraw my prior statements as it was posted in ignorance, and I've striked them out in case anyone reviews them in the future to avoid misleading them.

I've always made assumptions about power supplies used in medical and consumer grade as fairly close together in terms of leakage current.  Never paid attention to it since we were happy with the small leakages akin to old style step down transformer based linear power supplies.

As jahonen linked, the medical grade Schaffner input filters have just 2uA leakage, without the Y cap, usually we get < 5uA leakage current on medical gear.

A theoretical problem is the increasingly large number of consumer grade office equipment making it into medical areas, and our internal labs have never tested leakage as acceptable just assume its adequate too; also there are so many personnel bringing their cellphone or other chargers [ e.g. laptop] into areas, and the eye opener is how leakage current can vary widely from model, make, and age.

Nothing has happened in regards to safety, but maybe we are simply unaware of the relationship, and its relatively simple to follow through. 



--- Quote from: jahonen on December 21, 2010, 06:29:43 am ---If you take a look at for example the datasheet of Schaffner inlet filter, then it can be seen that specified leakage current for standard types is around 370 µA (@230 VAC). Those 2.2 nF Cy-capacitors form a capacitive voltage divider.  My personal experience is that it can really kill most consumer-grade interfaces, when plugging or unplugging them while connected to the mains. The fact that most usual consumer connectors stupidly connect the signal first(!), and ground last, makes it even easier. The leakage feels also extremely nasty, for example when connecting an grounded antenna to floating system. It is not a surprise that medical types don't have Y-capacitors.

Problem is also multiplied if there are several devices plugged in same extension cord. That can cause much higher leakage currents. I guess that it is far from safe to disconnect the safety ground wire at big building where there are hundreds if not thousands of such devices.

Regards,
Janne

--- End quote ---

saturation:
Thanks hero999 for the graphic and commentary.  Of concern in theory, are multiple consumer grade PSU in parallel in a medical setting all riding the same branch circuit in line with medical equipment.  Labs working in < 1V and mA/uA range projects need be aware of the potential for noise into their power supplies from sources within the line voltage introduced by their accessory gear or others using the branch circuit on the same floor.

So far testing hasn't shown leakage above the limit of 500uA, it typically less than 10% of that but survey continues.



--- Quote from: Hero999 on December 21, 2010, 06:28:16 pm ---I don't see what all the fuss is about. It's pretty normally for switch mode power supplies and is not an issue unless it's a medical device, lots of the PSUs are connected in parallel or you're using it to measure stic sensitive components.

As I keep saying, it's the Y1 rated capacitor which is leaking a tiny current from the mains to the DC side. If you care to measure it, you'll probably find it's probably under 250µA which the acceptable leakage for class 2 domestic appliances.

--- End quote ---

--- Quote from: Hero999 on December 21, 2010, 06:28:16 pm ---I don't see what all the fuss is about. It's pretty normally for switch mode power supplies and is not an issue unless it's a medical device, lots of the PSUs are connected in parallel or you're using it to measure stic sensitive components.

As I keep saying, it's the Y1 rated capacitor which is leaking a tiny current from the mains to the DC side. If you care to measure it, you'll probably find it's probably under 250µA which the acceptable leakage for class 2 domestic appliances.

--- End quote ---

Zero999:
SMPSes typically leak more current than old laminated transformer PSUs.

A medical SMPS will probably be a class 1 device because the EMI will most likely be leaked to earth rather than to the mains. I would imagine they use an earthed screen between the primary and secondary windings of the transformer.

Isolation transformers are used to ensure leaky devices such as commercial grade SMPSes comply with medical requirements. I would advise you get an isolation transformer which can be used for testing mains powered devices safely using a 'scope.

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