[R-390] Electrical safety leakage

[Perry Sandeen]

Roy Morgan wrote: There is a specified leakage limit for medical equipment

Having done those tests for 25 years I think I remember the values.  

     There are 3 tests performed. First is a ground resistance check from the instrument to the
power plugs ground pin.  The limit is 500 miliohms.  Normally you could easily get between 85 to
100 miliohms.  Anything close to the limit indicated a loose ground connection.  Equipment with
the IEC removable plugs was the biggest problem the constant removal of the line cord caused you
to have to twist the ground pins slightly to meet specs.  This shouldn’t be a problem  with BA’s
as they are almost always plugged in.

    The second test was in two parts.  You measured ungrounded leakage with the power turned on.
Once with normal polarity and then with reverse polarity.  The leakage current limit was 500
microamps.  Except for very large pieces of equipment the normal range of leakage was 80 to 150
microamps.

This is the rational for the 500 microamp limit.  The least amount of AC current that was able to
put the human heart into ventricular fibrillation by direct application was 200 microamps.  Since
the body is a big salt water sponge, the decision was made to codify the limit as 500.

Beginning of safety rant!
IMHO one is foolish to leave the original line filters on the R39XX, SP 600’s and any other BA
equipment.  The price for new or used  "medical  grade", I.E. low leakage, is less that half the
price of a 26Z5.  I’ve been "bit" badly by a 2 wire line cord SP 600 when I touch something
grounded.  NOT FUN.  The difference between nasty words and DEATH is the moisture on the surface
of you skin. A 4,000 to 5,000 microamp leakage can kill you with no problem.

Another reason to use the newer Pi balanced filters is that THEY HAVE BETTER SUPPRESSION
CHARACTERISTICS THAN 50 YEAR OLD CAPS.  Hide a new filter in the old case if you want.
If you vehemently disagree, have your will, life insurance, final arrangements up to date and make
 a value list of you BA stuff so you heirs don’t get ripped to badly on epay.  You are never lucky
forever.

In addition, one should make a shorting stick for your RECEIVERS using a 10 Kohm  5 or 10 watt
wirewound resistor.  One, your bleeder resistor may not really be working well (if it’s even
there!) and newer caps, both electrolytic and film capacitors have MEMORY.  They can come back to
bite.  

When I was in "sets" (hands on: live power) at Keesler  AFB I was one of the students who some of
the most "sparks".  But I never got hurt because I kept one hand in my pocket and didn’t wear a
ring even though I probably had the sweatiest hands.  Heck, my hands are getting sweaty just
writing this remembering the past.

Be safe, live long. Enjoy our temporary custodianship. End of Rant  Regards, Perrier


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