Author Topic: Electrical Safety  (Read 1370 times)

Offline K9DJT

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Electrical Safety
« on: July 17, 2014, 09:36:20 AM »
This is my 12th installment of “Understanding Test Equipment” and it just dawned on me I hadn’t addressed anything relating to Electrical Safety.  I am ashamed because it should have been one of the first things, and something none of us shouldn’t take lightly no matter how much experience one may have.

Did you know there is actually a correct way to connect and disconnect a piece of test equipment to a unit under test (UUT)?  Especially if you are using leads with alligator clips on them.  The correct way is to connect the low side of the test equipment, i.e., the ground lead (negative) to the UUT ground first, and then connect the high side, i.e., the positive lead to the potential (voltage).  When you disconnect from the UUT you just reverse the process.  Disconnect the positive first and then the negative.  So what difference does it make?  If the UUT is already powered up, it makes a big difference.  Let’s take a look at why.  During my travels, I had met an electrician who was working on a motor-drive system and disconnected the negative lead first and let it drop.  He created a lot of unnecessary damage in doing so because the negative was in reality “HOT”, i.e., it was at the same potential (voltage) as the positive lead of the test instrument.  The SCR’s the lead struck on the way down were not happy.  Remember the meter or scope you are using has an internal impedance (resistance), which no matter how large, has the same voltage at both ends until the negative side of that instrument is connected to the negative side of the voltage you’re measuring.  Let’s say you didn’t have an insulated boot around the negative alligator clip (dumb).  You grab it and remove it from the chassis and you are now holding onto the voltage you were measuring.  And yes, you might be unwilling holding it.

During my early years in electronics, age 12 to 20, it wasn’t uncommon to see many cartoons of some character being electrocuted and supposedly unable to let go of the wires.  It was always common to hear that we should work with one hand behind your back, which is still a very good practice, and stand on a rubber mat, which is also a good thing to do.  Well, of course I didn’t believe in this stuff.  I had already had so many shocks working on five tube radios and TV’s and was always able to pull away.  What nonsense I thought.  Well, it caught up to me while attending MATC.  My lab partner and I were setting up an experiment and all the stuff we were using was bread boarded.  The mature (older) hams will remember transformers, inductors, potentiometers, resistors and capacitors mounted on wooded boards with their leads to some type of spring connector which you pushed down and slid a wire trough.  It turns out my lab partner, who’s name I still remember and called on as a customer at GE Medical, had taken a power transformer, which had a line cord permanently attached to the primary, and plugged it in…WITH NOTHING CONNECTED TO THE SECONDARY!!!  No one would do anything like that.  Right?  Being fearless as I was, I proceeded to set up the circuit without ensuring the transformer was dead…and in doing so had a pair of fingers of one hand holding one end of the secondary while only one finger of the other hand just touching the terminal of the other end of the secondary.  400VAC hand-to-hand!  Unable to let go…unable to speak…everything going dark.  The momentary passing out is what save me as I fell off the lab stool.  I still remember the helpless feeling when trying to talk and all the other students laughing at me while thinking I was pretending and acting silly.  The above is a long story to make the point of; never trust anyone else regarding a circuit being dead or disconnected!  An example being a coworker or friend who may say, “I turned the breaker off.”  Don’t believe them…test all dead circuits yourself to ensure they are in fact dead.  How do should we test for a dead circuit?  No matter what you use, a VOM, DMM or Volt-Alert wand device, always test the instrument first on a known live circuit first.  This lets you know it is in fact working,  Then test the circuit which you believe to be dead.  Assuming it is dead by showing a zero voltage/potential, take the instrument back to the know live source again and ensure it is still working.  In this way you know the measuring device is working before, during and after the test of the dead circuit.  This is most important when using an instrument having test leads which may become intermittent.

I met many many people during my years as a sales engineer who experienced many close calls or knew someone who wasn’t as lucky as I.  The thing is, I never met a dumb electrician…all the dumb ones are dead!  Safety, relating to anything, is being smart and aware of your surroundings.  THINK about what you are doing!!!

Be safe out there!

73, Gary
K9DJT