Sorry Greg,
<Hot button warning...I need to put this out there before I go to sleep
tonight...>
I have to speak up... As you are well aware, there are real hazards working
with the AC mains... lethal hazards... hazards only to be approached with
proper training, experience and caution. The hazards go way beyond fried
electronics up to and including the risk of nasty and potentially lethal
shock. Mains can and do kill...
I know you know this, that you respect the hazards, and that you trust your
level of experience and training. I respect your designs... I respect your
skills! I respect your training!
In contrast, this thread was started by a student with what seems to be
little practical experience and likely no training on working with mains or
other high energy circuits. My strong belief / conviction is that they have
no place working with a non-isolated mains design prior to acquiring
appropriate training. I personally will not do a non-isolated mains powered
device. The risk/reward equation doesn't begin to work for me.
What follows is only intended to give an idea of what's involved... The
list is not complete and may have inaccuracies, but it gives an idea of
some of *my* concerns! Old maps used to say "Here there be dragons" in
areas where little was known... for me, that saying applies to mains
connected non-isolated designs. Go there if you must, but go with caution
and the knowledge to properly understand the risks and how to mitigate them.
Many of the hazards can be avoided or significantly mitigated by using an
appropriately safety-certified isolated power source. Given a choice, I
choose to let someone else do the mains connected part of a design and have
that design tested and certified for compliance on their dime... and fully
documented as part of what I check both on my hobby projects as well as
when I compile a documentation package that will accompany a device sent
for compliance testing.
There are safety standards for compliance when connected to higher voltages
(not safety extra low voltage "SELV" which covers most of what I do - in
part SELV will be under 60 V and ISOLATED by current standards...)... you
mention some. Here are some of those standards in no particular order and
absolutely not a complete list: we have multiple means of patient and
operator protection (MOOP, MOPP: 60601-1 3rd... if you happen to be in
medical... other standards apply for other devices...) - multiple failures
have to happen for a person to connect to the mains. There are component
ratings for various applications of mains connected devices ("X" and "Y"
capacitors as an example... these are not just 1 kV rated caps...),
creepage and clearance, leakage limits... The list goes on for quite a bit.
Then what happens when the device is knocked off a shelf and someone goes
to pick up the pieces? What happens when the babysitter plugs her
headphones into the jack? There are reasons why some odd and unusual
connectors show up on equipment - they are highly unlikely to be
accidentally connected to an improper device.
There are loads of accessible articles out there that give a starting
place on mains electrical safety... this two part article was the first hit
off a Google search (search "working with mains voltage safety") and seems
like a nice easy intro...
https://hackaday.com/2016/05/11/looking-mains-voltage-in-the-eye-and-surviving-part-1/
https://hackaday.com/2016/05/16/looking-mains-voltage-in-the-eye-and-surviving-part-2/
Low voltage also deserves respect... As an example, I remove my ring and
watch before reaching into a chassis or circuit regardless if there's
hazardous energy inside. This is a habit I choose to always enforce so that
it's there when it matters.... Low voltages may not be a hazard for
electrocution, but a short across a ring can result in a nasty burn if
there's sufficient energy available. There was the time I was reaching into
an old Heathkit tube device with both hands... violating multiple safety
practices. I'd acquired new habits over a bunch of years working with 15
VDC and below very low energy circuits... My pulse jumped... a lot... when
I noticed what I was doing. I decided then that I would have only one set
of habits...
Anyway... had to say something... NOW I can sleep!
I'd welcome others to chime in... Please call out or fix anything I have
wrong, etc... This is a worthy topic for discussion!
As always, best regards,
Bob
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