Re: [CAD_CAM_EDM_DRO] Re: Remember Mr. Bill's servo lite?

On 01/14/2012 09:56 PM, Andy Wander wrote:

> If you can explain why it is dangerous, please do. But just saying "I can't make it any clearer" doesn't help me understand.
>
>
> Andy Wander

I have been shocked and lived to tell of it. The problem is that too
much current through the heart muscles will kill you.

It is good design practice to NOT tie the AC return to the safety
ground, but to use a parallel capacitor and resistor, often something
like 0.1 uF and 4.7K to 47.K Ohms. If the chassis is floating, the
current is limited, if the chassis is grounded, the AC return should
apply little (or no) voltage across the RC combination. But if there is
a large load (say a 20 Ampere starting load for a motor), and a longish
common run (50' of #14 wire), you see (2.5 Ohms/1,000') 0.125 Ohms and
a voltage drop of 2.5 Volts. What does this do to your noise margins on
the TTL circuits? This explains the practice.

Why is it dangerous?

Again, let's look at the situation where the chassis is tied to line
neutral. Line neutral can float to levels you would not believe (the
worst I ever saw was 13.8 KV, more often 100-200 Volts), and can have a
lot of energy available.

Safety grounds should be tied to ground rods and are generally tied at a
single point to neutral (usually in the main power panel.) That does
not guarantee they are connected.

Tie the chassis to the line return and use a plug and the ground can be
broken and the plug can become reversed. One Ampere can kill you, ten
Amperes can be drawn from a fifteen ampere circuit without blowing the
breaker/fuse.

Dave 8{)

--
/"Political Correctness is a doctrine, fostered by a delusional,
illogical minority and rabidly promoted by an unscrupulous mainstream
media, which holds forth the proposition that it is entirely possible to
pick up a turd by the clean end."/
(quoted from http://www.wattflyer.com/forums/showthread.php?t=30060)

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