RE: [CAD_CAM_EDM_DRO] Re: variable transformer for dc power supply

Mike,

I designed industrial machines with DC. Like I said we did it both ways. I
liked it more when we didn't let the DC float. On the larger voltage
systems we ALWAYS grounded the DC bus from a center tap on the transformer.
Those were 600 volt DC systems so each side was either +300 VDC or -300 VDC.
If we didn't ground the center tap and let it float through isolation, then
whenever a DC wire touched the machine frame the other side was at a full
600 volts relative to ground. If we did ground the center tap, then
whenever a DC wire touched the machine frame we would blow a breaker and
everything stopped as it should.

To my way of thinking, isolation does not make the system safer. In
particular, don't think that by isolating the system you can be less careful
with the DC wiring. Wires should never get loose in the system and any
loose wire that contacts the machine should shut the machine down, not
electrify it.

For anyone that is really concerned with safety, just get a $6 GFI and wire
it into the main supply of the machine. This is a standard industrial
practice on 115 vac industrial circuits and UL encourages it.

GJ

-----Original Message-----
From: n4onl [mailto:umrk@...]
Sent: Tuesday, November 26, 2002 12:04 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Re: variable transformer for dc power supply


I bet if you check those industral machines with DC in them they
aren't rectifying the AC line directly, their using an isolation
transformer. They don't want to kill their customers.