Re: 48 V AC to case of cable box
- From: "Michael A. Terrell" <mike.terrell@xxxxxxxxxxxxx>
- Date: Fri, 09 May 2008 20:41:55 -0400
ineedcoffee wrote:
Voltage question for some electronics repairs techs...
After feeling electricity when connecting two cable wires via a
coupler, I traced the source back to my Scientific Atlanta 8300HD
cable box. This cable box has no ground prong. When all wires expect
the power are disconnected, my meters reads 48 V AC from the case to
ground. For ground, used both my hot water base board and the neutral
and ground from the receptical. The outlet polarity is not
reversed.
When touch ground and the box with my hands, I can intermittently feel
slight tingling, about the strength of a dying 9 V on the tongue.
I've also verified the cable wire is grounded. When it is connected
to the box, the voltage on the case of course disappears. Obviously
it cannot source much current, since this is effectively shorting a
voltage source to ground.
The cable company has had two other boxes out, and both have the same
symptoms.
Am I missing something here?
Yes. A lot.
This cannot be normal.
Yes, it is.
For a two
pronged appliance, I assume the case should be electrically isolated
from both the hot and the neutral. Does this sound right?
No. How would _YOU_ isolate the chassis when the RF connectors are
grounded to the chassis without causing other problems, and keep the
price reasonable? How will you pass the strict EMI requirements if the
RF can radiate out of the case?
Any ideas or info would be appreciated. Thanks.
That is normal capacitive leakage in the power supply. If you put a
1 Kohm resistor across your voltmeter you will see almost nothing.
Do things the right way. You are supposed to hook up all of the RF
cables BEFORE you plug it in. Then the chassis is grounded, and there
will not be enough voltage for you to feel anything.
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