Hi John:
Nothing wrong. When the ground is opened, you get
about half the mains on the (formerly) grounded
parts. All equipment does this, including two-
wire equipment. Even a three-wire power cord does
this!
The voltage is due to a capacitive voltage divider,
line-chassis-neutral. The capacitances are in the
line filter and the strays due to the wiring.
The key is the current. It is (or should be) less
than that specified in the standard. This is
"touch" nee "leakage" current. Measure the current
to ground and you will find any where from 0.5 mA
to 5 mA.
When you touch the chassis, the voltage drops to
about 20 volts or less, and you may feel a tingle.
The source impedance (to mains) is about 200 k
(capacitive reactance), maybe more, maybe less,
depending on the equipment.
Best regards,
Rich
On 9/5/2014 12:49 PM, John Cochran wrote:
*From:*John Cochran
*Sent:* Friday, September 05, 2014 3:47 PM
*To:* 'emc-p...@ieee.org'
*Subject:* UL 60950-1 clause 2.2.3
I am doing compliance testing on an outdoor computer/display and am
having problems with complying with clause 2.2.3 of UL 60950-1 and
clause 6.2 of UL 60950-22. When the earth ground is disconnected,
there is an AC potential on the chassis ground that is ½ of the line
voltage. At 240VAC, there is 120VAC on the ground. Since everything
is grounded in the system and it is in an aluminum enclosure, I have
this potential everywhere. I cannot find an open frame AC power
supply that does not do the same thing. What is it that I am doing
wrong. The UL 60950-22 standard says the maximum AC voltage that is
acceptable is 15VAC.
*/John Cochran/*
425 Caredean Drive
Horsham, PA 19044
PHONE: 215-443-3400 X219
FAX: 215-443-3002
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