Hi Dan:
> with a three prong NA cord set. I have found that if I bypass the earth
> ground plug I can measure a 80 VAC potential from my chassis to earth
This is normal.
Consider the circuit:
L >------------o-------------> 120 V rms
|
|
2200 pf -----
-----
|
|
o------- ~60 V rms
| | (depending on the
| | tolerances of the
2200 pf ----- | capacitors)
----- |
| |
| |
N >------------o-------)-----> 0 V rms
|
|
PE >--------------------o
(open) |
|
|
-----
/ / /
The capacitors form a 2:1 voltage divider. So,
if the chassis is not grounded, then about 1/2
of the supply voltage appears on the chassis.
(You need a 10-megohm input meter to measure
this voltage; otherwise, the meter impedance
affects the measurement.)
The current is:
I = E / Xc
Xc = 1/(2*pi*f*C)
= 1.206 megohms (for 60 Hz)
I = 120/(1.206 x 10*6) or ~100 uA
This confirms what the manufacturer told you.
> grounded bench and got zapped. Is there guidelines regarding this? I see
> the UL mark on this power supply. I want to use a two prong NA cord set
This current is well below the two typical
values permitted by safety standards:
500 uA
3500 uA
Some people can feel this current (i.e. 100 uA)
when they are solidly grounded and they lightly
touch the chassis. If they hold on firmly, most
people cannot feel the current. Disclaimer: I
am not suggesting that you do this.
(We had a discussion about a month ago as to
the physiology of the light touch.)
> the UL mark on this power supply. I want to use a two prong NA cord set
> not a three prong NA cord set and I have been told OK by the manufacturer.
I disagree with the manufacturer on this point.
The safety of the product was designed on the
condition that it be connected to ground. If
the unit is used without a ground, then one of
the two safeguards against electric shock is
defeated.
With respect to electric shock, safety standards
require a principal safeguard and a supplemental
safeguard. The ground is one of several
supplemental safeguard schemes. Without a ground,
then the customer or user has only the principal
safeguard providing protection. If that principal
safeguard should fail, then there is a risk of
electric shock.
If you want a two-wire product, then I urge you to
use a product whose safety is expressly designed
for two wire connection. Such products are known
as "double-insulated" and bear the "square within
a square" mark.
Best regards,
Rich
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