Hi Georg:
You pose an interesting question, "Is a GFCI allowed on
an IT system?"
I will answer a different question: "Will a GFCI work
on an IT system?"
I will answer yet another question: "Does a GFCI provide
a protective function on an IT system?"
A GFCI measures the difference current between the two
supply conductors. Under normal load conditions, the
difference current is zero. If the difference current
exceeds the GFCI setting, usually 5 to 20 mA, then the
GFCI disconnects the supply.
The GFCI requires that one of the two supply conductors
(i.e., the neutral) be grounded. Or nearly grounded.
This provides a current path AROUND the GFCI such that
a difference current is created and thereby causes the
GFCI to operate.
The IT system has no direct connection to ground, i.e.,
the neutral is not grounded. The "I" in IT means the
neutral is grounded through an "I"mpedance. I understand
that the value of the impedance is 1000 ohms or greater.
This impedance has the effect of setting the maximum
difference current in the system. The maximum difference
current for a 230-volt system would be:
230 V
I = ----------- = 230 mA
1000 ohms
Clearly, if the impedance is 1000 ohms, then the GFCI
will work.
If the impedance is 100,000 ohms, then the maximum
difference current is 2.3 mA, and the GFCI will not work.
So, whether or not the GFCI will work depends on the value
of the grounding impedance of the IT system.
However, if the IT system is grounded through 100,000 ohms,
then the maximum current that can flow through a body
connected between one pole and ground is 2.3 mA. While
unpleasant, it is not unsafe!
So, if the value of the impedance of the IT system ground
is high enough to not allow the GFCI to operate, then the
IT system does not reguire a GFCI to be safe!
Best regards from San Diego,
Rich
"You may have to fight a battle more than once to
win it."
-- Margaret Thatcher
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Richard Nute Quality Department
Hewlett-Packard Company Product Regulations Group
San Diego Division (SDD) Tel : 619 655 3329
16399 West Bernardo Drive FAX : 619 655 4979
San Diego, California 92127 e-mail: [email protected]
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