Rich Nute wrote:
> 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
>
Hi Rich
Thanks for your comment.
You're right:
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 require a GFCI to be safe!
According to HD384.4.41 S2:1996 (I only got copies of 2 pages) RCD
(GFCI's) are
allowed as protection.
They are right, under following circumstances:
- each consumer (or consumer group) is connected via GFCI
- there is one fault on one consumer.
If now a person would touch an active wire (or you would have a second
fault)
BOTH GFCI's would trip.
I.e. GFCI's could offer a protection in case of two faults.
Best regards
George
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