ed.pr...@cubic.com wrote:
> --- On Mon, 22 Jun 1998 00:44:00 -0700 "Liew, Shih Perng"
> <shih.perng.l...@intel.com> wrote:
> > Hi All,
> >
> > I have some different machines in one production line sharing a
> same
> > power source, namely IX testers, ovens etc. I encountered a
> problem
> > where some of the ovens tripped-off intermittently. The
> earth-leakage
> > current was found to be excessive and causes the earth-leakage
> circuit
> > breaker to trip. To find a short-term solution, the affected
> ovens
> > were rewired from a new power source and the metal structures
> were
> > insulated. The problem seems to be solved...however, I am not
> so
> > convinced.
>
> > rgds,
> > sp
>
> Shih:
>
> Be careful! The ovens which caused the leakage path to exist have now
> "been connected to a different power source" which I assume does not
> have ground fault current sensing? And you have "insulated the metal
> structures"?
>
> Seems like this creates a hazard, since the new arrangement will allow
> a dangerous voltage potential to exist between the newly insulated
> metal structures and earth ground.
>
> Better find out where that fault path is in the ovens, and get the
> structure grounded again. Look at the heater elements, look for
> corrosion and contamination on the insulator supports. Check for
> corroded sensing and control elements, and look at the fan mo!
> !
> tors for insulation degradation.
>
> Ed
>
> --------------------------
> Ed Price
> ed.pr...@cubic.com
> Electromagnetic Compatibility Lab
> Cubic Defense Systems
> San Diego, CA. USA
> 619-505-2780
> Date: 06/22/98
> Time: 08:48:59
> --------------------------
We performed many investigations with ground fault current interrupters
(GFCI, RCD etc.).
One of the problems we identified was that they trip not only according
to the
rms value of the leakage current. They also trip if you have current
spikes.
You will have current spikes under following conditions (provided the
load has a
power line filter):
* in the moment you connect the load
* when you have a voltage spike on the power line (you will have a
lot)
* if you have frequency adjustable drives or switched power supplies
You will also encounter problems, especially in power line networks
where the
central point is not earthed, like IT networks or networks with earthed
outer conductor
(I encountered such networks only in Japan)
This is the reason why we came to the conclusion that we have to
eliminate the GFCIs.
In order to increase safety we place a second earthing conductor
(also see IEC950, sublcause 5.2).
Good luck and best regards
George
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