Ground Fault Isolation


by ivannewman
Tags: fault, ground, isolation
ivannewman
ivannewman is offline
#1
Jun18-07, 08:55 PM
P: 1
We are using 120V commercial grade electronic ballasts, properly grounded, rated at 2Amps to drive two low pressure UV lamps submerged in water rated at 800mA. System requirements are to use GFI barkers with each ballast. This installation in a wastewater treatment plant, where there are motors, pumps and air compressors. GFI’s are randomly tripping at 5mA from ballast electronic current leakage up to 9.2mA. Can we resolve our nuisance GFI tripping by using 120V to 120V isolation transformers between the GFI and electronic ballast?
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xez
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#2
Jun18-07, 10:27 PM
P: 175

Well maybe the isolation transformers will help.
It depends on what you're trying to achieve.

a) you mention the facility has motors, pumps, compressors,
etc. and there's some implication that maybe the power
line fluctuations / noise from these may be relevant
in causing the GFI's to trip. If the GFIs trip because of
EMI / noise present on the power line, then they're
erroneously tripping due to noise and not a 'real' ground
fault. To solve problems relating to power line coupled
noise, try installing power line EMI filters and/or
filtering surge suppressors and/or filtered or "online" UPS
units BEFORE the GFI so that the power supply INTO the
GFI is clean and stable and the GFI won't be more likely
to trip due to noise present on its INPUT.

b) If EMI coupled through induction to the leads on the
OUTPUT of the GFI (between the GFI and its load,
the ballasts) is causing enough noise so as to contribute
strongly to tripping the GFI, then you'll need to minimize
the circuit loop inductance and EMI noise coupled from
the ballast circuit loop to the GFI's output side. This
can be done by using more closely spaced wires, twisted
pairs of wires, series ferrite / iron chokes which each of
the individual circuit wires is separately wound around,
and by installing a series EMI filter (capacitor / inductor)
unit just on the outside of the GFI before the leads run
out toward the ballast load(s). Using more heavily
shielded metallic conduit and boxes can also
help somewhat in reducing the amount of induced
EMI coupled to the GFI and its associated wiring.

c) if you install an isolation transformer after the GFI
and before the GFI's load, so that the transformer IS
the GFI's only load then you've just created a somewhat
distinct partition of your circuitry. Circuit A is between the
GFI's output and the isolation transformer.
Circuit B is between the isolation transformer and the
ballast(s). The GFI will ensure there are no ground faults
between it and its load, Circuit A, the isolation transformer.
If you draw a significant imbalanced current from either
the hot or neutral lines attached to the GFI to ground, the
GFI will trip. If the current in the hot and neutral leads
of the GFI is balanced (equal within a couple milliamps),
the GFI should not trip. By using a transformer between
the GFI and the ballast, and by (for the sake of
example) locating the isolation transformer right next to
the GFI, you're almost ensuring that there WON'T be any
ground faults that the GFI will 'see' since unless you do
cause a ground fault in the short circuit path between the
GFI and the transformer primary circuit, there will be no
ground fault for the GFI to detect IN THE CIRCUIT IT IS
MONITORING.
Now the isolated SECONDARY winding of the isolation
transformer is CIRCUIT B and consists of the secondary
of the iso xfmr and the ballast(s) which load it. One
could create a legitimate ground fault on CIRCUIT B,
relative ONLY to the SECONDARY of the ISO XFMR and
the GFCI on CIRCUIT A would not detect it since the
current in CIRCUIT A is still balanced (through the
ISO transformer PRIMARY) and isn't ground-faulting.
There could be unbalanced current from the secondary of
the ISO XFMR but that's not what the GFCI is attached to
and therefore it's not monitored.
Certainly in such a case you'd be reducing the nuisance
tripping of the GFCI due to a legitimate ground fault
in the circuit leading to the ballast(s), but in such cases
of a 'true' ground fault imbalance in CIRCUIT B, the GFCI
on circuit A is providing no protection / detection.
So could it help solve nuisance tripping? YES.
Could it help to circumvent the very protection that
the GFCI is supposed to provide in the major way it's
supposed to work? YES, in that your ultimate circuit would
include portions of it that wouldn't be GFCI protected
against real ground faults.

d) However since isolation transformers usually are
somewhat effective line noise filters for some kinds of
high frequency spikes, surges, EMI, the mere presence
of an isolation transformer in the circuit COULD help reduce
EMI related GFCI tripping due to the other motors /
compressors / etc. in the vicinity. For maximum EMI /
line noise protection, of course you should use an isolation
transformer that also incorporates EMI filtration / surge
suppression / choke type circuitry. That'd help against
nuisance tripping that ISN'T due to a true ground fault.
dlgoff
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#3
Jun19-07, 05:54 PM
Sci Advisor
PF Gold
dlgoff's Avatar
P: 2,633
Are you sure that the UV lamps and wiring aren't leaking through the water? I would think this is the most likely reason for seeing a ground fault.

Have you checked with other water treatment facilities to see if they have experienced such problems? And what they have done?

If you use an insolation transformer then will your GFIs still be effective? I think not.


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