7
Instruction Leaflet
IL019226EN
Effective October 2018
C-HRG technical information
and adjustment procedure
EATON
www.eaton.com
There are two important factors that should be considered when
choosing which resistor setting to use in a system:
1. The charging current of a system.
2. The amount of ground return current through the neutral resistor
that should be allowed on the system.
It is rather important to consider these factors in this order because
the capacitive charging current will provide the minimum amount of
ground return current that should be let on the system to make the
C-HRG effective. That, in turn, relates to the maximum grounding
resistance that can be chosen.
ote:
N
The ground return current to be allowed on the system should be
higher than the charging current of a system. In other words, the grounding
resistor value to be chosen should be lower than the capacitive coupling
to ground.
The reason behind choosing a grounding resistor value that is
lower than the charging capacitive coupling is so that in case of a
ground fault, the return path of the ground current will be assured
to flow through the grounding resistor and not through the alternate
capacitive path.
ote:
N
For most systems, a resistor tap setting of 5 A will exceed
the capacitive charging current of the system yet still prevent
equipment damage.
Determining the charging current
WARNING
HIGH VOLTAGE!
MAY CAUSE DEATH, BODILY INJURY, OR EQUIPMENT DAMAGE.
TURN OFF POWER AT THE SOURCE BEFORE WORKING INSIDE.
There are different methods to determine the capacitive charging
current of a system, but they all prove to be tedious and/or
impractical.
It is advisable to use the industry accepted method
of estimating the charging current instead of calculating and/
or measuring it.
It is important to note that the capacitive charging
current of a system is the current that flows toward the unfaulted
phases during a ground fault.
This current is different from the leakage current that flows to
ground from the phases during normal operations. The leakage
current, typically of very small magnitudes, will continually flow
through insulation.
Estimating the charging current
THIS IS THE PREFERRED METHOD FOR DETERMINING THE
CHARGING CURRENT OF A SYSTEM.
IEEE
T
Standard 141 (IEEE “Recommended Practice for Electrical
Power Distribution for Industrial Plants”) recommends a resistance
value that limits the maximum ground current between 5 A and 10 A
for high resistance ground systems.
It also recommends for high resistance grounding to be used only
in systems of 5 kV or lower, charging currents of 5.5 A or lower, and
never in 15 kV systems.
A way to estimate the capacitive charging current for low-voltage
systems is as follows:
480 V systems:
•
0.5 A per 1000 kVA
•
0.5 A per set of surge capacitor used
Calculating the charging current
Both of the methods described below are either tedious or
impractical for industrial applications.
To calculate the capacitive charging current of a system:
1. De-energize the system completely.
2. Allow enough time for all the components to cool down to
room temperature.
3. Develop an impedance model by either measuring cable lengths,
field data collection, information on one-lines, and so on.
4. Calculate the Thévenin equivalent of the entire system.
5. Using Ohm’s law and with the aid of a power system software,
calculate the capacitive charging current.
Measuring the charging current [1]
DANGER
HIGH VOLTAGE!
WILL CAUSE DEATH, BODILY INJURY, OR EQUIPMENT DAMAGE.
TURN OFF POWER AT THE SOURCE BEFORE WORKING INSIDE.
THE METHOD BELOW IS FOR INFORMATIONAL PURPOSES.
IT IS RECOMMENDED TO ESTIMATE THE CHARGING CURRENT
AS SUGGESTED ABOVE WHEN DETERMINING THE CHARGING
CURRENT.
Only qualified personnel should attempt to measure the
capacitive charging current of a system.
The system needs to be
ungrounded
and the equipment
rated
properly for the system voltage
.
If system is grounded, disconnect all grounding equipment by
opening the appropriate disconnect switch or breaker. Make sure
no other ground is connected to the system.
1. De-energize the system or part of the system under test.
2. Take one phase to ground in the de-energized part of the system
using a 6 A current-limiting fast acting fuse, a circuit breaker, a
variable resistor, and an ammeter, all in series.
3. Make sure the circuit breaker is open and the resistance is set
to maximum.
4. Re-energize the system and make sure that the entire system is
energized and under maximum load conditions.
5. Close said breaker protecting the circuit under test and slowly
reduce the resistance to zero.
6. The reading on the ammeter should be showing the capacitive
charging current at this time.
7. Take the resistance back to the maximum and open the breaker.
