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Why HRG and LRG used beside transformer, what is the purpose
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Answer Posted / the_dabi
Resistance Grounding falls into two categories: Low
Resistance and High Resistance
1) In Low Resistance Grounded (LRG) Systems the current is
limited to 25 amps or more. Generally the range is from 25
to 600 amps, although in some systems it may be even
greater.
2) In High Resistance Grounded (HRG) Systems the current
is limited to 10 amps or less.
A) On systems with a voltage of 1000 volts or less (phase-
to-phase) grounding resistors are not used. Normally these
system use a solidly grounded configuration with no
intentional impedance being introduced into the system.
B) Resistance Grounding is recommended on Medium Voltage
Systems from 1000 volts to 15,000 volts phase-to phase.
C) Because of the cost, Resistance Grounding is not usually
used on systems above 15,000 volts phase-to-phase.
Additionally, the use of a solidly grounded system allows
for use of equipment which is insulated for the phase-to-
neutral voltage of the system.
D) When a system has protective relays which will trip the
circuit if a ground fault occurs, a grounding resistor with
a 10 Second Rating is often specified, because the relays
will trip the system in less than 10 seconds. However, One
Minute or Ten Minute ratings are sometimes used for an
extra margin of safety, even though the cost will be
greater.
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