
85
3
Selection and application
3-16 Ground fault protection in system applications
Notes: *1 The TN system is mainly used in Europe (except for France) while the
TT system is mainly used in Japan and France for direct grounding.
A TN system is grounded to one point. Here, all exposed conductive
parts at the load side are grounded at one point through protective
conductors. The neutral conductor and protective conductor are
handled in one of the following ways.
s 4.34HENEUTRALCONDUCTORANDPROTECTIVECONDUCTORFORTHEOVERALL
system are completely separate.
s 4.#4HENEUTRALCONDUCTORANDPROTECTIVECONDUCTORFUNCTIONSFOR
the overall system are combined into a single conductor.
s 4.#34HENEUTRALCONDUCTORANDPROTECTIVECONDUCTORFUNCTIONS
are combined into a single conductor in one part of a TN-S system.
Table 3-31 shows a comparison of various characteristics for
grounding systems as well as precautions regarding their use.
A TT system is grounded to one point in the system. All exposed
conductive parts at the load side are grounded to a ground electrode
that is electrically separate from the ground terminal for the system.
*2 A system that uses no grounding is referred to as an IT system.
Table 3-30 Grounding systems for low-voltage circuits
Method
Description
Main circuit
Sample applications
Direct
grounding
*1
Direct grounding systems run a ground to a neutral
point in order to minimize any increase in the electric
potential to ground with mixed high and low voltage
grounding.
Since ground fault current on a single wire is rather
large with direct grounding systems, it is much easier
to detect ground faults, and any increase in the electric
potential to ground is relatively small with a sound
phase. This is quite helpful for system safety. There is
also no real danger of abnormal voltage occurring due
to resonance or intermittent grounding as is often the
case with no grounding at all. As a rule, a faulty circuit
is shut down as quickly as possible when a ground
fault occurs.
L
L
M
E
Main transformer Low voltage
Fig. a
Single-phase
load
Three-phase
load
Used to reduce voltage
to ground for safety in
building wiring.
Resistor
grounding
(low to mid
impedance
grounding)
Ground fault current is significant with direct
grounding, and no grounding systems may be to
blame for circuit problems. However, low-voltage wiring
systems tend to have low charging current to ground
and the sensitivity of protective relays makes it hard
to choose a feeder for ground faults in that case. The
resistor grounding method shown in Fig. b then is a
better choice for this application. Unfortunately, resistor
grounding does not completely eliminate possible
abnormal voltages or higher electric potential to
ground even with appropriate phases if a ground fault
occurs.
M
E
Main transformer
Low voltage
Contact prevention
plate
Fig. b
Ground
resistance
Three-phase
load
Factory wiring
Circuits shutting down
immediately when a
ground fault occurs
very often shuts down
operations in factories,
so greater emphasis
is being placed on
suppressing ground
fault current to prevent
fires and explosions.
No grounding
(high-
impedance
grounding)
*2
A ground fault in a factory power supply instantly trips
circuits that can shut down operations. No grounding is
generally used in 3 to 6kV high-voltage wiring systems
because ground fault current continues to flow here
without high-speed tripping as long as the ground
fault current is kept low enough to prevent damage to
the equipment and to prevent the fault from becoming
more widespread.
Nearly all motors in a factory are 3-phase loads that
do not normally require a neutral line, and even an
ungrounded system like that shown in Fig. c can be
used in a 400V wiring system. This makes it hard to
detect ground faults, however, so steps must be taken
to handle abnormal voltages when they occur.
M
E
Main transformer
Low voltage
Contact prevention plate
Fig. c
When using a
grounding
transformer
Three-phase
load
Factory wiring
Circuits shutting down
immediately when a
ground fault occurs
very often shuts down
operations in factories,
so greater emphasis
is being placed on
suppressing ground
fault current to prevent
fires and explosions.
3-16 Ground fault protection in system applications
3-16-1 Grounding methods and ground fault protection in
system applications
There are three possible grounding systems for low-voltage circuits: direct
grounding, neutral point resistor grounding or no grounding at all.
Direct grounding systems are widely used in Europe and the United States.
Unfortunately, direct grounding causes ground circuit impedance to drop
and ground fault current to rise rather steeply when there is a ground fault,
and the result is often a malfunction.
Table 3-30 provides an overall description of all grounding systems.