Ungrounded systems
Wolfgang Hofheinz, E.Eng.
Torsten Gruhn, E.Eng.
Andy Moeschl, P.Eng./MBA
the heart of a reliable supply of electrical power
The advantages of modern manufacturing and process installations can only be exploited if the supply of electrical power is also reliable. Unexpected
ground faults in the installation can result in undesirable failures of the supply. As early as the selection
of the system used for the supply of electrical power
along with the related protection devices and monitoring equipment, it is possible to lay the basis for a
high level of protection for personnel and equipment,
as well as for trouble-free operation.
Electrical safety in the electrical power supply
A virtually trouble-free, reliable electrical installation that
ensures a high degree of safety for personnel, operational
safety and installation safety is the primary objective of
all installation managers in industrial facilities, hospitals
and commercial buildings. The concept for the electrical
installation must therefore
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Take into account the safety of personnel and installations,
Improve operational continuity and
Contribute to the performance of the installation.
For this purpose, optimally selected electrical power supplies and the related protection devices and monitoring
equipment can:
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omprehensively protect individuals and the installation
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against hazards due to electrical power,
Provide immediate signalling and reaction to critical
installation and operating states,
Reduce maintenance, servicing and failure costs, Prevent
or minimise interruptions in operation,
Manage installation data to suit specific needs.
Correctly selecting a power supply system
The common requirements for installation safety and
safety in operation are defined in EN 61140:2002+A1:2006.
The following aspects are of crucial importance for the
selection of a suitable electrical power supply system and
the related protective measures in accordance with CEC
22.1:2012, NEC 250:2014 and HD 60364-4-41:2007:
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Probability of isolation faults,
Basic protection and fault protection,
Continuity of the supply of electrical power,
Technical and commercial aspects,
Available experience.
The layout of power supply systems and their connection
to ground is described in more detail in CEC 22.1:2012,
NEC 250:2014 and in HD 60364-1:2008.
The main forms are the solidly grounded system (midpoint grounded), the impedance grounded system (midpoint grounded) and ungrounded system.
In solidly grounded systems, one point is connected
directly to ground and the exposed conductive parts of
the electrical installation are connected to this point via
protective ground conductors.
In impedance grounded systems, one point is connected
to ground through an impedance (CEC describes the
needed impedance as grounding resistor, grounding
transformers, ground-fault neutralizers, reactors, capacitors or a combination of these and the exposed conductive parts of the electrical installation are connected to the
ground via protective ground conductors.
In ungrounded systems all active parts are isolated from
ground. The exposed conductive parts of the electrical
installation are grounded either individually or jointly.
To ensure an adequate level of protection for individuals
and the installation, it is always necessary to co-ordinate the ground connection and the characteristics of
ground conductors in conjunction with the type of system. The protective measures allowed are defined in CEC
22.1:2012, NEC 250:2014 and HD 60364-4-41:2007.
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For the solidly grounded systems, residual current devices
(RCDs / GFCIs) and overcurrent protection devices (e. g.
circuit breakers) are the most common protection devices, the impedance grounded systems use monitors
which can control the fault current to 10A or less and
which de-energize the system when the continuity of the
connection between the neutral point and the grounding electrode through the impedance is no longer given;
while in an ungrounded system the usage of isolation
monitoring devices (IMDs) with an active measuring
principle (EN 61557-8:2007) is also required in the HD
60364-1:2008. CEC 22.1:2012 and NEC 250:2014 only
speak of minimum standard “Wiring systems supplied by
an ungrounded supply shall be equipped with a suitable
ground fault detection device to indicate the presence of
a ground fault” – this still allows the extremely unreliable
three-light-system (dummy lights) to be used.
Figure 3: ungrounded system in accordance with CEC 22.1:2012
In principle, the protection of personnel is to be given the
highest priority in electrical installations. However, the
control of the risk of the lack of availability of electrical power is becoming increasingly important. If an installation is unexpectedly shut down due an isolation fault, there
are further aspects that should be taken into account:
Placing individuals at risk, e.g. due to:
• The sudden failure of the lighting,
• The shutdown of equipment that is required for safety
in operation.
Figure 1: solidly grounded system in accordance with CEC 22.1:2012
Commercial risks, e.g. due to:
• High cost as a consequence of a loss of production, in
particular in areas in which re-starting is protracted
and expensive,
• Data loss in the information technology area,
• Increased costs due to malfunctions and damage to
installations or loads.
Furthermore, sensitive loads can be disrupted by high
fault currents; a shut down can cause overvoltages and/or
electromagnetic effects, malfunctions or even damage to
sensitive equipment. In relation to the availability of the
supply of electrical power, the behaviour of the electrical
power supply in the event of a first isolation fault is of particular interest.
Figure 2: impedance grounded system in accordance with CEC
22.1:2012
Comparison between grounded and ungrounded
electrical power supplies
Ungrounded systems are supplied either by a transformer
or by an independent supply of power (e.g. battery, generator, etc.). The special aspect of these systems is that no
active conductor is connected directly to ground. In the
case of a fault to frame or ground fault, no short circuit
current can flow as it would do in the grounded systems.
Instead, as a consequence of the lack of a complete circuit,
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there will only be a small fault current with a magnitude
defined by the isolation resistances RF and the capacitance
Ce of the conductors to ground. The difference between
the grounded and the ungrounded system in the case of a
fault can also be seen by comparing Figures 4 and 5.
On the occurrence of a direct ground fault RF in a grounded system (here a solidly grounded system), a ground fault
current IF equal to the short circuit current IK flows. The
upstream fuse trips and operation is interrupted (Figure 5).
The ungrounded system does the opposite. Here it is easy
to see that in the case of an isolation fault 0 ≤ RF ≤∞ only
the mostly very small, capacitive current flows through the
cable capacitances Ce. The upstream fuse does not trip in
this case such that the supply of power is also safeguarded
in the case of a single-pole ground fault (Figure 4).
ges. For this reason it is also used in many areas where
a high degree of reliability and safety is required in the
electrical power supply. Examples are control circuits in
accordance with CEC 22.1:2012, NEC 250:2014 and EN
60204-1:2006+A1:2009, electrical power supplies for areas in the medical sector in accordance with CSA Z32:2008
and DIN VDE 0100-710 (VDE 0100-710):2002-11, mobile
generators in accordance with HD 384.5.551 S1:1997 and
many other areas. However, ungrounded (or floating) systems with isolation monitoring are also becoming more
widespread in other areas such as electric vehicles, photovoltaic installations, industrial installations with regulated drives (ASD / VFD), complex manufacturing installations, information technology facilities (data processing
centres), as an unexpected failure of the supply of electrical power can have enormous financial consequences,
among others.
On the operation of an ungrounded system it must be
taken into account that on the first isolation fault, the originally ungrounded system changes into a system comparable with a solidly grounded system, and a second
isolation fault may result in the tripping of the short circuit protection and as a result in shut down. However, it
is clear from more than 60 years of our experience that a
single-pole fault (first isolation fault) is the most probable
type of fault (> 90%) and hazardous situations due to a
second isolation fault are more unlikely. Nevertheless HD
60364-4-41: 2007 recommends rectification of the isolation
fault quickly after its occurrence.
Figure 4: ungrounded system with isolation monitoring
(IMD = Isolation Monitoring Device)
Figure 5: solidly grounded system with isolation fault RF
In relation to the reliability of the supply of electrical power, the ungrounded system offers the most advanta-
Information edge due to isolation monitoring
In accordance with CEC 22.1:2012, NEC 250:2014 and HD
60364-4-41:2007 an ungrounded system is always to be
equipped with a ground fault detector. Bender invented
and uses the isolation monitoring device with an active
measuring principle, so that symmetrical and unsymmetrical faults can be detected. The isolation monitoring device is connected between the active supply conductors
and ground; it superimposes a DC measuring voltage on
the system.
In the event of an isolation fault, the isolation fault RF
closes the measuring circuit between the system and
ground, such that a DC measuring current Im proportional
to the isolation fault is generated. This DC measuring current Im generates at the measuring resistor Rm a corresponding voltage drop that is evaluated by the electronics. If this voltage drop exceeds a specific value, which
is equivalent to a drop in the isolation resistance below a
specific value, a signal is output on annunciator LEDs and
alarm contacts. The small system leakage capacitances
Ce are simply charged to the DC measuring voltage and
have no effect on the measurement after a brief transient
effect. (Figure 6).
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EN 61557-8:2007 contains detailed requirements on the
isolation monitoring device. The isolation monitoring device provides operators with the information edge they
need to initiate appropriate planned maintenance measures in good time (Figure 7).
ring devices that utilise the measuring technique of the
superimposed DC measuring voltage. The reason for this
situation is that in the case of a fault, these DC voltages
from external sources are additional to the DC measuring
voltage and therefore either result in a higher measuring
current - and as a consequence in increased response sensitivity - or a lower measuring current - and as a consequence in the failure to trip (Figure 8).
A further source of interference for isolation monitoring
devices is system leakage capacitances that are often present in the form of interference suppression filters (EMC)
between the grid and ground. On powering up the ungrounded system, these capacitances represent a low
impedance connection to ground such that a high DC
measuring current (charging current for Ce) flows briefly;
as a consequence there is an alarm from the isolation monitoring device.
Figure 6: Operating principle of isolation monitoring device
Figure 8: Effect of DC voltages from external sources on the measuring voltage
Figure 7: Information edge due to isolation monitoring
Modern measuring technique for modern loads
The measuring technique described above is appropriate
if the connected loads are only purely AC loads. However,
regulated drives (ASD / VFD) or loads with switched mode
power supplies (e.g. PCs, dimmers, and electronic ballasts)
are common these days. On the one hand this situation
offers the advantage of less power loss, smaller dimensions
and less weight, however, on the other hand the harmonics produced by switched mode power supplies and
possible DC effects have become a problem. The DC components cause incorrect responses from isolation monito-
To eliminate the effects on the isolation measurement
due to DC voltages from external sources and system
leakage capacitances, modern isolation monitoring devices use a pulsed DC measuring voltage. This measuring
technique reacts to system leakage capacitances with variable pulse times that take into account the charging curve of Ce. The magnitude of the DC voltage from external
sources is determined in a measuring cycle and can be
taken into account in the measurement of the isolation
resistance. In practice, this means that both DC voltages
from external sources or even high system leakage capacitances no longer degrade the function of the isolation
monitoring device or the result of the measurement. As a
consequence it is possible to precisely determine the exact isolation resistance.
One point: so-called ground fault relays that measure the
displacement voltage to evaluate an isolation fault or the
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ones using three lights or three voltmeters connected to
the 3 phases to check for phase loss are not considered
isolation monitoring devices in the context of EN 615578:2007 and are completely forbidden in the same standard because they do not detect symmetrical ground
faults which can result for example by humidity.
Devices for isolation fault location EDS
In complex installations, i.e. in installations with a widely
distributed supply of electrical power (such as the control
voltage in a power station or substation, a large industrial
application or railway signalling application), locating isolation faults can involve a high level of personnel resources
and a considerable amount of time. This problem can be
minimised by the usage of devices for isolation fault location in accordance with EN 61557-9.
These devices locate isolation faults automatically during
operation and indicate the faulty circuit via an LCD or other
type of visualisation unit. The operator does not need to
interrupt operation and the location of individual isolation
faults is precisely indicated. With a separate version of this
detection device it is easily possible to shut down a certain
circuit when the user wants that.
Summary
These days’ complex manufacturing and process installations place high requirements on the supply of electrical
power, as even a short power failure can result in downtimes and high costs. However on the usage of ungrounded systems with isolation monitoring, a tool is available to
effectively and efficiently counter this problem.
References:
CEC 22.1-12 Canadian Electrical Code, Part 1:2012 – Safety
Standard for Electrical Installations
NEC 250:2014 National Electrical Code, Edition 2014 –
The practical safeguarding of persons and property from
hazards arising from the use of electricity
IEEE C2:2007 National Electricity Safety Code, Edition 2007
EN 61557-8:2007 „Electrical safety in low voltage distribution systems up to 1 000 V a.c. and 1 500 V d.c. Equipment
for testing, measuring or monitoring of protective measures - Part 8: Isolation monitoring devices for IT systems
EN 61557-9:2009 „Electrical safety in low voltage distribution systems up to 1 000 V a.c. and 1 500 V d.c. Equipment
for testing, measuring or monitoring of protective measures
- Part 9: Equipment for insulation fault location in IT systems
HD 60364-4-41:2007 „Low-voltage electrical installations
– Part 4-41: Protection for safety – Protection against electric shock
EN 60204-1:2006+A1:2009 Safety of machinery. Electrical
equipment of machines. Part 1: General requirements
HD 384.5.551 S1:1997 Electrical installations of building
- Part 5: Selection and erection of electrical equipment;
Chapter 55: Other equipment; Section 551: Low-voltage
generating sets
Figure 9: General layout of a device for isolation fault location for
ungrounded systems
EN 61140:2002+A1:2006 Protection against electric
shock. Common aspects for installation and equipment
HD 60364-1:2008 Low-voltage electrical installations –
Part 1: Fundamental principles, assessment of general characteristics, definitions
Wolfgang Hofheinz: Schutztechnik mit Isolationsüberwachung, VDE-Schriftenreihe 114, 2nd Edition 2007, VDE-Verlag GmbH, Berlin – Offenbach
Wolfgang Hofheinz former CTO of the Bender Group
Managing Director of Bender Canada
Andy Moeschl Torsten Gruhn Technical Support Engineer of Bender USA
Figure 10: Isolation monitoring device for use in photovoltaic
installations (Bender works photograph)
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