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Ungrounded systems as reflected in the standards

Unearthed power supplies (IT systems) are always used in situations where an initial insulation fault must not cause a protective device to respond, thereby interrupting the power supply. Classic fields of application, for example are in medically used locations, sensitive processes in the industrial sector, and feedback-controlled drives in heavy industry. New applications in electromobility and DC systems have come along as part of the paradigm shift in energy usage occurring now. This article provides an overview of the prevailing situation with respect to the standards.

Network types

The basic power network types are illustrated in DIN VDE 0100-100 (VDE 0100-100):2009-06 In 131.1 of this standard, reference is made to the fact that, with these requirements, the safety of persons, livestock and property is provided against dangers and damage, provided that electrical systems are used in the intended manner. The risks involved include the occurrence of dangerous shock currents and interruptions in the power supply.

The IT system is described in IEC 60364-1:2005-11, 312.2.3 (AC) and 312.2.4.5 (DC). The definition states that all active components must be isolated from earth or that one point must be connected to earth via an impedance. The exposed-conductive-parts (of electrical equipment items) on the electrical system are either earthed individually, in groups or collectively with the system's earth (also refer to IEC 60364-4-41:2017-03, 411.6). The system may also be connected to earth via a sufficiently high impedance. In Germany, this is only applied for measurement or functional purposes.

Figure 1: Comparison between an IT system (left) and a TN system (right) during a first insulation fault

The power source

A distinction must be made between the power source for a "normal" IT system and for a medical IT system. Disconnection in the form of basic insulation is required for the power source. In practice, this usually takes the form of an isolating transformer. This function can also be performed by a battery, by a standalone photovoltaic (PV) system or by a mobile power generator. In a medical IT system, the leakage currents during intracardial interventions (e.g. open-heart surgery) must be extremely low given the potential risk to the patient. The required isolating transformer is described in IEC 61558-2-15:2011-11.

Figure 2: Layout of a medical IT system in acc. with IEC 60364-7-710

Earthing

In practice, the IT system is frequently known as "the unearthed power supply system". "Unearthed" in this context only refers to the connection between all active conductors and the earthing system. In accordance with IEC 60364-4-41:2017-03, 411.3.1.1 "Protective earthing", the exposed-conductive-parts must be connected to a protective conductor, depending on the type of earth connection. What this means for the IT system in accordance with 411.6.2 is that the bodies must be earthed individually, in groups or collectively and that the following conditions must be satisfied:

In alternating current systems R_A × I_d ≤ 50 V

Where

R_A is the sum of the resistance in Ω of the earth electrode and the protective conductor for the respective exposed-conductive-parts;

I_d the fault current in A of the first fault with negligible impedance between a line conductor and an exposed-conductive-part.

No touch voltage limitation is considered in d.c. systems as the value of I_dcan be considered to be negligibly low.

Figure 3: Comparison of earthing in the IT system and TN system

The 1st fault in the IT system

The fault current I_d following the occurrence of a first fault against an exposed-conductive-part or earth is very low, and automatic shutdown is not necessary (IEC 60364-4-41:2017-03, 411.6.1), presupposing that the earthing requirement defined in Section 411.6.2 has been satisfied. This means that the protective conductor resistance R_A is parallel to the body resistance and that the already very low fault current flows through this protective conductor and that the touch voltage remains significantly below the maximum permitted value of 50 V in AC systems. This is of particular benefit in the medical sector.

The value of fault current I_d at a first fault is determined by the nominal voltage, nominal frequency and the parallel circuit comprising the system leakage capacitance and insulation resistance of the electrical system to earth. The fault current, in the event of a first fault, flows with negligible impedance between a line conductor and an exposed-conductive-part. With a good level of insulation in an electrical system, I_d can be approximated by the system leakage capacitance and can be calculated as follows:

For a 3-phase system

I_Ce = U⁄√3 ×3ω × C_e = U ×√3×ω ×C_e

For single-phase system

I_Ce = U × ω × C_e

Figure 4: Fault current Id with a first insulation fault in the IT system (backup circuit diagram)

Figure 5: Example of touch voltage UT after a first fault

Protection and monitoring devices

IEC 60364-4-41:2017-03, 411.6.3 states that the following monitoring and protective devices can be used in IT systems:

Insulation monitoring devices (IMDs);
Residual current monitoring devices (RCMs);
Insulation fault locating systems (IFLS);
Overcurrent protective devices;
Fault current protective devices (RCDs).

Subclause 411.6.3.1 defines that an insulation monitoring device (IMD) is required to report a first fault between a live part and an exposed-conductive-part or to earth. This device must issue an audible and/or visible signal that must last for as long as the insulation fault exists. It is advisable that a first fault is remedied as fast as practically feasible. "As fast as practically feasible" depends on the practical parameters relating to the system. Fundamentally, the IT system has a significant advantage in that an insulation fault does not have to be eliminated immediately but can instead by delayed, e.g., until the next maintenance interval for the system is due.

Insulation fault locating systems (IFLS)

With an insulation fault locating system (IFLS), faulty outputs and/or devices can be located during operation, i.e. there is no need to shut down the system. For troubleshooting purposes, measuring pulses are superimposed on the IT system that are in turn picked up and evaluated by measuring current transformers. Based on the assignment of the measuring current transformer/output, it is then easy to identify the faulty output.

Figure 6: Principle diagram of IT system with an IMD and several IFLS

Residual current monitoring equipment (RCM)

<p">Residual current monitoring devices (RCM) can only function with some limitations (see fault current protective devices (RCD))

Overcurrent protective devices

The configuration of overcurrent protective devices takes due account of the contents of IEC 60364-4-43:2008. For IT systems, the following points must be taken into account:

In medical IT systems, it is not permissible to have an overload protective device in the output circuit (secondary circuit) of the transformer, i.e. only short circuit protection is required. Therefore, the load current and temperature of the transformer must be monitored, and any variances must be reported (IEC 60364-7-710:2002, 710.411.6.3.101)
In accordance with IEC 60364-4-43:2008-08, 431.1.1, all live conductors must be protected (all poles). With reference to IEC 60364-5-55:2011+A1:2012, clause 557 ("Auxiliary circuits"), 557.3.6.1 states that "Unearthed a.c or d.c auxiliary circuits shall be protected against short-circuit currents by protective devices interrupting all line conductors". The same message can also be found in IEC 60364-4-43:2008-08, 431.2.2.
If overcurrent detection is needed for the neutral (N) conductor of 3-phase/N IT systems, this must cause all live conductors to be disconnected (IEC 60364-4-43:2008-08, subclause 431.2.2). This overcurrent protective device can be dispensed with if, for example, the N conductor is protected from overcurrent on the supply side.
A shutdown on all poles also helps to "disengage" all poles in the IT system in accordance with the 5 safety rules
At this point, a comment on the generally applicable comment from IEC 60364-4-43:2008-08, 433.3.3, that overload protective devices can be omitted for circuits supplying current-using equipment if an unexpected disconnection of the circuit constitutes a risk. In such cases, an overload alarm should be considered.

Figure 7: Explanation of the need for an all-pole overcurrent protective device in IT systems

Fault current protective devices (RCD)

In acc. with IEC 60364-4-41:2017-03 subclause 411.3.3, additional protection must be provided in AC voltage systems in the form of fault current protective devices (RCDs) for < 32 A sockets that are intended for use by non-specialists and for general use. However, this requirement is not practicable in allowing RCDs to achieve the desired protective action in IT systems. For a start, the operating principle of an RCD after a first fault requires a fault current I_d that must be above the rated fault current I_Dn of an RCD (e.g. > 30 mA). However, in practice, this is not the case. Even two independent insulation faults at both live conductors or connected equipment items do not cause an RCD to trip because these two faults act like a load.

In addition, IEC 60364-4-41:2017-03, states that additional protection for socket-outlet circuits using RCDs with I_Dn £ 30 mA is also required for IT systems if a fault current I_d > 15 mA flows in the event of a first fault. On closer inspection, however, it becomes apparent that this requirement is technically questionable:

Apart from the fact that sockets are the exception rather than the norm in IT systems, how should the fault current I_d be even determined in the planning phase? The fault current I_d is determined mainly by the cable length and the number of loads as well as the leakage currents of the electrical system to earth. However, no planner can determine these variables with even the best of planning.
RCDs currently available on the market do not have a response value of I_Dn < 15 mA. In addition, the German DIN VDE 0100-530 (VDE 0100-530):2018-06, 538.4 states that discriminating (direction selective) residual current devices are recommended in AC IT systems to avoid unwanted notifications/alarms due to leakage currents when high leakage capacitances are likely to occur downstream of where the residual current device is connected. The problem here is that such discriminating RCDs are also not available.

Para. 8: Two insulation faults on different live conductors "behind" an RCD

Para. 9 Division of system leakage capacitances "before" and "after" an RCD

Para. 10 Use of RCDs in branched IT systems

Use of AFDDs in IT systems

The primary objective governing IT systems is that they should not shut down unexpectedly when an initial fault occurs. There is therefore no point in using electric arc protective devices for final circuits up to 16 A in IT systems. This is also the subject of the latest DKE announcement in Germany in November 2017. This includes the following provisions:

The German standard DIN VDE 0100-420 (VDE 0100-420):2016-02, Section 421.7 does not include any requirements for electrical systems used in the field of application of DIN VDE 0100-710 (VDE 0100-710):2012-10, subclause 710.1. Areas used for medical purposes in senior citizen homes and care facilities in which patients receive medical treatment are not therefore included within the scope of DIN VDE 0100-420 (VDE 0100-420):2016-02, 421.7.
There is no need to use arc fault detection devices (AFDDs) on circuits that supply electrical consumables where an unexpected interruption on the power supply poses the risk of causing damage. This applies, for example:
- to IT systems that were installed to improve security of power supply or
- for electrical systems for safety purposes in accordance with DIN VDE 0100-560 (VDE 0100-560), in particular in safety lighting systems.

Further notes and requirements for installation and device selection

Selection of insulation monitoring devices

An insulation monitoring device is selected on the basis of the following criteria:

Maximum nominal voltage
Network type, i.e. AC, DC or AC / DC
Main circuit, control circuit or specialist application
System leakage capacitance
Response values
Inclusion of an insulation fault location system
Specific environmental conditions

To simplify the process of selecting insulation monitoring devices for the planner and user, the product standard for insulation monitoring devices, IEC 61557-8:2014/CORI:2016 includes further stipulations:

Designation of the corresponding application
A distinction is made between insulation monitoring device types based on possible components in the IT system:
- type AC IMD for pure AC IT systems
- type DC IMD for pure DC IT systems
- type AC/DC IMD for AC IT systems with directly connected rectifiers and for pure DC IT systems and for DC IT systems with directly connected AC inverters
A distinction is made between insulation monitoring device types for specific applications:
- Medical
- Photovoltaics

Figure 11: Examples for the identification of IMDs

Requirements on insulation monitoring devices (IMDs)

Important notes for the project planning of IT systems can be found in subclause 538.3 of DIN VDE 0100-530 (VDE 0100-530):2011-06¹⁾ "Selection and erection of electrical equipment– Isolation, switching and control". IMDs must comply with IEC 61557-8: 2014

Measuring results must not be affected by DC current components
IMDs must be connected symmetrically between external conductors and earth, or single-pole between any desired external conductor and earth (alternatively also to the N conductor in 3phase/N systems)
In cases where several IT systems are connected together, only one IMD must ever be active.
IMDs must be rated for maximum mains voltage
It is advisable to use IMDs that report any interruption in the measuring connections to power circuit conductors and earth.
Insulation fault location systems must comply with the requirements of IEC 61557-9: 2014-12 clause 4

Setting the response values

The response value must be set appropriately for the affected system. In accordance with the German standard subclause 538.1.3, a value of 100 Ω/V and, for prewarning, a value of 300 Ohm/V is recommended. In the previous edition of the standard, a guide value of 50 Ω/V is recommended. Both parameters are correct in principle and are influenced by the number of consumers and by the quality of the installation (e.g., humidity, dust etc.). In practice, the value displayed on the screen of the IMD is used to set an alarm value that is less than this displayed value, and that therefore represents the desired minimum value, leaving sufficient latitude for service and maintenance work. Also bear in mind here that all important system outputs are also in operation.
However, another advantage is that any significant change to the insulation resistance by switching on or off a load or system component is displayed by the IMD, enabling potential weaknesses to be identified.

Monitoring of offline loads

In accordance with VDE 0100-530 (VDE 0100-530):2011-06, subclause 538.3, an insulation monitoring device in TN, TT and IT systems can be used to monitor circuits that are switched off. This can, for example, include motorised winches, elevators and slide valve drives. This requires that the monitored electrical circuits are isolated from all poles of the system.

Figure 12: Off-line monitoring of a motor, e.g. on a crane

Correct response to a second insulation fault

In accordance with IEC 60364-4-41:2017-03, subclause 411.6.4, after an initial fault occurs, the conditions for an automatic disconnection following a second fault occurring on a different live conductor must be satisfied. In practical terms, this means that a defined level of loop impedance must be achieved. For IT systems without a neutral conductor, the loop impedance is defined as:

Z_S ≤ U/(2 × I_a )

Where:

U = Nominal AC voltage between the line conductors
I_a = Current (in A) that causes a protective device to be triggered within a time period defined in subclauses 411.3.2.2 / 411.3.2.3.

If for reasons of availability, if the system is not intended to shut down when an initial fault occurs, ensure that the prevailing fault current I_d is < 0.4 I_Δnwhen using RCDs. For a 30 mA RCD this means that I_d is < 12 mA.

Additionally, it should be stated that symmetrical faults on different live conductors should never cause a fault current that causes a system to shut down.

An RCD can be used for each consumable if and only if the shutdown conditions for overcurrent protection cannot be satisfied, because for example:

it is not possible to define the loop impedance precisely (cable lengths difficult to estimate, metallic materials close to lines)
the fault current is so low that the maximum permitted disconnection time when using overcurrent protective devices cannot be achieved
the loop resistance is too high to ensure an automatic shutdown
and additional equipotential bonding is not possible

Figure 13: The second fault in the IT system

N conductor in 3-phase IT systems

IEC 60364-4-43:2008-08 subclause 431.2.2 includes a comment to the effect that it advisable not to route an N conductor in IT systems. This should be taken into account whenever single-phase loads are also connected to a 3-phase/N IT system. If an insulation fault occurs in L1, the voltage on conductors L2/L3 to earth is increased to the phase-to-phase voltage, e.g., 400 V. This could damage suppression capacitors that are switched to earth. Ensure that the voltage offset only affects the voltage to earth. No voltage offset between active conductors arises. Single-phase equipment must be configured appropriately, i.e., they must be suitable for operation in 3-phase /N systems. In practice, two separate IT systems are frequently set up, one for single-phase loads and one for 3-phase loads.

At this point, a comment on the generally applicable comment from IEC 60364-4-43:2008-08 sub-clause 433.3.3, that overload protective devices can be omitted if an unexpected disconnection of the circuit constitutes a source of risk. In such cases, an overload alarm should be considered.

Summary

IT systems are always of greatest benefit when they protect against the disconnection of the power supply in the event of a first fault. The fundamental basis for fault-free and safe operation comprises setting the system up in accordance with the standards and the correct choice of protective and monitoring devices.

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