The IT system is little known in practice and is also hardly used in universities and training centers. As a result, the grounded system has become established and continues to spread. The IT system is primarily used where human lives are ultimately at stake - for example in operating theaters and intensive care units or in railroad signaling technology. However, the advantages listed below can also be used for many other applications.

The advantages of the unearthed system (I)

Inherently safe

The IT system offers inherent protection against high touch voltages. It differs from the TN or TT system mainly in that there is a conductive connection between the star point of the transformer supplying the system and earth. This conductive connection is present in the earthed system, but is omitted in the unearthed system. As an alternative to the supply transformer, an IT system can also be created using a power source such as a battery. In an unearthed system, all-pole protection is required for all active conductors. The same applies to the N conductor (if present). As neither conductor is earthed in a single-phase system, there are two live conductors instead of the classic "phase and neutral".

The insulation monitoring device 'iso685' is used to monitor unearthed power supply systems (IT systems).

© Bender

But what is the big difference in the effect if there is only such a small difference in the design?

If a person touches an uninsulated live conductor or a live conductive housing in an intact unearthed system, nothing happens! The reason for this is that current can only flow in a circuit and the circuit has not been closed because the star point of the transformer is not earthed. It's like a bird on a high-voltage line - it's safe.

The grounded system is different: Here, a closed circuit is provided in advance and, to a certain extent, waits for the fault. In this case, if a person touches a live conductor or a live conductive housing, a fault current immediately flows through the person due to the low-resistance connection to the supply transformer. This would be dangerous without functioning, fast-switching protection technology. This circuit is therefore protected by fuses and residual current devices (also known as RCDs or RCDs): in the event of a fault, the circuit is switched off quickly enough before the person suffers any significant damage. To ensure that this works, the protection technology must be checked regularly. For example, the RCDs in electrical systems must be tested for functionality every six months - even in private households. But how often is this actually done?

The IT system is therefore inherently protected against high touch voltages. The only exceptions to this are AC systems with very large system leakage capacitances and asymmetrical loads. Countermeasures here are, firstly, splitting into smaller subsystems and, secondly, measuring the capacitance and calculating the maximum body current in the event of a fault, which is possible, for example, with the "Isometer iso685" insulation monitoring device from Bender. Of course, insulation faults in the IT system must also be eliminated promptly in order to keep the system at a high level of insulation.

The advantages of the unearthed system (II)

Fault localization

Using so-called insulation fault location systems (IFLS), insulation faults can be localized during operation or when switched off. Devices for stationary installation and mobile devices are available for this purpose. In principle, this is also possible in earthed systems based on residual current technology (RCM) - but with the restriction that this only works in switched-on systems and, in contrast to the IT system, is limited to asymmetrical insulation faults.

No unwanted interruptions to operation

As already mentioned, the IT system is inherently safe, which has the side effect that no shutdown is required in the event of an insulation fault. This is also the reason why IT systems are mandatory in intensive care units, for example: In the event of an isolation fault, the life-support equipment continues to be supplied. The IT system is therefore particularly suitable for all applications in which shutdowns are undesirable, serious or costly - for example in the process industry, in data centers or in automation.

Control circuits of all kinds are of particular importance. Malfunctions and failures of control circuits - for example in a transformer station or a nuclear power plant - can have serious consequences. Based on the information provided by the insulation monitoring device, maintenance and repair measures can be planned in the IT system for the long term and unplanned operations to rectify faults can be avoided.

The advantages of the unearthed system (III)

Detecting deterioration at an early stage

Another decisive advantage is that deterioration in the insulation level can be detected immediately. In an earthed system, residual currents can be resolved in the single-digit mA range using highly developed residual current technology (RCM technology) - but no further. Even if only the ohmic components of the residual current could be selected, this means that at a system voltage of 400 V and a resolution of 10 mA, the deterioration of the insulation level can be detected below 40 kΩ. Compared to an earthed system that is not monitored and simply switches off unintentionally at some point, this is an enormous improvement. However, an insulation value of 40 kΩ already corresponds to the recommended main response value for the IT system. Measurements can be made in the IT system in the mega-ohm range and above; compared to the earthed system, this means a factor of at least 1000. This means that insulation deterioration in the unearthed system can be measured and rectified much earlier.

Detecting symmetrical faults

Symmetrical faults can be detected in an IT system using an active insulation monitoring device in accordance with IEC 61557-8. Symmetrical faults are insulation deteriorations of the same magnitude on all phase conductors. Such faults are not uncommon - for example, the insulation values in photovoltaic systems often deteriorate equally on the positive and negative sides.

The advantages of the unearthed system (IV)

Measurements in DC networks

RCDs for pure DC systems such as battery systems are not currently available. Options for this are either devices for residual current monitoring (RCM) with a DC supply voltage or the design as an IT system with insulation monitoring. The "Isometer iso685" also offers the advantage that it indicates in DC systems whether the fault is on the positive or negative side.

The insulation monitoring device 'iso865' in an electrical installation.

© Bender

Measurements in mixed AC systems with DC components

If there are battery systems, converters, switching power supplies etc. in the AC network, DC residual currents are possible. The widely used type A RCDs for pure AC systems are unsuitable here. This means that only type B RCDs can be used in earthed networks or it must be ensured by other means (using RCM technology) that DC currents above 6 mA are switched off. A sensible alternative to this is to operate the system as an unearthed system and monitor it with an insulation monitoring device.

Offline monitoring

As an insulation monitoring device in accordance with IEC 61557-8 actively measures in the IT system, it can also monitor IT or TN systems with all poles disconnected. This is important, for example, for point heating systems, fire pumps on ships and redundant cooling systems in nuclear power plants. In summer, for example, it is possible to detect whether a point heating system has an insulation fault and repair it in good time. Otherwise, the fault would only be noticed when it is switched on in winter - in the form of an immediate failure of the system exactly when it is needed.

The advantages of the unearthed system (V)

Closing the gap between periodic tests

The insulation monitoring device prescribed in the IT system permanently monitors the insulation value. In contrast, only the current insulation condition is recorded during regular tests (keyword: test in accordance with DGUV regulation 3). This can deteriorate dramatically immediately after the test and then remain unnoticed for a long time. Permanent monitoring is also possible in the earthed system through the additional use of residual current monitoring systems (RCM technology).

Fire prevention

Insulation faults in electrical installations are the most common cause of fire. In the IT system, the probability of fire is very low: Firstly, insulation faults can be detected and rectified at an early stage. Secondly, due to the lack of a low-resistance return path, no current flows in the event of an insulation fault that would be high enough to cause a fire. Here too, the restriction to systems with not too large a system leakage capacitance applies.

Long-term observation

The "isometer iso685" and "iso1685" can seamlessly record grid parameters with date and time assignment over many years. In conjunction with other recorded system events, this enables event-based fault analysis and facilitates the detection and elimination of sporadically occurring faults as well as improving the decision-making basis for future investments. The analysis can be carried out on the device itself or via Ethernet.

The advantages of the unearthed system (VI)

Safe handling of non-linear loads

Today's grids contain fewer and fewer linear (resistive) loads. The light bulb has been replaced by energy-saving lamps or LEDs, computers and televisions are connected to the mains via switching power supplies, the washing machine contains a frequency converter and frequency converters are used in large numbers for motors in industry. A powerful insulation monitoring device in the IT system has no problem with this and measures the insulation value of the entire system correctly. The IT system is particularly suitable for use with inverters: In the IT system, DC currents and the associated saturation effects in iron cores cannot destroy the inductive elements of feeding generators and transformers, even in the event of a saturated insulation fault in the DC link of large inverter drives.

The insulation monitoring device 'isoPV1685' is suitable for unearthed DC systems up to 1500 V in photovoltaic systems.

© Bender

The "isometer iso685" was developed for monitoring networks with frequency inverters and enables a logical linking of system parameters in order to trigger an automatic shutdown of drives in the event of a critical system status. Fault differentiation in inverter drives according to DC link and motor side is possible with the "iso685" without additional expenditure and other devices.

No stray currents

In earthed systems, stray currents - currents that do not flow via the L, N and PE conductors but seek other paths - often cause problems. They cause corrosion and pitting in pipelines, lightning protection systems, ball bearings, foundation earth electrodes and other conductive parts. They can also destroy the shields of signal cables and even cause fires. In addition, magnetic interference fields can occur that cause problems with IT and communication systems.

As the return path to the star point of the transformer is not closed in the unearthed system, stray currents cannot propagate in unearthed systems.

More stable during transients

IEC 62109-1:2010 describes the possibility of reducing the overvoltage category from ÜK IV to ÜK III through isolation via isolating transformer, optocoupler or similar galvanic isolation, because transients do not result in currents as high as in earthed systems. The practical consequence of this is that components in the electrical consumers in the IT system experience less stress from voltage peaks and therefore have a longer service life.

The disadvantages of the unearthed system

But of course there are also disadvantages to the IT system:

Size limitation

Very large IT systems can become unwieldy and have an unintentionally high grid leakage capacity. It is therefore advisable to divide very large IT systems into separate units using isolating transformers, which can result in additional costs and power losses, although these are usually negligible overall. The division into electrically isolated subsystems also has advantages, such as the filtering effect against interference or the possibility of individually adapting the voltages to the consumers supplied.

The insulation monitoring device 'isoPV' with the coupling device 'AGH-PV' monitors unearthed power supply systems in photovoltaic systems.

© Bender

What exactly constitutes a very large system must be assessed on a case-by-case basis and depends on the system parameters. For example, the world's largest PV fields can be completely monitored by individual "isometers" of the "isoPV" type - which means that a single "isometer" will not miss a faulty plug connection, a damaged cable or a defective PV module, despite the system size of ten or more soccer pitches.

Voltage increase in the event of an insulation fault

In an IT system with an insulation fault on one conductor, the phase-to-phase voltages of the other conductors increase in relation to earth potential. In the event of a full earth fault on a conductor in the 230 V system, the voltages of the other conductors increase to around 400 V with respect to earth potential. System components in which the potential to earth plays a role, in particular Y capacitors and overvoltage limiters, should therefore be suitable for the maximum rated voltage. The voltage increase can be avoided if the secondary side of the transformer is designed in delta form.

The IT system as a good choice

Compared to earthed systems, the IT system has many advantages as described above. It is not only suitable for the high requirements in the operating room or in a nuclear power plant, but for almost any application. In many cases, it is not even considered today, although it would be the better choice. The current generation of isolation monitoring devices offers many economic and technical advantages that benefit the operator. Sometimes the cost of an isolation monitoring device is cited as an argument against an IT system, but the opposite is true: given the benefits listed above and the associated economic impact, deployment in the commercial sector is always an option worth considering.

Author:

Dr. Dirk Pieler is the former Managing Director of Bender in Grünberg.