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A significant advantage of this integrated data acquisition is its speed and the comprehensive overview of all data. This facilitates the detection of faults, which would only be partially perceived – or even entirely missed - by a single system. The user is, therefore, able to react before fuses or residual current devices (RCD) switch off affected systems or socket power circuits. This applies in particular to hidden rising residual currents (e.g. triggered by an insulation fault), highly operating currents and any other overloading of system parts or users (image 1).
Other sources of faults are massive grid feedback effects or resonance effects due to a growing number of non-linear electrical loads. If irregular grid parameters such as excessively high harmonics or residual currents are detected intime, it is possible to initiate repair measures before a device fails and in order to avoid downtimes, or at least plan or reduce them
As previously mentioned, RCM is playing an increasingly important role with high availability power supplies, which are nowadays found in almost all market segments. Constant processes and especially sensitive applications such as computer centres, hospitals and semiconductor factories are depending on RCM in particular. Furthermore, RCM measurement offers a good alternative in all areas in which it is not possible to utilise insulation resistance measurements and residual current devices due to local or operational circumstances. The described „foresighted“ monitoring also helps to reduce alarms, as required for example with alarm management according to EEMUA 191 or NAMUR NA 102.
However, RCM can do even more – in particular to reduce the risk of fire! Residual current, triggered by defective insulation, can be insidious. The current level is determined by the power of the supply network, the insulation fault resistance and the resistance to ground. With a sufficiently high current flow the upstream protective device disconnects the electrical consumers from the mains. However, if the residual current is too low then the protective device will not trigger. If the recorded fault power exceeds a value of approx. 60 Watt (approx. 261 mA at 230 V), a risk of fire exists. Residual current monitoring, therefore, also serves as fire prevention. The next section explains how RCM works in detail.
The basic functionality of the residual current principle is shown in image 2. Here, the phase and neutral conductor of the protected output are fed through the summation current transformer, the ground wire is left out. To provide a better overview the image shows a simplified circuit. In practical terms, all three phases and the neutral conductor run through the summation current transformer. If the system is in fault-free condition, the summation current is zero or close to zero (within a tolerable range), meaning that the current induced in the secondary circuit is also zero or close to zero. If, however, residual current flows away to ground due to a fault, the residual current in the secondary circuit will result in a current being logged and evaluated by the RCM measuring device (image 3).
Modern RCM devices accept different threshold value settings here (image 4). A static threshold value has the disadvantage that it is either too high with a part load, or too low with a full load, i.e. either insufficient protection is provided or erroneous alarms are issued, which may have over time negative effects on the attentiveness of the monitoring personnel. For this reason it is advisable to use RCM measuring devices with dynamic threshold value formation. In this case the residual current threshold value is formed on the basis of the actual load conditions and is, therefore, optimally aligned with the respective applicable load (image 5). Through parameterisation (i.e. stipulation of the typical residual current in „GOOD“ condition) of the system in new condition and constant monitoring, all changes to the system state after the point of start-up can be detected. This also enables detection of creeping residual currents.
Examples of „modern fault sources“ include collapsing polypropylene PFC capacitors. These serve to compensate reactive currents, which can be generated for example with three-phase motors. Paradoxically, a fault, therefore, arises due to equipment that is actually intended to improve the energy supply. With these capacitors, an overload or excessively high temperature frequently results in melting of the PP winding. The melt in turn causes a high-resistance short circuit to ground. It is not possible to shut off such short circuits to ground with conventional protection measures (HRC fuse, circuit breaker). The constant residual current usually leads in the mid-term to a dead earth short circuit and may pose a considerable risk of fire or endanger safety under certain circumstances (image 6). The residual current measurement detects such faults and enables rapid countermeasures. In this way it is possible to avoid costly and dangerous system failures.
Errors such as impermissible connections between the N and PE phase also frequently arise during installation. Sometimes the two phases are simply interchanged. Image 7 shows a typical connection error, which can easily result in a residual current of 5000 mA. With RCM, such errors are detected immediately during the installation and are reported via the alarm management.
A further and rather new type of fault source is a large number of single-phase loads, such as switched mode power supplies from servers in computer centres or PCs in office buildings. These generate a high proportion of 3rd harmonics. These harmonics lead to interferences on the neutral conductor rather than being nullified via the transformer windings. This may result in overloads on the N phase. Integrated measuring devices, such as the UMG 96RM-E, enable comprehensive monitoring of all phases and are, therefore, able to report increased neutral conductor currents in a time.
In this connection we also refer to the safety specifications of the VdS (association of insurers in Germany) for electrical systems up to 1000 Volt: „VdS 2046 : 2010-06 (11) 3.2.4 In order to increase the safety of electrical systems in which numerous non-linear loads (such as frequency converters, phase angle-controls e.g. in lighting systems) are operated, measurement of the current in the neutral conductor should take place regularly - e.g. once annually and additionally after any significant changes to the electrical system or the type and quantity of electrical loads. If the safety of the system is at risk due to excessively high harmonic currents, measures must be implemented in order to protect the harmonics according to the publication ‘Lowfault electrical installations’ (VdS 2349).“
Image 6: Destroyed PP reactive power compensation capacitor: A creeping high-impedance ground fault has led to the complete melting of the capacitor and a local source of fire.
Image 7: N- and PE have been interchanged here
IT technology itself places high demands on the supply. However, particularly critical are applications in which the loss of data simply cannot be allowed to occur. BITKOM, therefore, writes the following in its guidelines for „Operationally reliable computer centres“: „In computer centres the maximum availability requirements apply. The energy supply must, therefore, be permanently guaranteed. Therefore comprehensible is the requirement that the power supply to the computer centre itself, and to all areas in the same building to which data cables run, must be designed as a TN-S system. Permanent self-monitoring of a “clean“ TN-S system and the issuance of signals to a permanently manned desk, e.g. in the control centre. The electrical engineer will detect any action required on the basis of signals received, and can avoid damages through targeted service measures.“
With the Janitza solution, the safety criteria „RCM residual current monitoring“ can be realised through this type of EMC-optimised TN-S system (image 8).
The VdS states the following regarding harmonics/ the installation of power supply systems:
„In the case of power supply systems with PEN phase, operational currents - which may cause damage - flow through the entire ground and potential equalisation system (see section 3.3). With new electrical system installations it is therefore necessary to plan TN systems as TN-S systems. In the case of existing TN-C systems, modification to a TS-S system is advised. TN-S systems must be realised from the supply (handover) point where possible.
In order to guarantee the functionality of a TN-S system on a permanent basis (no conductor short between the N and PE phase, interchanging of the N and PE phase) this must be monitored by a residual current measurement device (RCM).
If the set trigger value is reached, a perceivable optical and acoustic error signal must be issued, in order that the defect can be eliminated immediately. In order that signal issuance is successful, this should be sent to a manned desk where applicable. If signalling is dispensed with then the forced shut-down of the faulty current circuit is required...“
In addition, the Vds prescribes the following, with regard to the safety regulations for electrical systems up to 1000 Volt: „VdS 2046 : 2010-06 (11)
3.2 Compliance with proper condition
3.2.3 In order to guarantee safety in electrical systems on a permanent basis, if it is not possible to carry out insulation resistance measurements due to local or operational circumstances then it is necessary to implement substitute measures. Such measures are described in the publication „Protection with insulation faults“ (VdS 2349).
An anderer Stelle, bei den Sicherheitsvorschriften für elektrische Anlagen bis 1000 Volt schreibt der VdS vor:
„VdS 2046 : 2010-06 (11)
3.2 Erhalten des ordnungsgemäßen Zustandes
3.2.3 Um die Sicherheit in elektrischen Anlagen auf Dauer zu gewährleisten, wenn Isolationswiderstands- Messungen aus örtlichen oder betrieblichen Gegebenheiten nicht durchgeführt werden können, müssen Ersatzmaßnahmen getroffen werden. Solche Maßnahmen werden in der Publikation „Schutz bei Isolationsfehlern“ (VdS 2349) beschrieben.“
Eine adäquate Ersatzmaßnahme ist hier die permanente RCM-Überwachung!
Image 10: Mains feedback effects due to frequency inverters
Image 11: Critical voltage drop with production standstill
Connections between the N and PE phase result in „stray“ operating currents being distributed across the PE system, via data lines and all metal building parts. Because these currents are not equalised, they generate electromagnetic fields. Various malfunctions in the electrical systems, IT networks and pipe systems of building installations are the consequence. Image 12 shows how the operating current can distribute at the PEN bridge and flow back via multiple paths, whereby the sum of the supply and return conductor current is no longer 0. This may lead to the following malfunctions:
The supply and return conductors, also in distribution systems, must be positioned close to each other in order to minimise magnetic fields. At every junction in a current circuit the sum of the currents must be equal to zero, in order to avoid residual currents. Additionally, the sub-distribution or current circuit should be monitored by an RCM. The UMG 96RM-E is very well suited for monitoring sub-distribution or larger loads. Individual current circuits, in which no residual current circuit breakers can be used due to operational reasons, can be monitored with the UMG 20CM. A signalling RCM in combination with the technical staff at site provide the maximum alternative safety.
We are based in Lahnau, a Hessian town located between Wetzlar and Gießen, where we develop and manufacture for the German and international market. For more than half a century, our hardware and software products have always remained a little ahead of their time. We introduce new technologies and combine existing applications into convincing, intelligent products and solutions. We serve different market segments in 60 countries with a network of competent sales partners who support our customers directly at their location.
Our portfolio
Janitza's extensive product portfolio ranges from current transformers to measurement devices, from communication equipment and IT environments through to software solutions and data analyses.
