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Friedrich Hendel
Siemens AG Stuttgart, I&S IS STG 17GL
Anton Kohling
Siemens AG Erlangen, A&D ATS SR
Diethard Möhr
Siemens AG Erlangen, I&S CTF
November 2008
Designing low voltage supply systems for
electromagnetic compatibility
1. Introduction
Electrical safety of low voltage supply systems is the subject of many national and international standards. Installation rules providing electromagnetic compatibility were introduced during the last decade, but still are not well known to installers and system designers.
From the perspective of electromagnetic compatibility, there must be no operating currents, not even stray currents (as defined in IEC 60364-5-54) flowing through the earth wire and earthing and equipotential bonding system.
In an earthed supply, this requirement can only be achieved by a TN-S system. This is the only way to prevent galvanic couplings with other circuits and cable shields and to reduce the sum of currents in one cable or bus system roughly to zero, thus minimising the line frequency magnetic fields. Practical measures necessary in order to reach these objectives are discussed.
2. The advantages of the TN-S system
In single-fed supplies, a TN-S system is easy to realize. Figure 1 shows two equivalent solutions to supplying a TN-S system from a single transformer.
When planning and implementing multiple supplies, special features must be used in order to match the electromagnetic compatibility objectives to be attained as initially formulated (Table 1).
Figure 2 shows two options for a single supply from two differently located transformers or generators. Option A consists of an insulated PEN conductor, while option B has a 4-pole switch for switching off the PE conductor.
The existence of a PEN conductor in a TN-S system initially caused some confusion. The reason for choosing this designation was because this conductor may carry operating as well as fault currents. Until recently it has been common practice to earth each neutral point in a multiply fed system individually, but this practice is at present abandoned by standards, since it allows both return currents as well as fault currents to split among the return and earthing conductors. This needs to be avoided by installing one and only one connection between return conductor and earth, however many feeding points there may be in an installation. As a side effect problems may arise when a fault occurs e. g. inside a generator or transformer leading to extraneous conductive parts to go live. The fault current no longer stays inside the device, but passes via the protective conductor, along the sole existing central earthing point (CEP) and back along the return conductor. Therefore, a return conductor is called PEN from one of the power sources to the CEP, while for the remainder is designated neutral conductor N.
The old prohibition to switch a PEN conductor remains valid as long as there is not, as in this case, a PE conductor installed in parallel.
Figure 3 shows the measures formulated in Table 1 in case of multiple supplies.
Figure 4 shows the stray current: operating currents split among the PEN conductors, the cable screens and the equipotential conductor in a TN-C system according to Kirchhoff’s Law: operating currents flow through the conductive parts of the building.
Figure 5 shows the same situation in a TN-S system planned and implemented to be electromagnetically compliant.
Figure 1b: TN-S system supplied by one transformer
The following measures need to be taken when installing multiply supplied systems: • Neutral points of the sources not to be connected directly to earth, but routed
towards the low voltage main distribution system.
• These conductors must be designated as PEN conductors, since they carry both operating current and have a protective function.
• PEN busbar has to be connected to earth only once at the main earthing terminal bar in the low voltage main distribution, and thus only once within the entire system.
• The protective conductor has to be connected to the PEN conductor only at one point. The N and PE conductors shall not be contacted to each other anywhere else within the system.
• The protective conductor may be multiply earthed; in fact it is recommended that it be connected to earth at as many points as possible.
• Attention must be paid to the spatial extent of the installation, so as not to impair the effectiveness of the protective measures.
Table 1—Contextual relationship between EN 50174 series and other relevant standards
Building design Generic cabling Specification Installation phase Operation phase phase design phase phase
EN 50310 EN 50173 series EN 50174-1 EN 50174-1 except
EN 50173-4
5.2: Common bonding 4: Structure 4: Requirements for 4: Requirements for
network (CBN) within a specifying installations specifying installations
building 5:Channel performance of information technology of information
cabling technology cabling
6.3: AC distribution 7:Cable requirements
system and bonding of 5: Requirements for
the protective conductor 8:Connecting hardware installers of information
(TN-S) requirements technology cabling
9:Requirements for cords and jumpers
A:Link performance limits
Planning phase
EN 50174-2 EN 50174-2 and
EN 50173-4
4 and 5: Structure 4: Requirements for 5: Requirements for the
planning installations of installation of information
6: Channel performance information technology technology cabling
cabling
8: Cable requirements 6: Segregation of metallic
information technology
9: Connecting hardware 6: Segregation of metallic cabling and mains power
requirements information technology cabling
cabling and mains power
10: Requirements for cords cabling
and jumpers
7: Mains power and
A: Link performance limits lightning protection
and and
EN 50174-3 EN 50174-3
and and
(for equipotential bonding) (for equipotential bonding)
EN 50310 EN 50310
5.2: Common bonding 5.2: Common bonding
network (CBN) within a network (CBN) within a
building building
6.3: AC distribution 6.3: AC distribution
system and bonding system and bonding
of the protective of the protective
conductor (TN-S) conductor (TN-S)
and EN 50346
4:General requirements 5: Test parameters for balanced cabling 6: Test parameters for optical fibre cabling
Figure 2: TN-S system alternatively supplied from various supplies
Figure 4: Stray and operating currents in the PEN, PE and N conductors in an installation with a TN-C system
The figures clearly show the necessity for professional quality planning and construction and maintenance staff training, particularly in order to avoid additional connections between PE and PEN to be made later.
3. Line frequency magnetic fields in the TN system
Blurred images, unsteady characters and color distortions can make it difficult to work with a CRT. These disturbances are often caused by external low frequency magnetic fields [1].
In the TN-C system the PEN conductor is often multiply earthed, i.e. part of the load current flows through the equipotential bonding and earthing system, as well as all across any earthed metal part (extraneous conductive parts), such as heating pipes, etc. These currents are missing in the feeder and in the cables to consumers and sub-distribution panels, so the currents within one cable do not cancel out. This results in a significant magnetic field in the proximity of cables, causing disturbances in particular to nearby CRT’s. These disturbances are noticed as an unsteadiness of characters due to the interference between the image deflection frequency and the 50 Hz of the supplied current. Depending on the screen size, CRT’s are unsuitable due to an external magnetic field strength of approximately H≈0.5 A/m. This may be expressed as a function of the current. If a current of just 10 A does not flow back along the cable it came through? As a first approximation, the cable can be seen as a single conductor of infinite length for carrying 10 A. The magnetic field strength is found from Ampere’s law:
For 0.5 A/m this results in a distance d of
Thus all areas closer than 3 m to the cable are unsuitable for CRT’s. This is merely a simple example. In buildings with a TN-C system, complete sections or entire floors may be unsuitable for CRT’s. These influences can only be avoided by implementing a TN-S, TT or IT system or by using flat screens instead of CRT’s.
In the TN-C system not only the fields discussed above have a detrimental influence on electromagnetic compatibility. The galvanically coupled share of the operating currents flowing through all conductive metal parts, including screens of signal lines and cable shields, may result in undesired influences. In addition, a high proportion of 150 Hz current flows through the building, since even under symmetric load they do not cancel out in the PEN conductor as they have a homopolar nature.
4. Alternative supply of a consumer from two directions
If for purposes of redundancy, a consumer is supplied from power sources installed at different locations, four-pole switching must be used so that the neutral conductor of the consumer is not wired parallel to the PEN conductor in the main low voltage supply system. Then the current cannot split between these two conductors and the total current in the supply cable becomes zero. Figure 6 shows the situation.
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Figure 6: Alternative supply of a consumer from different power sources
5. Concentric earth wire
Even in a fault-free TN-S system, significant currents can be measured in the earthing conductor. The reason is that the cable earthing conductor is connected to the equipotential bonding system or earth at both ends, so that it acts as the short circuited winding of a transformer. These currents can be reduced significantly through the use of cables with a concentric earth wire. This is illustrated by an experiment [2].
Example:
•
Cable length 10 m, rated current 80 A•
Earth wire connected at both ends to the equipotential bonding system a. Cable type with one core used as earthing conductor:An induced current of 1.6 A was measured in the earthing conductor. b. Cable type with screen:
An induced current of 27 mA was measured in the earthing conductor.
6. Leakage currents from filters
Unfortunately this term is inappropriately chosen. It is technically sound to speak of leakages against earth through faulty insulators and parasitic capacitances, but the currents to be discussed here mainly arise from the Y-type capacitors that connect a certain filtering capacitance from L to PE and from N to PE. Especially in inverter drives relatively large capacitors are used for filtering the high frequency disturbances. Leakage currents with plug-in-ready equipment are also limited by limiting the Y-type capacitors in the appliance standards. Higher leakage currents are to be expected only with hard-wired
devices or those connected by CEE plug connectors. To reduce the leakage currents, it is advised to use so-called low-discharge filters. In these filters the capacitors are wired from the phase conductors to the N conductor, and a common capacitor from the N conductor to the earth wire. In this way the leakage current is determined by the difference in potential between the N and PE conductors. This makes it possible to reduce the leakage currents substantially in the entire grid. Leakage currents can be regarded in the potential equalization system as partial operating currents.
7. Residual current devices (RCD)
The majority of the above measures are necessary even when RCD’s are used. If the sum of currents in a protected circuit deviates from zero, the difference is registered as a fault current and the protective device performs its protective function and trips.
8. Summary
Planning and setting up low voltage supply systems for proper electromagnetic compatibility is not trivial, and the rules and their backgrounds are certainly new for many of those involved. The basis for ensuring electromagnetic compatibility in buildings and facilities is an earthing and equipotential bonding system, free of operating current. The indicated measures must be taken into account and coordinated with the safety requirements of IEC 60364, but setting up low voltage supply systems for proper electromagnetic compatibility involves more than earthing for voltage protection in accordance to the standards.
9. Literature
[1] G., Zimmer, Beeinflussung von Bildschirmarbeitsplätzen durch Magnetfelder; EMV von Gebäuden, Anlagen und Geräten ['The influence of magnetic fields on CRT workstations; electromagnetic compatibility of buildings, facilities and equip-ment']. VDE-Verlag, pp. 357-366; ISBN 3-8007-2261-5.
[2] B. Jäkel and R. Messer, Niederfrequente Streufelder von Energiekabeln und deren Kopplung mit Masseschleifen ['Low-frequency stray fields of energy cables and coupling them with earth loops']. EMV 96, 5th International Trade Exhibition and Congress on Electromagnetic Compatibility, pp. 71 – 78.
