THE IMPORTANCE OF POTENTIAL EQUALIZATION
by Armin Gärtner Dipl.-Ing. [MEng.]
The following article describes the importance of supplementary potential equalization (SPE) for medical technology. A distinction must be made between permanently installed, general potential equalization in accordance with DIN VDE 0100 Parts 410 and 540 and SPE, which is to be separately connected via a flexible cable with special plug, for mobile active and non-active medical devices which are mobile and mobile at a specific location. SPE is standardized in accordance with VDE 0107 (old) and VDE 0100 Part 710 (new).
Due to their design, normally only very small touch voltages exist on touchable device parts, which become device leakage currents when touched. In the event of a fault, larger device leakage currents can occur in the form of fault currents. In the first fault, these residual currents can lead to high touch voltages if no supplementary potential equalization is available.
Supplementary potential equalization thus represents a preventive measure to protect the patient, user and third parties from touch voltages that can occur due to voltage potentials between touchable conductive parts and mobile equipment.
In the following, the necessity of potential equalization and the associated safety philosophy of supplementary potential equalization (SPE) are described so that the measure, which is often neglected or ignored in practice, is better understood and applied.
1. PERMANENTLY INSTALLED SUPPLEMENTARY POTENTIAL EQUALIZATION
Fig. 1: Schematic cable routing of the protective conductor and the potential equalization in operating theaters with indication of possible flow directions of compensating currents
In the buildings of a hospital, in addition to electrical installations, a large number of conductive pipelines are installed, e.g. made of copper for water or also for gases, which can conduct electricity well due to their large cross-sections. These pipelines are foreign conductive parts according to DIN VDE 0100 part 200. These also include the conductive building constructions or the mounting which is inserted into the concrete.
Since the scope of heavy current installations has increased more and more in recent years, it is to be expected that electrical currents will flow not only via the intended protective conductors but also via conductive pipelines according to Kirchhoff's laws. Such currents may flow not only in the event of a fault but also during normal operation.
For this reason, a pipeline system, for example, can be supplied with voltage during normal operation, and even more so in the event of damage. If no potential equalization is present, an electric current can flow through a person touching two different piping systems, one of which is earthed. Depending on the contact resistances, the contact voltage for a simple earth fault is below the 230 V mains voltage.
These touch voltages (potentials) must be reduced to values that are harmless to humans by connecting all conductive systems such as pipelines or equipment housings using potential equalization conductors. For complete potential equalization, foundation and lightning earthing plugs, inner gas pipes, water pipes, heating supply and return lines, steam pipes, medical gas pipes, etc. are connected to the main potential equalization busbar near the low-voltage main distribution board.
The illustration shows how possible compensating currents, which during the operation of motors, lifts, air conditioning systems, etc., can endanger the patient via ferrous-reinforced armoring from other building areas via operating theaters, operating theater supply units, equipment, etc., if no potential equalization exists or is not connected.
The voltmeter drawn above the operating table indicates possible potential differences between the individual pieces of equipment. These can allow compensating currents to flow during surgery over and through the patient's heart.
Requirements of the previous VDE 0107 power current systems in hospitals and rooms used for medical purposes outside hospitals.
VDE 0107 divides medically used rooms into application groups (AG) 0, 1 and 2. For AG 2 rooms, a medical IT network with floating potential and a supplementary, special potential equalization is generally required. Medical IT networks are used so that no disconnection occurs during the first body or earth short. Only in the event of a short circuit or excessive overload is it not possible to continue operating the medical electrical equipment.
Note: The term IT network as a designation for this particular form of power supply should not be confused with the identical term IT network of information technology (IT).
In AG 2 rooms (operating theaters, intensive care units, left heart catheter rooms) in accordance with DIN VDE 0107, in addition to the protective measures in accordance with DIN VDE 0100 Part 410, all foreign conductive parts within the patient environment are electrically connected to each other and to the protective conductor busbar. With this measure even very low touch voltages can be achieved.
This means, for example, for an operation, that all:
- permanently installed operating tables, unless they are protection class I devices,
- the conductive surgical floor,
- ceiling mounted pendants, unless they are protection class I devices
- sinks provided they are installed in the patient's vicinity and are foreign conductive parts,
- metal frames provided they are in the patient's vicinity and are foreign conductive parts, and
- stainless steel worktops, provided that they are installed in the patient's environment and are foreign conductive parts
must be included in the potential equalization. The establishment of a supplementary potential equalization requires the functionality of the main potential equalization. The nominal cross-section of the main potential equalization conductor is calculated in accordance with DIN VDE 0100 Part 540.
Behind these measures are considerations and experiences to avoid possible voltage differences, which can occur as voltage sources, in the vicinity of the patient or on the patient. These voltage sources can cause currents via the body resistance, which not only flow through the patient but can also impair or even endanger the doctor and nursing staff. In addition, the function of active medical equipment is also impaired by draining currents or even by their malfunction.
REQUIREMENTS OF THE NEW VDE 0100 PART 710
The previous VDE 0107 was replaced by the new VDE 0100 Part 710 at the end of 2002. The supplementary potential equalization is required unchanged, as prescribed. The metrological verification of potential differences in rooms of group 2 is omitted. This requirement has been abandoned because it is a standard for the construction of new installations or the adaptation of old installations after substantial modification. The new standard contains a strict PEN prohibition, i.e. PE conductors and N conductors may no longer be combined up to the floor distributor as TN-C networks but must be separated from the main distributor as shown in Fig. 2. The new VDE 0100 Part 710 prescribes a TN-S network for new buildings and conversions in which there is no connection between PE and N when correctly installed. Figure 2 shows a TN-C network on the left and the TN-S network to be installed for conversions and new buildings on the right.
In the TN-C network, a parallel connection to pipes, shields of EDP networks, reinforcements and other conductive bodies must be observed, whereby a division of the reverse currents results depending on the conductivity and the diameter of the materials. Partial currents flow back to the source via the PEN. A current clamp can be used to determine the current flow in the main potential cable. Currents flow over all conductive parts, i.e. over all materials that are conductive and connected to earth, i.e. even a metallic pipe system represents a potential conductor.
A connection between PE and N or caused by an insulation fault can be established unintentionally, which nobody would notice without measurement and monitoring by a so-called RCM procedure (RCM = Residual Current Monitoring). RCM monitors are residual current monitoring devices according to DIN EN 62020, which enable targeted monitoring of individual devices or system components.
The new standard also requires that the TN-S network must be installed in its entirety from the main low-voltage distribution board and no longer only from the main building distribution board; with the aid of RCM monitoring, an alarm is triggered if an insulation fault or bridging occurs between PE and N faults.
In principle, according to VDE 0100 Part 710, vagabonding currents should no longer occur if the TN-S network is cleanly monitored from the source. For this reason, the measurement of the 10 mV voltage difference between touchable, conductive parts in the operating theatre has also been omitted from the new standard, because the philosophy of the standard is based on the assumption that monitoring the return current means that there is no longer any danger.
Therefore, it is recommended to continuously monitor the TN-S systems. Under these conditions, no harmful vagabonding currents can occur in normal operation. However, it is important to note that monitoring by means of RCM does not limit the current, but only gives an indication when an alarm limit has been reached! It is therefore not a protective measure and does not indicate where the source of potential equalization currents or voltage sources is.
The current in the PE conductor will never be zero, as there are always device leakage currents due to insulation limits and natural changes due to ageing of the insulation. The more equipment connected to a source, the higher the leakage currents. Thus, depending on room use, the alarm limit of the RCM circuits must exceed the sum of the leakage currents in order to detect spontaneous events that cause the leakage current to rise or indicate increasing ageing of the insulation.
The limit value for measurement in a cardiological surveillance area group 2 with Left Heart Catheter Device should be adjusted accordingly. The increase in the compensation currents measured in the RCM method can therefore be an indication of dangerous potential differences. The increase in the total leakage currents can be controlled by following the change in the measured value display when the instruments are switched on.
Thus, if the type of use of rooms used for medical purposes is changed or the room installation is significantly changed, the electrical installation must be adapted to the new VDE 0100 Part 710; in such cases there is an obligation to adapt.
The implementation of and compliance with the requirements for electrical installation according to VDE 0100 Part 710 only makes sense if the installation philosophy of VDE 0100, which ends at the mains socket, is consistently continued in the rooms used for medical purposes and when medical equipment is connected. However, it is essential to note that unintentional and faulty connections between the various room groups 1 and 2 with IT networks via antennas, intercom systems, video/audio lines, EDP networks or lightning protection systems can invalidate the safety philosophy of VDE 0100 Part 710.
In all buildings with rooms used for medical purposes, it must be checked whether at least the general, local potential equalization is structurally present in accordance with the predecessor standard VDE 0107.
When considering the installation problem, the following points should be checked:
- loop formation
- vector addition of currents
In the medical areas of group 2, connecting bolts for potential equalization cables in accordance with DIN 42801 must be fitted near the patient position in order to include mobile medical electrical equipment and mobile surgical lights in the supplementary potential equalization.
EQUIPOTENTIAL EARTHING BAR
The equipotential bonding rail for the supplementary potential equalization must be located in or near the medically used area (room or room group). An equipotential bonding rail must be arranged in or near each assigned distributor to which the equipotential bonding conductor and the protective conductor must be connected. The connections must be such that the two conductors are clearly distinguishable and separable.
2. SUPPLEMENTARY POTENTIAL EQUALIZATION FOR MOBILE MEDICAL EQUIPMENT
In the discussion about the importance of potential equalization for mobile, portable medical equipment, the still valid VDE 0753 Part 2/02.83 application rules for electromedical equipment for intracardiac interventions must also be observed. VDE 0753 stipulates that when using active, mobile medical equipment in safety class I design in Group 2 medical rooms, the correct connections of the supplementary potential equalization cables and the potential equalization connection equipment in the room must be checked before medical equipment is used.
Leakage currents from electrical equipment, potential equalization currents between room-related metal parts as well as measuring circuit currents from electrical equipment can flow through the heart via intracardiac catheters or pickups placed on the exposed heart; the same problem applies to the brain.
According to VDE 0753 Part 2, direct current or low-frequency alternating current up to 1,000 Hz of 10 mA are still considered physiologically compatible. All electrical equipment that emits energy in any form to the patient or is only conductively connected to the patient can emit these tiny amounts of energy and therefore represent a source of danger at any time.
The supplementary potential equalization for mobile or portable medical equipment still represents a preventive measure to avoid a compliance voltage, which can arise from voltage (potential) differences between touchable equipment and the patient, and to bring all voltages to a common potential.
According to Ohm's law, I=U/R, the low values of 10 mV or 10 mA are required for the operating theater, since the protection factor of the skin is usually canceled with an assumed average value of approx. 1 kOhm for medical applications.
Fig. 4 shows the schematic representation of the electrical laws according to Ohm's law (touch voltage Ub = 10 mV, average body resistance Rbody = 1 kW, limit value for cardiac fibrillation = 10 mA) and thus the electrotechnical basis for limiting currents and voltages in the patient vicinity.
Fig. 4 shows the still prevailing necessity of measuring and maintaining the maximum touch voltage of 10 mV, even if it is no longer included in the new VDE 0100 Part 710 for the reasons mentioned above.
During the application of equipment with direct contact to the patient, an area with potential equalization must be created at least around the patient (so-called patient environment) via a central potential equalization point close to the patient, to which the potential equalization conductors of the equipment are connected.
In group 2 rooms, connection bolts for potential equalization conductors must be installed near the patient position, via which mobile medical electrical equipment for intracardiac interventions and mobile operating tables must be included in the potential equalization when HF surgery is being performed. Potential equalization must be limited to the area immediately surrounding the patient, i.e. within 1.5 m of an operating table or bed in an intensive care unit.
If there is more than one patient in this area, the various potential equalization points must be connected to a central potential equalization rail, which is connected to the protective conductor of the power supply for the area in question.
In the case of medical devices (Fig. 6) with safety class I and II versions, the touchable metal parts of the devices must also be connected to the supplementary potential equalization connection points of the room via flexible potential equalization cables using connecting elements in accordance with DIN 42801 (see Fig. 7).
Potential equalization can consist of fixed permanent connections or a number of individual connections, which are established when the equipment is installed close to the patient. The necessary connection points for the potential equalization cables marked green/yellow (standard designation GREEN-YELLOW) must of course be provided both on the devices and on the system. Figure 8 shows an example of the correct installation or retrofitting of the potential equalization cables on a mobile equipment cart in the operating theatre.
In the case of mobile, mobile medical equipment, the supplementary potential equalization assumes several tasks:
- avoidance or compensation of potential differences between bodies of electrical equipment and permanently installed conductive parts in the patient vicinity
- discharge or reduction of increased leakage currents according to Appendix BBB of DIN EN 60601-1-1:2001(system standard)
- redundancy to the protective conductor connection according to DIN EN 60601-1-1:2001 in case of interruption of an equipment protective conductor
EQUIPMENT FOR SUPPLEMENTARY POTENTIAL EQUALIZATION ON ACTIVE MEDICAL EQUIPMENT
DIN VDE 0750 / EN 60601 / IEC 601 (areas of validity: Germany / Europe / World) do not explicitly require the presence of an earthing pin for every piece of equipment. Although the standard describes the mechanical dimensions of the bolt, it does not contain any installation requirements, so that medical equipment without this connection can also be offered on the market.
This means that the operator may have to retrofit the connection to a medical product or appliance or may have to make this requirement in the selection/tendering or service specifications.
Mobile equipment carts which are intended to accommodate a medical electrical system in accordance with DIN EN 60601-1-1 should always contain a suitable device for supplementary potential equalization in the form of a star (see literature reference no. 5).
Supplementary potential equalization may be required if the equivalent equipment leakage currents exceed the permissible limit values and no isolating transformer is installed.
This means that in practice a medical device without earthing bolts cannot be used and therefore only medical devices with potential equalization connection should be procured. This requirement also applies to non-medical devices such as video monitors, video printers and recorders used in Group 1 and Group 2 medical rooms.
How is a medical device or appliance to be included in the supplementary potential equalization without the bolt? A manufacturer would therefore be well advised to attach such a device from the outset to components that are, or could be, connected to invasive components in the broadest sense.
If the application (room group 2, invasive, total leakage current > 1 mA) requires it, the bolt must be retrofitted to mobile equipment trolleys or medical products.
In the case of old devices which were procured before and during the validity of the MedGV (Medical Devices Regulation), this can be done by an experienced medical technician (taking into account all applicable technical regulations - in particular air and creepage distances must be adhered to, which can sometimes be problematic with daughter monitors), if necessary the commissioning must be carried out by an expert.
SUPPLEMENTARY POTENTIAL EQUALIZATION FOR SYSTEMS SUCH AS MEDICAL ELECTRICAL EQUIPMENT ACCORDING TO DIN EN 60601-1-1
The informative Appendix AAA of Standard 60601-1-1 adds to Section 19.201 leakage currents by pointing out that for non-medical equipment in the relevant standards higher housing leakage currents may be permitted than the limit values in 60601-1-1. If non-medical equipment is operated outside the patient environment, these increased housing leakage currents can be accepted.
Supplementary potential equalization for systems of medical electrical equipment
Within the patient setting, it is necessary to limit potential differences between different parts of a system. In limiting this potential difference in a system of protective conductors, the quality of the connection and the central consolidation play an essential role. It is therefore important to prevent interruptions to the protective measure at any part of the system.
- Supplementary potential equalization conductors can be used if the housing leakage current exceeds the permissible limits at the first fault.
- Supplementary protective conductors for medical electrical equipment that comply with IEC 60601-1-1 are not required. However, in the case of non-medical electrical equipment, this measure may prevent the permissible limits of the housing leakage current from being exceeded.
As shown in Fig. 9, the green-yellow supplementary potential equalization conductor should always be compulsorily connected to the mains connection cable up to the mains plug, so that the user plugs both cables and thus also the potential equalization cable into the intended connection socket.
The necessity of potential equalization to prevent potential differences as a motive voltage source with hazard potentials is still evident.
Monitoring the return current in the PE conductor with RCM technology does not replace the philosophy of precautionary prevention of voltage differences as a driving force for voltage sources with potential hazards for patients and users in medically used rooms. Therefore, all users of medical equipment have to be informed in detail about the importance and necessity of supplementary potential equalization. For mobile equipment, the potential equalization cable should be routed to the mains plug with the mains cable and only medical products and equipment should be purchased which have a connection bolt for supplementary potential equalization.
- VDE 0753 Part 2/02.83 Rules for the use of electromedical equipment in intracardiac surgery
- VDE 0100 Part 710: 2002.11.01 Installation of low-voltage systems, areas used for medical purposes
- VDE 0107 High-voltage systems in hospitals and medical rooms outside hospitals
- Gärtner, A.; Medical Equipment Safety - Guide for the Operator, TÜV-Publishing Company Cologne, 2001, ISBN 3-8249-0672-4
- Gärtner, A.; Requirements for mobile equipment trolleys, mt medical technology, 6/2002, S. 211 - 217
- Hofheinz, W.; Electrical safety in rooms used for medical purposes, VDE Publishing Company 2001, ISBN 3-800-2527-4
- Sudkamp, N.; Electrical systems in hospitals, TÜV-Publishing Company Cologne, 2001, ISBN 3-8249-0533-7
- DIN EN 62020:1999-07, Electrical installation material - Residual current monitors for household and similar uses (RCM`s)
- DIN 42801 Edition:1980-04, Connection bolts for potential equalization cables
- DIN 42801-2, Edition: 1984-01 Potential equalization cables; connection socket