Continuous availability, operational reliability and cost efficiency through permanent residual current monitoring

Savings potential through adapted test intervals for permanent residual current monitoring

The protection targets of the BetrSichV and DGUV Regulation 3 with regard to the periodic tests to be carried out are always met if it is ensured that the electrical equipment is free of defects. In addition to the fixed test interval, fixed electrical installations can also be subject to "continuous monitoring". For mobile electrical equipment, the DGUV Regulation 3 implementation directive "Electrical Installations and Equipment" permits, in addition to the strict reference to the assigned test intervals, an extension to the intervals if a fault rate of less than 2 % is determined. Solutions are conceivable if it is ensured that only proper equipment in full working order is made available to the employee.

Using the differential current measurement methods presented here, it is possible for the electrician (or qualified person according to the BetrSichV) to determine clear and meaningful test intervals and to define them for specific applications. This definition can include both a reduction and extension of the test intervals. Depending on the "degree of use" of the equipment, it is possible to carry out a periodic test that is adapted to safety and economic aspects (determination of time intervals).

Switching off, even for short periods, is a thing of the past thanks to the targeted use of residual current monitoring systems (RCMS). The availability of an electrical system is increased and the cost of the periodic testing of electrical systems and equipment is minimized.

Advantages

The main advantages are summarized below:

Optimized maintenance

Permanent monitoring according to DGUV regulation 3 and BetrSichV at longer intervals instead of costly intensive plant inspections
Maintenance measures can be carried out on a plant-specific basis.
Information about changes in the plant status is centralized
Modern facility management through remote diagnosis via Internet/Ethernet

Higher operational/plant safety

Preventive safety to protect people from the hazards arising from electrical current
Faults and unexpected operational interruptions of sensitive equipment are minimized
Insulation faults in newly installed systems and devices are detected immediately
TN-S systems are monitored for additional unwanted N-PE bridges

Higher economic efficiency

No expensive and unplanned plant shutdowns
Less time and personnel required for maintenance
Possible cost savings through lower insurance premiums

Higher fire safety

Residual currents are already detected in the development phase
Overload on neutral conductors is detected
Fire hazards in electrical systems are reduced

Assembly and installation of a residual current monitoring device (RCMS)

The following chapter describes the basic structure of an RCMS system (residual current monitoring device with measuring current transformer and evaluation and display option) in an electrical power supply system. (Fig. 3 – Fig. 5).

Fig. 3: Residual current monitoring system composed of a number of RCMS460 and measuring current transformers in an installation

The basic structure (Fig. 4) shows the circuit monitoring of the power supply with up to 12 outputs (circuits). The preferred installation location of the required measuring current transformers for determining the fault current is also shown and simultaneously serves for fault location detection (circuit assignment).

Fig. 4: Schematic diagram for power supply monitoring for up to 12 individual circuits

Fig. 5 shows an overview of supply circuits whose maximum number is limited to 1080 circuits. The measuring current transformers are selected according to the cable cross-sections of the circuit to be monitored.

Fig. 5: Schematic diagram for power supply monitoring for up to 1080 single circuits

Application example

The RCMS460 residual current monitoring system in an EMC-compliant TN-S power supply, e.g. at the central earthing point and in important sub-distributions

Fig. 6: Schematic diagram of power supply monitoring

Individual measured values from the residual current monitoring systems can be easily viewed from the electrician's workplace. Changes or fault currents that occur in the power supply to be monitored are shown graphically and documented (Fig. 7). Thus, the fault location/final circuit can also be determined beyond any doubt.

Fig. 7: Time characteristic of the fault current in the monitored circuit

Summary

Within fixed electrical systems and equipment, residual current monitoring systems offer an ideal opportunity for assessing insulation degradation. The measured residual currents can be clearly assigned to the respective circuits and thus a plant-related safety assessment is possible. Therefore, the measuring method of the residual current monitoring devices (RCMs) to be used and the installation of the measuring current transformers must be selected to meet the system requirements. Within the framework of continuous monitoring, such monitoring equipment can ensure the required "continuous" metrological testing.

Similarly, in the case of circuits that are monitored by residual current monitoring systems, the connection of faulty hand-held electrical equipment is measured, visualised and, if necessary, the circuit is switched off. This assumes that the permissible threshold values of the differential current are specified by a qualified electrician for the specific system. The chronological documentation and graphic representation of the system behaviour, e.g. a reduction of or increase in the differential currents, make it possible to define adapted test periods for the electrical equipment to be connected.

If it is determined within the organisation which equipment is to be used for operational reasons, the test intervals can be corrected on the basis of the measurement results. Practical experience to date has clearly shown that this measurement method makes it possible to assign individual periodic tests in a tailored manner and therefore meet the protection objective of the BetrSichV "Festlegung gefährdungsbezogener Prüffristen" (Specification of hazard-related test intervals)

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I∆	Residual current/Fault current
I∆n	Response residual current
RF	Fault resistance
RL	Load resistance
IF	insulation fault
I1, I2	Operating current
US	Supply voltage
PE	Protective conductor

Disadvantages of an insulation measurement with DC measuring voltage.	Advantages of RCM measurements
Ageing and previous damage of varistors by insulation measurement with high DC measuring voltages.	RCMs are passive monitoring devices - this source of error is excluded.
Test currents which are too high or which are applied for too long can damage (i.e. burn) protective relays.	RCMs are passive monitoring devices - this source of error is excluded.
Loads that have not been disconnected from the mains during the insulation measurement can be damaged.	RCMs are passive monitoring devices - this source of error is excluded.
UPS supported system components cannot be measured - no switch-off possible.	RCMs are passive monitoring devices - this source of error is excluded.
In extensive networks with large conductor cross-sections and meshed networks, the time required for coordination, insulation testing and evaluation is often not justifiable for economic reasons.	RCMs can perform the task of detecting insulation degradation at a cost that is both technically and economically acceptable.
Periodic verification is very often only random checks. They are not representative and can display different values due to climatic conditions, for example.	RCMs monitor online. A trend analysis and a statement about future failures can be made when measured values are logged and evaluated. Defects that pose a danger to persons, livestock and property must be rectified immediately.
Electronic equipment can be damaged by the test voltage if the neutral conductor is not disconnected or if some form of connection due to insulation faults, for example, is present. In these cases, there is a risk that the test voltage of the insulation measuring device may occur at the input terminals of the equipment.	Disconnection of the neutral conductor is not necessary. Terminals necessary to disconnect the neutral are therefore not required..
Internal shutdown problems; not all system components are checked.	No switch off is necessary.
Operating processes and downtime costs are influenced due to shutdowns.	No costs incurred due to downtime.
Connecting the neutral conductor disconnecting terminal as a hazard point for interruption, or forgetting the connection after measurement.	The disconnecting terminal for the neutral conductor does not have to be used.
N conductor disconnecting terminals are known to be a weak point in the event of high harmonic currents. An interruption causes a voltage increase.	The disconnecting terminal for the neutral conductor does not have to be used.
In the de-energized state, measurements can only be made up to the protective separator.	RCMs measure the complete installation, including powered fixed and mobile equipment.
If the insulation resistances of several circuits are tested together, coupling between different circuits may remain undetected. Possible consequences are feedback voltages and the function of the RCD may be restricted.	RCMs detect such couplings when each circuit is individually monitored.
No meaningful measurement results in the presence of surge arresters.

Disadvantages of an insulation measurement with DC measuring voltage.	Advantages of RCM measurements
High staff retention and considerable cost factor.
Qualified personnel with knowledge of the test equipment are required. They must be familiar with the relevant safety requirements, regulations, operating instructions and the hazards associated with the work. A sufficient number of persons working on electrical systems must be trained to be able to provide first aid. Correct documentation is necessary but in many cases not available.	No expert and knowledgeable personnel are required for monitoring. An electrician is only required when the fault current exceeds the limit value.
Possible hazards and stresses the tester may encounter include: 1. inadequate preparation, risky improvisation, working under time pressure 2. working with safety devices that have been switched off or dismantled 3. working under difficult conditions in the surroundings 4. working on conductors 5. working when the system is energized or in the vicinity of live conductors.	No dangers.

Further distinctive features of RCM measurements	Advantages of insulation measurement with DC measuring voltage
An insulation fault between active conductors is not detected. Insulation faults between N and PE can be measured if loads are connected and switched on.	Can be measured.
RCMs can only be used in TN-S and not in TN-C systems.	Response value from approx. 5 mA: Insulation faults smaller than 40 kΩ between phase and earth can be measured.
Detection by the response value - no high-impedance insulation faults according to DIN EN 5110-1 (VDE 0105-1):2014-02.	The insulation test detects symmetrical and asymmetrical ohmic insulation faults. Capacitive fault currents are not fire-hazardous because they are evenly distributed over the length of the cable.
Detects only unsymmetrical capacitive and ohmic insulation faults.	Bei der Isolationsprüfung werden symmetrische und unsymmetrische ohmsche Isolationsfehler erfasst. Kapazitive Fehlerströme sind brand-ungefährlich, da sie sich gleichmäßig auf die Leitungslänge aufteilen.
There are no standardized response values yet. The response values can vary greatly depending on the mains leakage capacitance and the system. The selection of the correct response value must be carried out by a qualified electrician.	Minimum values of the Insulation resistance are specified in DIN VDE 0100-600 (VDE 0100-600): 2017-06 , subclause 6.4.3.3, Table 6.1.
Conditionally applicable in system parts where no protective conductor is distributed. For a reliable assessment of the insulation resistance, the PE should always be distributed.
In IT systems, the residual current cannot be assigned to an insulation fault downstream of the measuring current transformer.	Can be used in all types of networks.

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Continuous availability, operational reliability and cost efficiency through permanent residual current monitoring

Insulation resistance monitoring measurement and/ or residual current measurement in electrical installations

Residual Current Monitoring (RCM) device measuring technique

Savings potential through adapted test intervals for permanent residual current monitoring

Advantages

Assembly and installation of a residual current monitoring device (RCMS)

Application example

Comparison of the necessary insulation resistance measurements with an RCM-monitored electrical system

Technical personnel

Tester

Further distinctive features

Summary