Usually unrecognized, a creeping danger develops in commercial and industrial operations as well as in office and public buildings: currents that flow via undesired paths due to an insulation fault.
In an ideal situation, all circuits within an electrical installation are insulated and there is no possibility of a current not flowing back via the desired path. Over time, however, dangerous leakage currents can develop. For this reason, the responsible electrician (VEFK) must carry out regular insulation measurements on equipment and systems, among other things. They are responsible for ensuring that these measurements are carried out and documented at regular intervals. If this VEFK does not exist in a company, the employer/entrepreneur is responsible for this.
There are laws, regulations, ordinances and standards that refer to these obligations and also require them.
In my opinion, the most important is the DGUV V3 accident prevention regulation for electrical systems and equipment. In addition to personal protection, there is another not insignificant aspect: the contributions for the property insurance of companies. The VdS (Association of Property Insurers) checks whether these measurements have been carried out. If no proof can be provided, there is a risk of higher premiums or, in the worst case, termination by the insurance companies.
Recurring insulation measurement
But why are these regular insulation measurements so reluctant to be carried out? The reason is the considerable effort involved and the risk of damaging equipment. During the insulation measurement, all systems and circuit components should be recorded, which means that all switching elements must be connected. At the same time, all surge protection devices and the majority of equipment must be disconnected. This work is time-consuming and costs are incurred for production downtime during the measurement period. These are considerable and therefore the biggest hurdle for companies.
Example in a data center of a company in which the regular DGUV V3 test must be carried out: In order to carry out the test, the software applications must be terminated, servers shut down and de-energized. In the period between shutting down the servers, measuring for DGUV V3 and putting the software applications back into operation, the entire company is at a standstill. Continuous monitoring and testing by residual current detection systems is used here.
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Alternative: Residual current measurement
If fixed electrical installations and equipment are continuously monitored with a residual current monitoring device (RCM), it is not necessary to measure the insulation resistance. The following processes must be ensured for this:
- Preparation of a risk assessment
- Permanent monitoring of the systems using residual current monitoring
- Establishment of a reporting chain in the event of an excessive residual current occurring
- Organization of prompt fault clearance
How does a residual current come about and how is it measured? In mathematical terms, a residual current is the mathematical sum of all current components of the active conductors L1, L2, L3 and N. Ideally, this should be 0 amps due to the phase offset in the three-phase system. If the value is greater, either unavoidable leakage currents are flowing or there is a fault in the electrical installation. This residual current can damage systems, cause fires and, in extreme cases, cause damage to people’s health.
What causes a residual current? There are many possibilities: Active current-carrying conductors are surrounded by insulation. Over the years, the plasticizers diffuse out of the insulation so that it becomes porous and can break. If the cable is in contact with a conductive housing, a current flows via the protective conductor: we have an insulation fault that causes a residual current. Residual currents can also be caused by mechanical damage to cables or systems, as well as by technical defects in systems. Human error can also lead to residual currents.
Personal protection and DGUV V3
The best-known residual current measuring device is the residual current circuit breaker (RCCB). This not only measures continuously, but also interrupts the circuit in less than 500 ms in the event of a fault. An RCD is used for personal or fire protection and interrupts the circuit at a defined residual current. Already standard in residential buildings, it is usually unthinkable in industry due to the consequences: an automatic shutdown of the system. However, there are also solutions in the commercial sector: the residual current can be measured with a so-called residual current transformer. If this is connected to an evaluation unit, an automatic alarm is sent to the responsible electrician.
The maximum residual currents for personal and fire protection are regulated by law. These limit values can be monitored using a residual current measuring system. There is one difference to note here: While the maximum permissible 300 mA is monitored with an RCM for fire protection and an alarm message is generated if this is exceeded, the circuit must be automatically interrupted by an RCD if 30 mA is exceeded for personal protection.
Residual current transformer type A or B/B+
While type A residual current transformers detect residual currents in the AC range, type B/ B+ transformers are all-current sensitive. This means that with all-current sensitive transformers, the currents in the DC range are also measured, so that continuous residual current measurement can replace regular insulation measurement in accordance with DGUV V3.
Using the example of a frequency converter, we show which leakage and residual currents can occur:
- 50 Hz residual current (fault in the system)
- AC residual current with many frequency components (fault in the system)
- DC residual current in the DC link (fault in the system)
- Residual currents via interference suppression capacitances (leakage currents that cannot be prevented)
- Residual currents via parasitic capacitances (leakage currents that cannot be prevented)
To illustrate the difference in residual current measurement with type A and type B/B+ transformers, we installed both types of residual current transformers in series in the supply line of a system at the KBR plant in Schwabach. In the diagram, the green bars show the energy consumption of the system, while the blue and red lines show the residual currents of the two different residual current transformers. During operation, the residual current increases – but differently. While the type A transformer (red line) only records low residual currents of a maximum of 11.43 mA, the type B residual current transformer (blue line) measures values of up to 91.43 mA. In this case, the residual current is mainly caused in the DC range by a fan with a frequency converter. The conclusion from this practical example: The selection of the residual current transformer type is important for assessing the residual current.
Conclusion
Setting up a residual current measurement relieves the responsible electrician of the task of measuring the insulation resistance. This reduces the amount of human resources required, reduces the high costs of downtime and prevents possible faults and damage to equipment caused by insulation measurement. The use of RCM measurement methods does not release the VEFK from the obligation to carry out periodic inspections of electrical systems and equipment by inspecting and testing the protective and equipotential bonding conductors and the effectiveness of the shutdown condition.
The measured values of the residual currents can be transferred to KBR’s visual energy energy data management system for analysis. This means that the responsible electrician always has a quick and complete overview of possible faults within his system. When planning a residual current system, it is important to ensure that a specialist develops the concept for the measurement setup. If you have any questions about your residual current measurement, please do not hesitate to contact our sales engineers.