RESIDUAL CURRENT MONITORING
•Residual current monitors RCM, RCMA |
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Any power Supply and Distribution Systems are chosen with suitable protective measures depending on the requirement of reliability, criticality of the equipment and process, considering the possibility of Insulation Faults and basic fault protections. The protective Earthing is a common practice for any System.
A protective earth (PE) connection ensures that all exposed conductive surfaces are at the same electrical potential as the surface of the Earth, to avoid the risk of electrical shock if a person touches a device in which an insulation fault has occurred. It ensures that in the case of an insulation fault (a "short circuit"), a very high current flows, which will trigger an overcurrent protection device (fuse, circuit breaker) that disconnects the power supply.
International standard IEC 60364 distinguishes three families of Earthing arrangements, using the two-letter codes TN, TT, and IT.
Distrinution Networks and Earthing
a) IT System – All active parts are insulated from earth or one point of supply is connected to earth through sufficiently high impedance. The exposed non-current carrying conductive parts of the electrical installation are earthed individually or collectively.
b) TN System –One point of the supply is directly earthed. The exposed non-current carrying conductive parts of the installation are connected to that point by protective conductors.
c) TT System – One point of the supply is directly earthed. The exposed non-current carrying conductive parts of the installation are connected to earth electrodes, which is/are electrically independent of the earth electrode of the system.
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Insulation Fault and Residual Current
An insulation fault may be due to insulation deterioration:
(i) between two live conductors,
(ii) between a conductor and the Functional Earth or the Protective Earth conductor,
(iii) on a single live conductor, making the conductor accessible to Human touch.
An insulation fault between live conductors becomes a short-circuit. In the above cases, a fault results in current flow to earth. This earth-fault current, which does not return through the live conductors, is the resultant of the instantaneous currents flowing in the live conductors is the "residual current", which quantifies the level of Human protection/safety.
BENDER Solution
Residual Current Monitoring Device
The AC and pulsed DC sensitive residual current monitor RCMA420-D (Type A) from Bender is designed for fault and residual current monitoring in earthed power supply systems (TN and TT systems) where an alarm is to be activated in the event of a fault, but disconnection must be prevented. In addition, the device can be used to monitor the current in single conductors, such as PE conductors, N-PE connections and PE-PAS connections. Due to the use of measuring current transformers, the device is nearly independent of the load current and the nominal voltage of the system.
The response value can be adjusted in the range of 10...500 mA. When the prewarning level is reached (50% to 100% of the response value), an alarm message is generated. This information achieved sufficiently well in advance can avoid unexpected disconnections and costly downtimes.
Operation
- An insulation fault RF leads to a connection of a phase to PE
- The fuse blows
- The load is automatically disconnected
- The fault current is equal to the short-circuit current of the fuse IF = IK
- High electrical energy may occur at the point of fault (fire risk)
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Applications
Typical application fields are: Residual current monitoring in earthed two, three or four conductor systems; construction site equipment; current monitoring of single conductors de-energized under normal conditions; socket-outlet circuits for devices which are operated unattended for a long time and which may not fail; alarm systems; safety devices.
Residual Current Monitoring System
BENDER presents the world's first multichannel, AC/DC sensitive, residual current monitoring system
The availability and reliability of the power supply is vital not only for the efficiency and productivity of our customers processes but also for the safety of traffic engineering and public facilities. Within the framework of preventive maintenance it is therefore very important that power supplies be continuously monitored to ensure that faults can be recognized and localized at an early stage.
The new residual current monitoring system RCMS460/490 from BENDER reliably monitors operating, residual and fault currents of 6 mA to 20 A AC, pulsed DC and AC/DC sensitive currents during operation. This applies for up to 1080 circuits while the max. scanning time for all channels lies under what could well be a record time of 180 ms.
The exchange of information between the devices takes place via a time- and cost saving RS-485 interface. Connection to a higher-level data management systems can easily be carried out via Ethernet or other field bus systems
