Preventing expensive unscheduled shutdowns by knowing the state of electrical supply systems
June 2000
Analytical Instrumentation & Environmental Monitoring
Anglo Allied Engineering
The sudden loss of electrical power, even in a small section of a modern plant, can result in the unscheduled shutting down of the whole plant which may prove to be very costly. Obviously, a good 'feel' for the state of the electrical wires and insulation in the plant would be of great help when planning scheduled maintenance and equipment overhauls.
In distributed IT TN and TT systems, insulation fault location can also be costly in terms of time and money. The Bender on-line insulation fault and residual current location system provides an automatic solution to these problems.
Time and money are saved by being able to monitor residual currents and verify insulation integrity while the plant is still running. The system will indicate the exact location of the fault, saving time looking for it. The resultant early preventative maintenance, without having to shut down the plant, makes it a cost-effective approach.
The system is modular and flexible, able to monitor the electrical supply system in any plant. Its flexibility permits easy retrofitting and extension, with up to 360 branches capable of being monitored by one system.
Its universal application is implemented through three system types. The EDS system is for use with all AC/DC IT systems, the RCMS system is for all AC, TN and TT systems and there are portable systems for working with small power systems and subcircuits. EDS devices comply with IEC/EN61557 \9 (draft).
Insulation fault
When an insulation fault occurs in IT systems, it will be detected and indicated by the A-Isometer insulation monitoring device. The test device generates a test signal for a defined period. This test signal will propagate via any insulation fault locations, and be able to be detected at any of the current measuring transformers within the insulation fault path.
Residual current location
All the conductors for a given circuit are fed through the core of a current transformer. In normal operation, the sum of all the currents will be zero. In a fault condition, there will be an imbalance in the total. This imbalance will be detected by the current transformer and the result will be evaluated by the RCMS470's electronics.
A maximum of 12 current transformers (ie 12 circuits) may be monitored by one RCMS470-12 evaluator, any faulty circuits are indicated by an LED. All the RCMS units work in parallel to minimise system evaluation.
The combination of an RCMS and PRC provides a central control and display of the measuring results and pre/main alarm warning set-up (10 mA to 10 A) for each circuit. The use of current transformers allows the RCMS system to be independent of both the load current and the voltages used in the installation.
Central control unit
The PRC470 is the central control unit for the system. All monitoring parameters and important operating states are displayed in a test window, providing a compete overview of the entire installation. Communication interconnection between controller and the evaluators is made by means of a cost saving two-wire interface which is easily extended.
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