In high-resistance earthed systems, the neutral point of one of several transformers or generators in connected to earth directly, is a resistor or a reactance. The impedances are sufficiently low so the transient oscillations are reduced and the conditions with regard to selective earth fault protection can be improved. Essentially, a distinction is drawn between:
solidly earthed system current-limiting, resistance impedance (resistance to earth, reactance)
In systems with current-limiting earthing, the fault current against an exposed conductive part or against earth is low under single fault condition, hence automatic disconnection is not required where exposed conductive parts are earthed individually, in groups or collectively.
In comparison to solidly earthed power supply systems, the resistance earthed neutral point connection provides several advantages regarding the protection of persons and power supply reliability as well as regarding consequential damage in case of phase to earth faults. The value of the current at the fault location during the occurrence of an earth fault contributes significantly to the rate of insulation deterioration. The value of the earth fault current depends on the resistance between neutral point and earth. Typical values of the maximum earth fault current are 5 or 10 A. The resistance (RNGR) is calculated according to the Ohm's Law RNGR=U0/IR (U0: nominal voltage, IR: fault current to the neutral point). The reduction of fault currents imposes exacting requirements on the performance and reliability of the monitoring equipment for selective detection and localization of earth faults, since over current protective devices are not operating in this case.
Why low-resistance earthing?
High availability achieved by reducing the earth fault current to no hazardous current values.
The fire risk is reduced.
The mechanical damage in case of earth faults is reduced.
Increased protection against electric shock in protective conductors in case of high transient currents.
Transient over voltages are limited.
Increased protection against material damage and increased plant protection by limiting the fault current.
Fast fault location in case of an earth fault without disconnecting the power supply.
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Functional description-Selective fault location using RCMS
When a phase-to-earth fault occurs in an electrical system with high resistance earthing, the earth fault current is limited by the resistor (RNGR) installed between the neutral point and earth.
The earth fault current is limited to no hazardous current values (5...10A) which does not lead to operating of an over current protective device. For fast fault location, residual current monitoring devices (RCMS systems) are used, which permanently detect and evaluate the fault current in the neutral point and in the load circuits (figure 2). The twelve-channel residual current evaluators RCMS460/490 can be interconnected and are capable of monitoring 1080 channels. Pulsed DC or AC/DC current sensitive measurements can be performed within 180 ms depending on the type of measuring current transformer.
Information exchange between all RCMS devices take place via an RS-485 interface (BMS protocol).
Benefits of Application
Early detection and localization of phase-to-earth faults by installing measuring current transformers in the earthing resistance path and in the load sub circuits.
Either alarm indication or fast disconnection of the faulty sub circuit.
Selective Time Delay
Pulsed DC or AC/DC sensitive fault current measurement depending on the type of measuring current transformer.
Information about the faulty sub circuit at a central location.
History memory, data logger, analysis of the harmonics.
Earth Resistance Monitoring.
In addition to permanent resiudal current monitoring the connection between the transformer neutral point and earth at the earthing resistance can be monitored by using an RC48N (figure 3) ground-fault and neutral grounding monitor.
This device combines the following monitoring functions:
Monitoring of the residual current between the neutral point and earth
Monitoring of the voltage between the transformer neutral point and earth
Monitoring of the earth resistance (NGR)
When the limit value is exceeded, the power supply can be disconnected via a
circuit breaker. An external RI2000 NC remote alarm indicator and and operator
panel can be connected to the RC48N.