Ground Fault Protection
PROTECTION AGAINST EARTH FAULTS
with Earth Leakage Circuit Breaker (ELCB) or Earth Leakage Switch.
These units are also called Residual Current Circuit Breaker (RCCB) or Residual Current Device (RCD).
ELCBs/RCCBs and Earth Leakage Switches/RCDs are devices capable of sensing earth leakage current and interrupting the circuit automatically when these currents exceed a predetermined value. An earth leakage current is the current flowing to earth from live parts of an installation.
CBI-electric: low voltage Earth Leakage Protection Devices are the core balance type which monitor the difference in the current flowing in the live and neutral conductors. Under healthy conditions the vectoral sum of the currents in the live and neutral conductor will equal zero and no operation is necessary. If an earth fault occurs the live and neutral currents will not balance and the difference will be detected by the Earth Leakage Circuit Breaker or Earth Leakage Switch. This imbalance is used to open the circuit and thereby stopping the current flow. This is also true for three or four wire circuits, whether or not the load is balanced or unbalanced, the vector sum equals zero.
EARTH LEAKAGE TRIPPING CURRENT (SENSITIVITY)
This is measured in milliamps (mA) and is referred to as the degree of protection in an accidental shock hazard situation.
CBI-electric: low voltage follow the IEC Standard 1008/1009 of 30 mA sensitivity for domestic and personal protection with the tolerance of 30 mA plus zero and minus 50%, that is, a range from 15 mA to 30 mA. CBI-electric: low voltage earth leakage units are manufactured to operate in the tolerance band of 19 to 26 mA.
For personal protection 30 mA offers a high degree of protection and will operate by cutting off the earth fault current well within the time specified in the IEC Publication 1008/1009.
Lower sensitivities above 30 mA are sometimes used for individual circuits where there is less chance of direct contact such as in hot water tanks in a roof or underfloor heating. These earth leakage units from 100 to 375 mA provide reasonable protection from the risk of electrical fires, but it should be noted that under certain circumstances a current of less than 500 mA flowing in a high resistance path is sufficient to bring metallic parts to incandescence and start a fire.
All earth leakage units are provided with a test button. When pressed, the test button causes an imbalance to simulate an earth fault operation. The test button should be pressed often to prove the unit is in working order. If the unit does not trip instantly, expert advice should be sought to check the electrical installation.
WHY ELECTRONIC AND NOT ELECTROMECHANICAL ELCBS?
South Africa's 40 years of experience in the manufacture and application of sensitive earth leakage protection devices, together with the more recent global experiences covering millions of installed units over several decades have resulted in a better understanding of not only the advantages, but also of the limitations of ELCBs.
In Europe, where electromechanical ELCB's are used almost exclusively, the question of reliability of installed ELCBs has become a subject of major concern and attention. The popular hype regarding perceived reliability of electronic components was not sufficient to prevent CBI-electric: low voltage, some five years ago, from changing their entire ELCB range from electromechanical to electronic technology ELCBs. The improvements in reliability that resulted from this decision are complemented by their freedom from safety performance limitations often found in lower specification electromechanical earth leakage circuit breakers.
FUNCTIONAL EARTH CONNECTION
The statistical probability of accidents resulting from broken neutral conductors is relatively low, since several fault conditions would be required to occur simultaneously for a hazardous situation to arise. In recognition, however, of even the remote possibility of hazardous situations arising, this potential shortcoming in ELCBs has been entirely overcome by the introduction of a FUNCTIONAL EARTHING connection into modern electronically operated ELCBs.
The functional earth provides an alternative path for ensuring the continuity of supply energy to the ELCB even in the event of a total break in the neutral supply conductor. The low energy requirements of modern ELCBs ensure safe and reliable operation of the ELCB even in extreme situations where the earth return path bond resistance is orders of magnitude higher than that normally permitted by mandatory or utility regulations.
Under normal operating conditions, the functional earth (FE) circuit is quiescent and does not influence the performance of the ELCB, whether the FE is connected to earth or not.
The connected FE circuit becomes operative immediately a total break or high impedance in the neutral conductor is detected, ensuring full protection even under abnormal conditions.
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