Excess current protection

The total earth loop impedance for ready operation of the largest rated excess current protective device relied upon for earth leakage protection. 

The winding insulation in electric motors of the enclosed ventilated type will overheat and may break down if the ventilation is impaired, e.g. by foreign matter clogging the air vents and dncts. The resultant short circnit between turns or to earthed metal may cause an ignition, but may not canse a sufficient increase in the supply current to operate the excess current protection. 

Cables should be protected by correctly rated excess current protective... 

Apparatus such as motors which may be subject to overloading should be provided with their own excess current protection and, where necessary,... 

The next consideration would be fault protection. If a short circuit fault occurred on the cable between the starter and the motor, is the starter capable of safely interrupting the fault current A loop impedance test may be needed to ascertain the fault level. If the chosen starter is incapable of safely interrupting the fault current, suitable additional protection would be needed on the supply side of the starter. This would ensure that the operation of the combined excess current protective devices would either avoid damage to the starter or so limit it that anyone nearby is not endangered. 

Regulation 21 requires excess current protection and evidently includes fault currents. Precautions against excess voltage are in Regulation 22 and against supply failure in Regulation 23. 

The prospective short circuit currents have to be determined at all relevant locations in the installation by calculation or measurement and protective devices selected to protect all conductors against thermal and mechanical effects. For new installations the designer will first have to ascertain the loop impedances and characteristics of the excess current protection at the intake from the supply company (see Regulation 313-01-01), except where private... 

If the supply system is PME, the supply company is precluded from providing an earthing terminal for outdoor sites because there is no equipotential zone, so the contractor has to provide his own earth. He may well then find that the earth loop impedance is too high for operation of the excess current protection, on the occurrence of an earth fault, within the specified time and he will have to resort to earth leakage protection using an RCD. 

To cater for the supply company s needs, the standard describes the requirements for an incoming supply assembly (ISA). It has two compartments with lockable access - one for the incoming cable termination, service fuses, neutral link, current transformers and meters, and another for the consumer s main switchfuse or circuit breaker fitted with excess current protection and, where appropriate, earth leakage protection. 

MCBs with excess current protection. The EGAs are similar but without the MCBs. The EGAs can be fed from the socket outlets on either the TAs or SGAs. The SGAs and EGAs are hght enough to be carried by one man. 

Where the supply company provides a site substation there should be no problem about using its earth terminal Ze will be very low and the excess current protection should be adequate for earth leakage protection except for the larger installations. 

Used as PTC thermistor, excess current protection element low-temperature heat, temperature sensor, condensor. 

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