IEC 60099-8:2017 pdf download

IEC 60099-8:2017 pdf download.Surge arresters — Part 8: Metal-oxide surge arresters with external series gap (EGLA) for overhead transmission and distribution lines of ac. systems above I kV.
This part of IEC 60099 applies to the externally gapped line arrester (EGLA)
This type of surge arrester is connected directly in parallel with an insulator assembly. It comprises a series varistor unit (SVU), made up from non-linear metal-oxide resistors encapsulated in a polymer or porcelain housing, and an external series gap (see Figure 1).
The purpose of an EGLA is to protect the parallel-connected insulator assembly from lightning-caused over-voltages. The external series gap, therefore, should spark over only due to fast-front over-voltages. The gap should withstand all power-frequency and slow-front over-voltages occurring on the system.
In the event of SVU failure, the external series gap should be able to isolate the SVU from the system.
specified short-term load of an SVU
greatest mechanical force perpendicular to the longitudinal axis of an SVU, allowed to be applied during service for short periods and for relatively rare events (for example, short- circuit current loads and extreme wind gusts) without causing any mechanical damage to the
mean breaking load of an SVU
average breaking load for porcelain or cast resin-housed SVUs determined from tests
high current impulse
peak value of discharge current having a 4/10 or 2/20 impulse shape, which is used to test the withstand capability of the SVU on extreme lightning occasions
salt deposit density
amount of salt in the deposit on a given surface of the SVU housing, divided by the area of this surface; generally expressed in mglcm2
verification test for coordination between insulator withstand and EGLA protective level test used to verify that the EGLA will exhibit correct sparkover operation and clamp the overvoltage caused by lightning considerably lower than the flashover voltage of the parallelconnected insulator assembly
The housing of the SVU shall withstand a lightning impulse voltage of
a) for “Series X”: 1,4 times the residual voltage at the nominal discharge current
b) for TMSeries Y”: 1,13 times the residual voltage at high current impulse, but not less than 1,3 times the residual voltage at nominal discharge current
NOTE The factor of 1,4 in case a) covers variations n atmospheric conditions up to 1 000m altitude and discharge currents up to three times the nominal discharge current.
6.1.2 Insulation withstand of EGLA with shorted (failed) SVU The EGLA shall have the following insulation withstand performance:
a) the EGLA shall withstand the specified switching impulse withstand voltage level of the system even if the SVU has been shorted due to overloading (failure);
b) the EGLA shall be able to withstand the maximum temporary over-voltages phase to ground for their maximum durations even if the SVU has been shorted due to overloading (failure).
6.2 Residual voltages
The purpose of the measurement of residual voltages is to obtain the maximum residual voltages for a given design for all specified currents and wave shapes. These are derived from the type test data and from the maximum residual voltage at a lightning impulse current used for routine tests as specified and published by the manufacturer.
The maximum residual voltage of a given EGLA design for any current and wave shape is calculated from the residual voltage of SVU sections tested during type tests multiplied by a specific scale factor plus a calculated inductive voltage drop across the SVU, the gap and.IEC-60099-8-2017

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