IEEE C62.55-2020 pdf free.IEEE Guide for Surge Protection of DC Power Feeds to Remote Radio Heads.
Flash strokes do not always occur in the same place. According to 2.7 ofCIGRF TB 549(2013). one-third to one-half of all lightning discharges to earth, both single- and multiple-stroke flashes, strike ground at more than one point with the spatial separation between the channel teminations being up to many kilometers. Clause 2.7 of CICIRE TB 549 (20l3) talks about strikes to ground. and it is not clear if this observation also applies to strikes to towers. The worst case is to assume that all strokes hit the tower.
Figure 10 illustrates the case of multiple flashes, and also includes continuing current (which can last from a millisecond to hundreds of milliseconds).
Annex A5.4 of MIL-STD-464-l997 [1119] shows an extreme case where a first stroke can transfer 10 C, a subsequent stroke can transfer 2.5 C, and continuing current can transfer 200 C (400 A for 0.5 s).
Depending on the interval between strokes and the thermal lime constant of a device, the thermal energy due to the strokes might accumulate in the device, which could impact its functionality. This view is supported in a paper by Sargent et at. 1B23i in which halfof a set oft 8mm MOV samples subjected to multi-surge burst 8/20 surges at rated current showed signs ofdamage: whereas the other half of the samples tested with a single 8/20 surge at rated current repeated at intervals of 60 s or more showed no damage. Analysis of the fiiilcd samples showed cracking and alteration of the material near the surge current conduction channel. Examination of this material suggested that local hot spots formed when a current pulse was passed through the MDV. The material in these hot spots likely resulted from a plasma formed during the current pulse. which rapidly cooled afterwards due to heat conduction to the surrounding ZnO grains. Creation of the altered material was thought to require a local temperature around 1000 °C, which would occur if the pulse energy was concentrated in about 2% of the MDV volume. This is an important observation, because a calculation of the energy absorbed in the multi-surge burst test showed that the temperature rise of the MOV would have been 231 °C if the temperature distribution were uniform, much less than the temperature thought to have caused the damage.
In another multi-surge burst test. Rousseau et at. [B22] subjected an MOV to sixty 20 kA 8120 surges spaced 60 s apart. with no failure. But when the same type of MOV was subjected to as few as five 20 kA 8/20 surges spaced 50 ms apart, liilure occurred.
In addition to forming hot spots, an MOV can fail by surfce Ilashover, as noted in a paper by Darveniza and Mercer [B4J. Surface flashover can occur due to plasma generation. manuliicturing defects of the MOV,surface coating, dielectric properties of the surface coating. and the electrode contact system. For multi-surge bursts. plasma enhancement and MOV surthcc coating were found to play a dominant role in surface flashover.
MOVs have a long themal time constant that allows energy buildup with closely spaced surges. Studies suggest the energy buildup results in failure due to a localized temperature rise in the MOV. A multi-surge burst also delivers repeated mechanical shocks to the SPD being tested (the “jackhammer effect”), and in the case of an MOV this mechanical stress might combine with the stress due to energy accumulation to cause failure. SPDs based on silicon technology generally have short thermal time constants, therefore heat accumulation in these devices is not expected to be an issue.
Figure II, Figure 12, and Figure 13 from Maytum, 2016 [B13] show how the thennal energy due to a flash consisting of a first stroke, subsequent strokes and continuing current can accumulate. In Figure lithe inter- impulse time is 60 ms.IEEE C62.55 pdf free download.