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T/F: SSR not below 1.8

Short answer

SSR is the ratio of peak switching surge voltage to peak normal phase voltage. With closing resistors and synchronized switching, the practical minimum is about 1.8–2.0; reducing it further is uneconomical and gives little insulation saving, so values below 1.8 are not chosen.

State whether the following statement is TRUE or FALSE. Justify your answer with a brief explanation: The value of SSR should not be chosen below 1.8.

TRUE (as a practical design rule).

  • SSR is the ratio of peak switching surge voltage to peak normal phase voltage. Switching a line always produces some transient over-voltage due to trapped charge, travelling-wave reflection and the Ferranti rise.
  • With closing resistors, synchronised (point-on-wave) switching and metal-oxide arresters the SSR can be reduced to about 1.8–2.0 pu; reducing it further needs very costly equipment and gives little saving in insulation.
  • Temporary over-voltages and arrester protective level also set a lower bound: the insulation must anyway withstand arrester residual voltage plus margin.

Hence in design an SSR below about 1.8 is not chosen; values of 1.8–2.0 are used for UHV and higher values for lower voltages.

✓ Answer checked by Claude Opus 5.5 (high effort)

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