True/false: protective ratio and grounding

TRUE.

Justification

The protective ratio (PR) is the ratio of the equipment insulation level (BIL) to the protective level of the arrester:

$$PR = \frac{\text{BIL of equipment}}{\text{Protective level of arrester}}$$

The arrester protective level depends on the arrester rating, and the rating depends on how the system neutral is grounded:

• During a line-to-ground fault, the voltage of the healthy phases rises. The arrester must not conduct (and must reseal) at this power-frequency voltage, so its rating is chosen as

$$V_{rating} = C_e \times V_{max(L-L)}$$

where $C_e$ is the coefficient of earthing.

• An effectively grounded system allows a lower-rated arrester with a lower protective level. For the same BIL this gives a higher protective ratio (more margin), or for the same margin it allows a reduced BIL.
• An ungrounded system needs a higher-rated arrester, so the protective level is higher and the protective ratio for the same equipment is smaller.

So the protective ratio obtainable depends on the type of grounding employed. This is why EHV systems are effectively grounded: it permits reduced insulation levels.