Transformer insulating oil — typical dielectric constant (relative permittivity) The dielectric constant (relative permittivity) of mineral transformer oil is approximately:

Introduction / Context:
Transformer oil provides insulation and cooling. One property of interest is its dielectric constant (relative permittivity, ε_r), which affects electric field distribution and capacitances in windings and bushings.

Given Data / Assumptions:

• Conventional mineral (naphthenic/paraffinic) transformer oil, clean and dry.
• Room temperature conditions; no significant additives altering ε_r.
• Selection among approximate, rounded options.

Concept / Approach:
Typical ε_r values for mineral transformer oils lie in the low-few range (significantly greater than air ≈ 1, and far less than ceramic dielectrics). In many practical calculations and multiple-choice keys, ε_r is taken as a small integer near this range.

Step-by-Step Solution:
Recognize that air/vacuum is ≈ 1, while common organics are a few units.Values like 10 or 20 are characteristic of certain polymers/ceramics, not mineral oils.Among the given rounded choices, “4” best represents the order of magnitude.Select option 4 as the approximate dielectric constant.

Verification / Alternative check:
Handbook data often cite ε_r ≈ 2–3 for mineral oils; exam options are coarse. The nearest choice above air is 4, which captures the “few” category better than 1.

Why Other Options Are Wrong:
(a) 1 corresponds to air/vacuum; (c) 10 and (d) 20 are too high for mineral oil.

Common Pitfalls:
Confusing dielectric constant with dielectric strength (kV/mm); mixing mineral oil with high-κ dielectric materials.

Final Answer:
4