Lumped system analysis assumes a uniform temperature distribution throughout the body, which will be the case only when the thermal resistance of the body to heat conduction is

Explanation:-

In general, the temperature of a body varies with time as well as position.

Lumped system analysis - 

Interior temperatures of some bodies remain essentially uniform at all times during a heat transfer process. The temperature of such bodies are only a function of time, T = T(t). The heat transfer analysis based on this idealization is called lumped system analysis. Which will happen only when the body possesses an infinite value of thermal conductivity or the body have zero thermal resistance to heat conduction.

The Biot number is the ratio of the internal resistance (conduction) to the external resistance to heat convection. Lumped system analysis assumes a uniform temperature distribution throughout the body, which implies that the conduction heat resistance is zero.

Thus, the lumped system analysis is exact when Bi = 0.

It is generally accepted that the lumped system analysis is applicable if  Bi <= 0.1.

Therefore, small bodies with high thermal conductivity are good candidates for lumped system analysis.