The TF coils were originally designed for operation at 3.45 T. Margins in the original design suggested that a higher field might be possible. A simple scaling up of the stresses in the copper showed reasonable stresses. Operation at 4 T appeared to offer substantial advances in plasma performance so in mid 1995 a detailed feasibility study was started. Upgrade of the power supplies was also studied and is reported elsewhere[1]. JET was conceived by the original designers with a high degree of built in flexibility. Among other aspects, it is the only tokamak in the world, designed in eight octants, each one including a section of the vacuum vessel, four toroidal coils and the corresponding section of the mechanical structure. This choice allowed nine complete octants to be procured, i.e. to provide a spare section of the vacuum vessel, of the mechanical structure and four spare toroidal coils. In addition, the central solenoid was designed in eight pancakes and two spares were also procured. These were used to upgrade the ohmic heating system, when JET was modified to increase the plasma current from 4.8 MA to 7.0 MA in limiter configuration and from 3.0 to 5.0 MA in X-point configuration (without divertor) in 1987-88. The 32 TF coils heat up adiabatically during the pulse and were originally water cooled in between pulses. As in other major tokamaks, water leaks developed. In JET these leaks occurred in three of the TF coils leading eventually to interturn short-circuits.