Generator failure is caused by many factors. One of the contributing factors is incorrect fan belt adjustment. This adjustment is very important to do correctly especially for those Lucas generators that were manufactured with a bushing in the commutator end plate. 
I personally believe that this type of generator was designed incorrectly due to the fact that the drive end plate (pulley side) was designed with a ball-bearing. This disparity causes the installer to forget about the rear bushing and loose awareness in the over tightening of the fan belt.
When the fan belt is over-tighten it cause the generator to actually pull to one side and become internally off-center. Once the generator is in that position the rotating armature will start to prematurely wear one side of the bushing causing the armature to drag against the field-coil pole shoes and short itself out causing generator failure. I have taken apart many generator cores in that condition were you can actually see the wall of the bushing only on one side completely worn-out. 
So now we address the question of: What is the correct belt tension for these bushing type generators?. 
You should tighten the fan belt just enough so the belt does not slip on the generator pulley. To test for possible pulley slippage with the vehicle’s motor stopped, take your hand and with one of your fingers apply rotating pressure on one of the generator’s fan blades and try to see if you are able to move the fan and pulley. If this action is possible and you can turn the fan and pulley without the belt moving you need to re tighten the belt a little more. I know this sounds tedious and time consuming, however if you want to save the rear bushing from premature failure I cannot stress enough the importance of correct fan belt tension for these types of generators. 
I think Lucas saw this problem all along and that is why on the later Lucas generator models they replaced the bushing with a ball-bearing and now the belt tension is not so critical because the bearings can handle a little more fan belt tension.