Some people incorrectly assume that dielectric grease is a conducting agent. It is not. It is infact in the true meaning of the name, an electrical currant blocker. It is in all cases a barrier to block electricity. Autotrad brand dielectric grease will block up to 80,000 volts.
We recomment it where you need protection from power leakage, such as at the bottom of the boot of Coil over plug arrangements. Used sparingly at the tip of the boot it will adequately prevent spark leakage and moisture penetration of the spark plug boot.
In some case, when too much dielectric grease has been added to the bottom of the boot and getting onto the coil spring it has stopped the engine. In another incident it has caused the engine to develope an anoying miss. The solution in both cases was to degrease the spark plug top and the spring, removing all traces of the grease in that area.
So be very careful when next you have the need to use dielectric grease on spark plug boots. There are some products on the market where this will not be a problem. They are mostly used in the food industry as a safe lubricant, not really useful in containg the spark energy in your ignition coils.