Spark testers is usually used for testing if there are leakage between insulating coating or membrane (such as PVC). The spark testers generate high voltage pulse, which is higher than dielectric voltage of air (but lower than tested coating or membrane). When there is leakage on the coating or membrane, spark will be generated between the spark tester and material behind (usually concrete or metals)
The dielectric strength is the maximum working voltage a material can withstand without breaking down. It is normally expressed in Volts/mm. Dielectric voltage of air is about 3kV/mm, depend on air conditions.