Your digital multimeter (DMM) should have a CAT rating on its case. The CAT rating gives you a transient voltage range for which that DMM (or any other electrical test instrument) is safe to use. If you look at your DMM and don't see this rating, then you cannot know its safety level. Destroy and discard that DMM.

The categories, in ascending order of transient voltage withstand capability, are CAT I, CAT II, CAT III, and CAT IV.

The manufacturer achieves a given CAT rating through various means. The basic strategy is to put higher resistance between potential points of crossover within the device. If you compare a CAT I multimeter to a CAT IV DMM, you'll notice the lead jacks on the CAT IV are spaced much farther apart than those on the CAT I.

We'll look more closely at CAT ratings in our next issue.