as mentioned just grounding it manually from the ground screw.But that shudnt be the case.It shouldnt have potential beacuse though its true theres a fuse inside which wud trip should hazardous amt of current flows thru but still still its not acceptable
Many a times, manufacturers do use a convenient and current style plug, as the one used here is used in Printers, PCs etc. However, inside they simply do not connect, (Like in Isolation Transformers). Hence I believe the separate ground outlet is provided. IMO, you should just peek inside and see whether both grounds are same, or touch the multimeter prod from outside to both, Earth pin and Ground outlet.