How to define BC ?

HI,

I am doing a 3D electromagnetic simulation. I am using magnetic field physics interface. We have here the magnetic insulation boundary condition (default). But I also want to use an 'electric insulation' BC in some boundaries. I think imposing the equation n.J=0 as a boundary condition on those boundaries should work. But I am not sure how to define this equation as a boundary condition.

Please Help!

Thanx in advance! :)