Helium is not a fire supressant. It may smother the fire in the same way as CO2, but I don't believe it interupts the chemical reaction in the way that Halon does. To smother the fire would require in excess of 35% by volume of the space to be protected as opposed to 7% for Halon. Even then it is unlikely to be as effective. When the concentration drops and the O2 level increases to 15% plus, the fire is likely to re-ignite!