How much draw does the MH light have in amps?
What are your relays rated at?
I ask because a metal halide ballast is a highly inductive load. When power is cut quickly on a highly inductive load, a voltage spike is produced that can damage a relay contacts.
To combat this, a diode can be connected across the load to limit this spike, its called a flyback or freewheeling diode.
With inductive loads, its better to oversize the relays and use a protection diode.
Not a diode (thats for the coil) but instead an RC Snubber circuit. Sizing these requires some calculations.