Ionisation can happen when tube blows and where semiconductors are used to switch the overload can cause the device to fry and become closed circuit. Sometimes semi-conductors fuses can stop this but MCB's are too slow.
Likely you need new sensor. Also be aware of restrictions on outside lights over 150W to leave without some form of auto switching will likely break the law. Sorry can't remember which "Part" of building regulations but looking at the output from a 75W metal halide light there is no need to exceed the 150W limit really.
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