I recently purchased some 400watt MH gear and would like to know if it’s safe to run the lamp out of an enclosure. I do know that MH lamps have the tendency to sometimes explode apon EOL.

Running them vertically reduces the likelihood.

Is f it is an exposed rated lamp, then I would say yes this is probably safe.

But assuming it is an enclosed rated lamp (as most are), then I would say maybe. If the lamp is used (or you don't know it's history), I would not at all run it in the open. Technically even a brand new lamp can explode with a severe enough manufacturing defect, but I haven't seen that.

Explosion is always a risk, but if you shield the lamp or shield yourself you should be relatively safe even with an old lamp.

I recently purchased some 400watt MH gear and would like to know if it’s safe to run the lamp out of an enclosure. I do know that MH lamps have the tendency to sometimes explode apon EOL.

All HID lamps (excluding HPS, as they run at below atmospheric pressure) carry some nonzero risk of explosion. Even mercury lamps. In practice, it is rather minor for a short test run. Be careful with heavily used ones with blackened or bulged arctubes.

Surprisingly, old style high powered MH lamps like 400W multi-vapor tend to run at rather moderate pressures and are less risky than some smaller MH in 20, 35 and 70 watts!

There are open rated lamps having an extra protection shroud around the arctube for the case it explodes, which can be run with a peace of mind openly. Rare in high wattages.