It seems that 150W and 250W lamps have almost the same electrical characteristics, except the voltage. Doesn't look risky, but is this really safe?

you could probably use this in the short term,such as testing a lamp,but it would be bad for the ballast and the lamp to be run like this continously,as you are overloading the ballast by driving a higher wattage lamp than what it was designed for,you are also damaging the lamp as the lower power means cooler than optimum electrodes.

thats my theroy.

You understood. That's only for a testing.

What about the opposite?  For example, running a 100 watt MV lamp on a 175 watt ballast.  How bad is this for the ballast & the lamp?

thats just as worse,overdriving the lamp over its rated power shortens the lamps life,as well as raising the risk of explosion.

it might be bad for the ballast,but maybe not as bad as as using a higher wattage lamp on a lower wattage ballast.

I just ran my 250W lamp in my S55 ballast for a bit more than 30 min without any problems. I could run it for much longer...

I just ran my 250W lamp in my S55 ballast for a bit more than 30 min without any problems. I could run it for much longer...

i mean by a signifigant margin(ex.running a 175 watt lamp on a 400 watt balast,or running a 400 watt lamp on a 1000 watt lamp).

in general forr best preformance most lamps should be opperated within a -/+ 10% margin of their rated power.

I'm surprised the 250W ran without cycling on the S55 ballast.

@Brian: I ran this 100W mercury lamp on a 175W ballast . The lamp still wouldn't start without a kick but it ran fine for over an hour,it would cut-out on a 100W ballast once it was almost full bright.

I'm surprised the 250W ran without cycling on the S55 ballast.

Don't forget that my 250W HPS lamp is brand new... 

That's true,I was going to say that it's probably running with no problems because it's new.

Overunning todays MV will probably explode them! But running older MV lamps is probably a different story because I have a 175 watt Sylvania lamp that was in the 400 watt PowerBracket for about 15 years or more......it still works today, but the arctube is bulged!!!

I ran my EYE 125W lamp on my H39 ballast for over 30 min. this afternoon and he worked well. And WEIRD thing, it looked like a /C lamp! Very strange for a /DX lamp...

I have run a chinese 100w merc on a 400w ballast, it ran very bright and the arc tube bulged, but it still works today! On the other hand, I destroyed a phileeps alto 50w HPS lamp by running it on a 1kw mercury ballast. :-P

By the way Vince, your claim that the 150w and 250w HPS lamps having almost the same electrical characteristics is incorrect unless you're referring to the rarer S56 150w lamp which runs at 100v like the 250w lamp. The 150w S55 lamp operates at only 55v. Understandly that your lamp didn't cycle cuz it was brand new, but if you ran it longer it'll eventually cycle at a lifestage much earlier than the rated 24,000 hour life.

Dave: I know the voltage of these 2 lamps is different, but I wanted to say that they are almost the same current (3.3 amps for the 150W and 3.0 for the 250W if I'm right.)

i ran a 100 watt regent delux white lamp on a 175 watt ballast,the lamp didnt seem to like it very well,as it kept flickering.