Hey bro, I think I know why they state that on there. Not all timers are created the same, and yours is probably listed as such because of what's call "inrush current."
Inrush current is defined as the maximum, instantaneous input current drawn by an electrical device when first turned on by solid state equipment (like your digital ballast.)
Peak inrush currents can be several orders of magnitude greater than the circuit's steady state current. It can be as high as 20 times normal operating current. Usually this inrush is reduced after 10ms, but could take up to 30 to 40 cycles until the current is at a normal value.
Inrush current can affect electrical components such as tripping circuit breakers and fuses. During start up, momentary contact bouncing in switches or relays may cause the contacts to become pitted due to arcing between contact points. This surge in current can also cause serious damage, such as welding switch contacts together.
So, what that means is the timer is going to be much more prone to failure if you use with a digital ballast (which is solid state technology.) That's not true for all mechanical timers, just in the case of yours or specifically when it's explicitly stated. Some are rated/designed to handle this inrush current.
Just some food for thought on timers:
In a mechanical timer, a motor turns the dial which mechanically actuates a switch that turns on or off the light at certain times.
In a digital timer, a microcontroller activates a relay which turns on or off the light.
Mechanical timers tend be a little more rigid in their time increment requirements; meaning the time intervals are set usually in 10 or 15 minute clicks. That means you can set it start at 7:00 AM, but the next peg to RUN or STOP will be either 7:10 AM or 7:15 AM.
A digital timer allows you to set much more accurate and frequent START/STOP times.
In terms of reliability, on a mechanical timer; the mechanical pawl and micro switch are usually the first to fail. On a digital timer, the relay or the micro controller is usually the first to go.
Hope that helps out a bit bro! :karma Cloud: