Rule, that check if last update is older than 5 Minutes

Setup, Configuration and Use Scripts & Rules
2

There are examples in this all over the Design Patterns.

I recommend using the Design Pattern: Expire Binding Based Timers and just put that on your HEARTBEAT Item and throw away the LASTUPDATE Item and Rule, unless you want to show it on your sitemap for some reason.

Switch E750_HEARTBEAT ... { ..., expire="5m,command=OFF" }

rule "No Heartbeat"
when
    Item E750_HEARTBEAT received command OFF
then
    E750_POWER_ALL_PHASES.postUpdate(0)
end