constant voltage from glow plug timer

hello all,, my missus informed me her megane was hard starter in cold...i removed all glow plugs which proved to be shot..i then put my voltmeter to glow plug feed at glow plug one ignition off..5.7v constant,, i understand that voltage is regulated by the oil temp sensor...but would this still supply 5.7 volts to glow plugs when ignition is off......when glow plug light shows on dash glow plugs are getting 12v ...when timer cuts out reverts to lower voltage ..is this the norm just dont want to destroy new glow plugs thanks